The importance of protein in nutrition and health cannot be overemphasized. It is quite appropriate that the Greek word chosen as a name for this nutrient is πρώτειος (proteios), meaning “primary” or “in the lead”, or “standing in front”. Proteins are found throughout the body, with over 40% of body protein found in skeletal muscle, over 25% found in body organs, and the rest found mostly in the skin and blood.
Proteins are essential nutritionally because of their constituent amino acids, which the body must have to synthesize its own variety of proteins and nitrogen-containing molecules that make life possible. Each body protein is unique in the characteristics and sequence pattern of the amino acids that comprise its structure. Amino acids may be classified in a variety of ways, including by structure, net charge, polarity, and essentiality.
Table of Contents
- Amino Acid Classification
- Amino Acid Biochemistry
- Essential And Non-Essential Amino Acids
- Protein Biochemistry
- Sources Of Amino Acids
- Protein Digestion
- Protein Absorption
- Amino Acid Metabolism
- Energy Production
- Ketone Body Production
- Other Uses Of Amino Acids
- Protein Synthesis
- Functional Roles Of Protein
- Nitrogen-Containing Non-Protein Compounds
- Glutamine Metabolism
- The Alanine-Glucose Cycle
- Skeletal Muscle
- Myths About Protein
- Meal Frequency
Amino Acid Classification
Structurally, amino acids have a central carbon (C), at least one amino group (—NH2), at least one carboxy (acid) group (—COOH), and a side chain (R group) that makes each amino acid unique.
Depending on the pH of the environment, the amino and carboxy groups can accept or donate H1, and thus the amino group may be represented as 1NH3 and the carboxy group as COO2. The distinctive characteristics of the side chains of the amino acids that make up a polypeptide bestow on a protein its structure and influence its functional role in the body. Amino acids can differ in structure in environments with different acidity levels.
These same distinctive characteristics determine whether certain amino acids can be synthesized in the body or must be ingested, as well as which metabolic pathways of the body the amino acids can enter.
Amino Acid Biochemistry
The tendency of an amino acid to interact with water at physiological pH—that is, its polarity—represents another means of classifying amino acids. Polarity depends on the side chain or R group of the amino acid. Amino acids are classified as polar or nonpolar, although they can have varying levels of polarity.
Polar-charged amino acids include both the dicarboxylic (aspartic acid and glutamic acid) and basic (lysine, arginine, histidine) amino acids. Polar-charged amino acids interact with aqueous environments, can form salt bridges, and can interact with electrolytes/minerals such as potassium, chloride, and phosphate.
The neutral amino acids interact with water to different degrees and can be divided into polar, nonpolar, and relatively nonpolar categories. The side chains of polar neutral amino acids contain functional groups—such as the hydroxyl group for serine and threonine, the sulfur atom for cysteine, and the amide group for asparagine and glutamine—that can interact through hydrogen bonds with water (the aqueous environment of cells). Polar amino acids are generally found on the surfaces of proteins. If not found on the surface, they are oriented inward and function at a protein’s (such as an enzyme’s) binding site.
In contrast, the amino acids containing side chains that do not interact with water are categorized as nonpolar or hydrophobic (water fearing). The aromatic amino acids are considered relatively nonpolar. Tyrosine, for example, because of its hydroxyl group on the phenyl ring, can to a limited extent form hydrogen bonds with water—hence the term relatively nonpolar. Because they do not interact with water, the nonpolar (and often the relatively nonpolar) amino acids are typically found tightly coiled in proteins or compacted (e.g., attracted by van der Waal forces) and oriented toward or within the central region or core portion of proteins.
Essential And Non-Essential Amino Acids
In contrast to chemists, nutritionists generally categorize the amino acids in terms of essentiality. In the context of nutrition, the term essential or indispensable means that the body cannot create the nutrient (in this case the amino acid) itself and so it must be supplied by the diet.
The 9 essential amino acids are:
Identifying amino acids strictly as nonessential/dispensable or essential/indispensable is an inflexible classification, however, that allows no gradations, even in decidedly different or changing physiological circumstances. Therefore, a third category exists: conditionally or acquired indispensable amino acids. A dispensable amino acid may become indispensable if, for example, there are rate limitations to its synthesis such as may occur if precursor availability is limited or, if an organ does not function properly, then amino acid metabolism may not proceed normally.
The 20 natural amino acids are proteinogenic, protein building. Amino acids strung together form peptidide chains that are in turn incorporated into proteins.
- Peptide is a short amino acid chain.
- Dipeptide: a peptide consisting of 2 amino acids.
- Tripeptide: a peptide consisting of 3 amino acids.
- Peptides with 2-20 amino acids are called oligopeptides.
- Peptides with more than 20 amino acids are called polypeptides.
These amino acids are folded into shape by the ribosomes to create complex structures.
Proteins have 4 levels of structure:
- The primary structure of a protein represents the amino acid sequence of the protein.
- The secondary structure is the coiling, folding and bending of the protein. This shape is created by i.a. hydrogen bonds, which are electrical attractions that keep atoms together and electrostatic/ionic attractions between amino acids (akin to magnets).
- The tertiary structure of protein is the overall or total three-dimensional configuration of the protein. This is basically the resulting total shape of the combined secondary structure shapes attached together.
- Some proteins also have a quatarnary structure as a result of interacting polypeptide chains that form a so called oligomer.
Sources of Amino Acids
Amino acids are derived from protein. Both dietary (exogenous) and endogenous proteins provide the body with amino acids. Dietary sources of protein include:
- Animal products such as meat, poultry, fish, eggs, and dairy (with the exception of butter, sour cream, and cream cheese)
- Plant products such as grains, grain products, legumes (including lentils, beans, and peas), nuts, seeds, and vegetables.
Following ingestion, exogenous proteins serve as sources of the essential amino acids, nonessential amino acids, and additional nitrogen needed to synthesize more nonessential amino acids, nitrogen-containing compounds, and protein in the body.
Endogenous proteins presented to the digestive tract represent another source of amino acids and nitrogen. Endogenous proteins include:
- Desquamated mucosal cells
- Digestive enzymes and glycoproteins
The digestive enzymes and glycoproteins are derived from secretions of the salivary glands, stomach, intestine, liver, and pancreas. The digestive tract’s mucosal cells contain a variety of proteins (such as apoproteins, structural proteins, and cytosolic enzymes) that when sloughed into the gastrointestinal (GI) tract are degraded. Most of these endogenous proteins, which typically total 70 g or more per day, are digested and provide amino acids that are available for absorption. Digestion of these proteins and the absorption of subsequently generated amino acids are crucial for protein nutriture.
That this is not “free protein” that you need to substract from your total protein need, as ultimately the building blocks of these proteins still have to be consumed in some form. Ultimately, your body derives all of the building blocks it needs to create proteins from exogenous sources, notably the consumption of food.
The digestion of protein begins in the stomach with the action of hydrochloric acid (HCl), which is found in gastric juice. The hydrochloric acid content of the gastric juice results in a gastric pH less than 3 and enables denaturation (disruption) of the quaternary, tertiary, and secondary structures of protein.
Denaturants such as hydrochloric acid break apart hydrogen and electrostatic bonds to unfold or uncoil the protein; however, peptide bonds are not affected by the hydrochloric acid. Hydrochloric acid does, however, begin pepsin activation from pepsinogen, which is secreted as a zymogen (inactive enzyme) by gastric chief cells. Pepsin, once formed, is catalytic against pepsinogen as well as other proteins.
Pepsin functions as an endopeptidase (meaning that it hydrolyzes interior peptide bonds within proteins or polypeptides) at a pH of ~3.5. Specifically, pepsin attacks peptide bonds adjacent to the carboxy end of a relatively wide variety of amino acids (pepsin has low specificity).
The end products of gastric protein digestion include primarily large polypeptides, along with some oligopeptides (short chains of amino acid peptide bonded to each other) and free amino acids. These end products subsequently enter the small intestine for further digestion.
Protein digestion yields two main end products: peptides (principally dipeptides and tripeptides) and free amino acids. To be used by the body, these end products must next be absorbed.
Absorption is the process by which the end products of digestion are transported from the lumen of the GI tract, most often the small intestine, into the body. To get into the blood for transport to tissues, amino acids must cross two intestinal membranes, the brush border (also called apical) membrane and the basolateral (also called serosal) membrane.
Amino acid absorption occurs along the entire small intestine, but most amino acids are absorbed in the duodenum and proximal jejunum. The absorption of amino acids into enterocytes requires carriers (integral membrane proteins); however, paracellular absorption—that is, absorption via passage through the tight junctions of enterocytes or transcellular endocytosis—can occur in occasional situations in which large volumes of hypotonic fluids containing some amino acids have been ingested.
The transport systems are responsible for carrying amino acids across the brush border membrane into the intestinal cell. Most amino acids are thought to be transported across the enterocyte brush border membrane by so called sodium-dependent transporters.
The affinity of a carrier for an amino acid is influenced both by the hydrocarbon mass of the amino acid’s side chain and by the net electrical charge of the amino acid. As the hydrocarbon mass of the side chain increases, affinity increases. Thus, the branched-chain amino acids typically are absorbed faster than smaller amino acids. Neutral amino acids also tend to be absorbed at higher rates than basic or acidic amino acids. Essential (indispensable) amino acids are absorbed faster than nonessential (dispensable) amino acids, with methionine, leucine, isoleucine, and valine being the most rapidly absorbed.
Ingesting large quantities of one amino acid or a particular group of amino acids that use the same carrier system may create, depending upon the amount ingested, competition among the amino acids for absorption. The result may be that the amino acid present in highest concentration is absorbed but also may impair the absorption of the other, less concentrated amino acids carried by that same system. Thus, amino acid supplements may result in impaired or imbalanced amino acid absorption.
The absorption of peptides is generally more rapid than absorption of an equivalent mixture of free amino acids. Although peptides, like amino acids, compete with one another for transporters, peptide transport appears to occur more rapidly than amino acid transport and is thought to represent the primary means by which most amino acids enter the intestinal cell. In other words, the majority of amino acids are absorbed as peptides.
Peptides, once within the enterocytes, are generally hydrolyzed by cytosolic peptidases to generate free intracellular amino acids. Intact peptides, however, can be found occasionally in circulation, and this is thought to result from entry of peptides into the body via paracellular (also called intercellular) routes. With illnesses, especially those affecting the intestines (such as inflammatory bowel diseases or celiac disease), the gastrointestinal tract can become more permeable, thus increasing the likelihood of peptides appearing intact in the blood.
Peptides found in the blood can be hydrolyzed by peptidases or proteases in the plasma or at the cell membrane (especially in the liver, kidneys, and muscle). Intracellular hydrolysis of the peptide, if absorbed intact, may also occur in the cytosol or in various organelles. The ability to administer peptides directly into the blood as in parenteral nutrition is of nutritional significance since some amino acids (e.g., tyrosine, cysteine, and glutamine) are insoluble or unstable in their free form. Administration of the peptide form, since it can be used by tissues, allows nutrients to be provided in situations in which traditional free amino acid parenteral mixtures are ineffective.
For amino acids to enter the blood and be used by other body tissues, the amino acids must be transported across the basolateral (serosal) membrane of the enterocyte and into interstitial fluid, where they enter the blood (through capillaries of the villi) for transport into the portal vein leading to the liver. The carriers found in the enterocyte’s basolateral membrane are generally sodium independent and similar to those found in other cell membranes of the body.
After amino acids are transported out of the enterocyte, they enter portal blood for transport to tissues. Uptake of the amino acids into liver cells (hepatocytes), as well as cells of the kidneys and other organs, occurs by some carrier systems similar to those found in the intestinal cell membranes.
Amino Acid Metabolism
The liver is the primary site for the uptake of most amino acids following ingestion of a protein-containing meal. The liver is thought to monitor the absorbed amino acids and to adjust the rate of their metabolism (including catabolism or breakdown of amino acids and anabolism or use of amino acids for synthesis) according to the needs of the body.
Amino Acid Catabolism
Catabolism of amino acids occurs to varying degrees in different tissues both during fasting periods and immediately after eating (the postprandial period). Liver cells have a high capacity for the uptake and catabolism of amino acids. In fact, after a meal, the liver takes up about 50–65% of amino acids from portal blood. The liver is the main site for the catabolism of the indispensable amino acids, with the exception of the branched-chain amino acids, which tend to be utilized to a greater extent by muscle and other organs such as the heart.
Within the liver, the periportal hepatocytes catabolize most amino acids with the exception of glutamate and aspartate, which are metabolized to a greater extent by perivenous hepatocytes. The liver derives up to 50% of its energy (ATP) from amino acid oxidation; the energy generated may in turn be used for gluconeogenesis or urea synthesis, among other needs, depending on the body’s state of nutriture. Amino acids are broken down in cells, including first the transamination and/or deamination of amino acids and then the disposal of ammonia.
Frequently (but not always), the first step in amino acid catabolism is the transfer or removal of an amino acid’s amino group. The process occurs by transamination and/or deamination. Transamination reactions involve the transfer of an amino group from one amino acid to an a-keto acid (also referred to as an amino acid carbon skeleton). The carbon skeleton/a-keto acid that gains the amino group becomes an amino acid, and the amino acid that loses its amino group becomes an a-keto acid. These reactions are important for the synthesis of many of the body’s dispensable amino acids.
The urea cycle functions in the liver and is extremely important for the removal of ammonia (and ammonium ions) from the body. Urea is formed by deriving one nitrogen from ammonia (from deamination reaction), a second nitrogen from aspartate (from transamination reaction) and carbon from CO2/HCO3-.
Once formed, urea typically travels in the blood to the kidneys for excretion in the urine; however, up to about 25% of urea may be secreted from the blood into the intestinal lumen, where it may be degraded by bacteria to yield ammonia. Activities of urea cycle enzymes fluctuate with diet and hormone concentrations. For example, with low-protein diets or acidosis, urea synthesis diminishes and urinary urea nitrogen excretion decreases significantly. Thus, substrate availability results in short-term changes in the rate of ureagenesis. In the healthy individual with a normal protein intake, blood urea nitrogen (BUN) concentrations range from about 8–20 mg/dL, and urinary urea nitrogen represents about 80% of total urinary nitrogen.
Several defects (genetic mutations) have been identified in genes coding for urea cycle enzymes. Urea cycle enzyme defects typically result in high blood levels of ammonia called hyper-ammonemia and necessitate a protein-restricted diet. Discarding urea is just waste disposal. The actual usefulness of amino acid catabolism lies in the produced carbon skeletons.
Carbon skeletons of amino acids can be further metabolized with the potential for multiple uses in the cell, depending on the original amino acid from which they were derived and the body’s physiological and nutritional state. An amino acid’s carbon skeleton, for example, depending on the particular amino acid, can be used to produce energy, glucose, ketone bodies, cholesterol, and fatty acids.
Amino acids are used for energy production when the diet is inadequate in energy. The production of glucose from a noncarbohydrate source such as amino acids is known as gluconeogenesis.
The carbon skeletons of several amino acids can be used to synthesize glucose. For example, oxaloacetate (the carbon skeleton of aspartate) and pyruvate (the carbon skeleton of alanine) may be used to produce glucose in body cells. In addition, the carbon skeleton of asparagine can be converted into oxaloacetate, and the carbon skeletons of glycine, serine, cysteine, tryptophan, and threonine can be converted into pyruvate for glucose production. Valine and methionine are also glucogenic, yielding succinyl- CoA. Thus, to be considered a glucogenic amino acid, catabolism of the amino acid must yield pyruvate or intermediates of the TCA cycle.
Essentially, the carbon skeletons of all natural amino acids except leucine and lysine, can be used to synthesize glucose.
Ketone Body Production
For an amino acid to be considered ketogenic, the catabolism of the amino acid must generate acetyl-CoA or acetoacetate, which are used for the formation of ketone bodies (also referred to as ketones). Some amino acids are both glucogenic and ketogenic. Leucine and lysine are the only totally ketogenic amino acids and upon catabolism generate acetyl-CoA.
The ketogenic amino acids are:
Lipid (Cholesterol and Fatty Acid) Production
The oxidation of several amino acids—including isoleucine, leucine, lysine, tryptophan, and threonine—yields acetyl-CoA, which can be metabolized to produce cholesterol. Leucine, however, is also the only amino acid whose catabolism directly generates b-hydroxy b-methylglutaryl-CoA, an intermediate in cholesterol synthesis. Moreover, leucine oxidation produces another metabolite, b-hydroxy b-methylbutyrate (HMB), which appears to promote de novo cholesterol synthesis in muscle, enabling cell growth and function.
In times of excess energy and protein intakes coupled with adequate carbohydrate intake, the carbon skeleton of amino acids may be used to synthesize fatty acids. Leucine’s carbon skeleton, for example, is used to synthesize fatty acids in adipose tissue.
Other Uses of Amino Acids
In addition to the above compounds that can be produced by the body by oxidizing amino acids, the compounds in the image below can also be produced.
Anabolism, including protein synthesis, increases in tissues following the ingestion of food, which provides the amino acids and energy necessary to build body proteins. For example, amino acids released following ingestion and digestion of “fast proteins,” which include whey protein, some soy proteins, amino acid mixtures, and protein hydrolysates (e.g., usually partially hydrolyzed whey protein), appear to be utilized differently in the body when compared with amino acids released following ingestion and digestion of “slow proteins” such as casein.
Ingestion of fast proteins better stimulates muscle protein and whole-body protein synthesis than slow proteins both at rest and following resistance exercise. Ingestion of foods providing slow proteins, however, is important as the lower and more prolonged plasma amino acid concentrations that result from ingestion of these proteins help reduce protein breakdown, which predominates in the postabsorptive state (between meals and overnight).
The hyperamino-acidemia (higher than normal amino acid concentrations in the blood) after fast protein ingestion can promote higher amino acid deamination rates and urea production than slow protein ingestion. In addition, amino acids from fast proteins appear to be used more for splanchnic (internal organs) protein synthesis versus peripheral (membrane proteins) or skeletal muscle protein synthesis.
Hormones play a major role in amino acid utilization for protein synthesis. During prolonged periods in which food is not eaten, such as during the overnight hours or a more prolonged fast, protein synthesis still occurs but at a much lower rate (than occurs postprandially), and protein degradation predominates. The degradative processes are stimulated by epinephrine and cortisol release and by the higher glucagon-to-insulin ratio in the blood. This higher glucagon-to-insulin ratio diminishes insulin’s ability to inhibit protein degradation and diminishes the overall rate of protein synthesis.
Yet, while skeletal muscle experiences more extensive protein degradation and limited protein synthesis during postabsorptive periods, the glucagon-to-insulin ratio favoring glucagon stimulates the hepatic synthesis of some proteins, such as enzymes for gluconeogenesis and ureagenesis. Higher blood cortisol concentrations (which typically rise with depletion of hepatic glycogen stores occurring with fasting or with injury, sepsis, burns, etc.) further promote muscle protein catabolism and hepatic use of the amino acids for gluconeogenesis and ureagenesis.
In contrast to the general catabolic nature of glucagon, epinephrine, and cortisol, other hormones, such as insulin, are anabolic. Insulin increases protein synthesis and decreases protein degradation.
Insulin is secreted in response to a rise in incretins (glucosedependent insulinotropic peptide and glucagon-like peptide 1 released in response to glucose in the digestive tract), a rise in blood glucose, and a rise in some blood amino acid concentrations, as occurs with food consumption. Insulin, upon binding to its receptors in cell membranes, exhibits multiple actions to promote protein synthesis.
For example, insulin generally stimulates the transcellular movement of amino acid transporters to the cell membrane for use in amino acid uptake and increases the overall activity of amino acid transporters, including systems A, ASC, and N in the liver, muscle, and other tissues. Insulin antagonizes the activation of some enzymes responsible for amino acid oxidation (degradation). Thus, insulin promotes the uptake of the amino acids into the tissues and inhibits the enzymes that are responsible for the degradation of the amino acids. Such actions of insulin facilitate the use of the amino acids for protein synthesis.
Insulin, along with the amino acid leucine (which by itself acts as an insulin secretogogue [i.e., it stimulates insulin secretion]), also plays other roles in stimulating protein synthesis.
Protein synthesis involves transcription of a gene into messenger (m) RNA followed by its translation. There are multiple factors that influence translation. Translation ican be affected by the amount and stability of mRNA, the ribosome number, the activity of the ribosomes, the presence of amino acids in the appropriate concentrations, as well as the hormonal environment, which in turn can be influenced by nutrients. Some amino acids may even regulate the expression of some genes through interactions with amino acid–responsive elements in the gene’s promoter region.
Amino acids can promote changes in cell volume and protein synthesis through intracellular signaling pathways. Leucine and its metabolite, β-hydroxy-β-methylbutyrate (HMB), promote protein synthesis through increased phosphorylation of mammalian target of rapamycin (mTOR). mTOR mediates the effects of insulin on protein synthesis through different multiprotein complexes but also exerts effects on protein synthesis that are independent of insulin.
The amount of tissue in the body tends to remain fairly constant, however those tissues are undergoing a continuous process of breakdown and synthesis. These two processes together are generally referred to as tissue turnover. Protein-based tissues such as plasma proteins and skeletal muscle undergo a continues process of breakdown and resynthesis. If synthesis exceeds breakdown, there will be an increase in the amount of that protein. If breakdown exceeds synthesis, there will be an overall loss in the amount of that protein. If breakdown equals synthesis, there will be no long-term change in the amount of that protein.
As an athlete that’s trying to build muscle what you want is skeletal muscle protein synthesis to be equal to or greater than protein breakdown. This means either increasing protein synthesis, decreasing protein breakdown, or doing both at the same time.
Different tissues turn over at drastically varying rates. Plasma proteins made in the liver may turn over in a matter of hours while skeletal muscle protein may take days to turn over; tissues such as tendons and ligaments may take months or years to turn over completely.
The process of protein synthesis requires that amino acids are to be pulled out of the free pool for incorporation into the protein being synthesized. Protein breakdown releases amino acids back into the free pool. Protein turnover is energetically costly, and it’s been estimated that protein turnover may account for 15-25% of basal metabolic rate.
Protein turnover might seem as a wasteful process for the body to undergo, especially since the net result is more or less maintenance of bodily tissue, however, it appears to play a crucially important role in dealing with stressful situations by providing amino acids where they are needed.
The amino acid content of a meal plays the major role in terms promoting protein synthesis, while insulin plays a secondary role. Assuming that sufficient amino acids are available, only very small amounts of maximal stimulation of protein synthesis via amino acids. Insulin also increases amino acid transport into skeletal muscle and some research suggests a direct role of insulin on protein synthesis.
Raising insulin levels without raising amino acid levels by consuming protein, has little to no effect on muscle protein synthesis. In fact, In fact, elevating insulin without simultaneously increasing amino acid availability tends to decrease protein synthesis, due to a decrease in circulating amino acid concentrations.
When it comes to protein breakdown, meal absorption appears to decrease protein breakdown via a combination of both increased amino acid availability (especially leucine), along with the increase in insulin.
Although insulin plays a minor role in promoting protein synthesis, insulin appears to play a primary role in decreasing protein breakdown.
Outside of the processes related to normal growth or aging, possibly the single greatest factor influencing skeletal muscle metabolism is training. Although the physiology of resistance and endurance training are significantly different, both impact profoundly on skeletal muscle metabolism. Resistance training affects both protein synthesis and breakdown with both being increased following training.
Resistance training has a net anabolic effect on the body, essentially “telling” it to maintain body protein stores at a higher level, eventually leading to increased levels of muscle mass.
Functional Roles of Protein
The molecular architecture and activity of living cells depend largely on proteins, which make up over half of the solid content of cells and which show great variability in size, shape, and physical properties. Their physiological roles are also quite variable and, because of this variability, proteins are categorized according to their functions.
Several proteins have structural roles in the body. Two groups of structural proteins include:
- Contractile proteins
- Fibrous proteins.
The two main contractile proteins, actin and myosin, are found in cardiac, skeletal, and smooth muscles. Skeletal muscle is found throughout the body and is under voluntary control. It is made of myosin (thick filaments) and actin (thin filaments). Contraction is calcium induced and involves not only actin and myosin but also troponin and tropomyosin.
Smooth muscle is found in many tissues including, for example, blood vessels, the lungs, the uterus, and the GI tract. Smooth muscle is under involuntary control and contracts in response to calcium-induced phosphorylation of the structural protein myosin. Fibrous proteins, which tend to be somewhat linear in shape, include collagen, elastin, and keratin and are found in bone, teeth, skin, tendons, cartilage, blood vessels, hair, and nails. Collagen is a group of well-studied proteins. Each type of collagen is made of three polypeptide (tropocollagen) chains that are cross-linked for strength.
Enzymes are protein molecules (generally designated by the suffix -ase) that act as catalysts, changing the rate of reactions occurring in the body. Enzymes are necessary for sustaining life and are found in the body both intracellularly and extracellularly (e.g., in the blood).
Enzymes are constructed so that they combine selectively with other molecules (called substrates) in the cell. The active site on the enzyme (a small region usually in a crevice of the enzyme) is where the enzyme and substrate bind and the product is generated. Some enzymes, however, require a cofactor or coenzyme to carry out the reaction.
Minerals such as zinc, iron, and copper function as cofactors for some enzymes. Metalloprotein is the name typically used for proteins to which minerals are complexed. Some, but not all, metalloproteins have enzymatic activity. B vitamins serve as coenzymes for many enzymes. Flavoprotein is the term generally used for protein enzymes bound to flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD), coenzyme forms of the B vitamin riboflavin.
Most human physiological processes require enzymes to promote chemical changes that could not otherwise occur. Some examples of different types of enzymes include dehydrogenases, which remove or transfer hydrogens; kinases, which add phosphate groups; and isomerases, which transfer atoms within a molecule. Some examples of physiological processes that depend on enzyme function include digestion, energy production, blood coagulation, and excitation and contraction of neuromuscular tissue.
Some proteins are hormones. Hormones act as chemical messengers in the body. They are synthesized and secreted by endocrine tissue (glands) and transported in the blood to target tissues or organs, where they bind to protein receptors on membranes. Hormones generally regulate metabolic processes, for example, by promoting enzyme synthesis or affecting enzyme activity. Whereas some hormones are derived from cholesterol and classified as steroid hormones, others are derived from one or more amino acids.
The amino acid tyrosine, for example, is used along with the mineral iodine to synthesize the thyroid hormones. Tyrosine is also used to synthesize the catecholamines, including dopamine, norepinephrine, and epinephrine. The hormone melatonin is derived in the brain from the amino acid tryptophan.
Other hormones are made up of one or more polypeptide chains. Insulin, for example, consists of two polypeptide chains linked by a disulfide bridge. Glucagon, parathyroid hormone, and calcitonin each consist of a single polypeptide chain. Many other peptide hormones, such as adrenocorticotropic hormone (ACTH), somatotropin (growth hormone), and vasopressin (also known as antidiuretic hormone, or ADH), have important roles in human metabolism and nutrition.
Proteins, because of their constituent amino acids, can serve as buffers in the body and thus help to regulate acid– base balance. A buffer is a compound that ameliorates a change in pH that would otherwise occur in response to the addition of alkali or acid to a solution. The pH of the blood and other body tissues must be maintained within an appropriate range. Blood pH ranges from about 7.35 to 7.45, whereas cellular pH levels are often more acidic.
In addition to acid–base balance, proteins influence fluid balance through their presence in the blood and in cells. More specifically, proteins help attract and keep water inside a particular area and contribute to osmotic pressure.
Immunoprotection is provided to the body in part by a group of proteins called immunoproteins, also called immunoglobulins (Ig) or antibodies (Ab). Immunoglobulins function by binding to antigens—which typically consist of foreign substances, such as bacteria or viruses that have entered the body. By complexing with antigens, immunoglobulins create immunoprotein–antigen complexes that can be recognized and destroyed through reactions with either complement proteins or cytokines. Complement proteins (which are part of the complement pathway and made primarily by the liver) are vital to the protection of the body against foreign microbes/organisms or substances after recognition by antibodies.
Transport proteins are a diverse group of proteins that combine with other substances (especially vitamins and minerals but also other nutrients) to provide a means of carrying those substances within the blood, into cells, out of cells, or within cells. Transport proteins in cell membranes, for example, carry and thus regulate the flow of nutrients into and out of cells.
A few transport proteins of clinical significance include albumin, transthyretin, and retinol-binding protein. Albumin, the most abundant of the plasma proteins, transports nutrients such as tryptophan, fatty acids, and vitamin B6 some minerals including zinc, calcium, and small amounts of copper; and some drugs. Albumin is synthesized by the liver and released into the blood.
Some proteins also function in cell adhesion, while others serve to transmit signals into and out of the cell.
Nitrogen-Containing Non-Protein Compounds
In addition to their use in the synthesis of body proteins, amino acids are used to synthesize nitrogen-containing compounds, that play important roles in the body. Such compounds are:
- Glutathione (a tripeptide synthesized from three amino acids — glycine, cysteine, and glutamate), a major anti-oxidant with the ability to scavenge free radicals (O2 • and OH•), thereby protecting critical cell components against oxidation.
- Carnitine, another nitrogen-containing compound, made from the amino acid lysine. In addition to being synthesized in the liver and kidneys, carnitine is found in primarily animal foods, especially milk, fish, poultry, and meats. Approximately 54–87% of carnitine intake is absorbed. It is needed for the transport of fatty acids, especially long-chain fatty acids, across the inner mitochondrial membrane for b-oxidation, as well for ketone catabolism for energy. Carnitine deficiency, though rare, results in impaired energy metabolism.
- Creatine, a key component of the energy compound creatine phosphate, also called phosphocreatine, can be obtained from foods (primarily meat and fish) or synthesized from three amino acids in the body. Creatine synthesis begins first in the kidneys and requires arginine and glycine. The second step occurs in the liver and involves the methylation of guanidinoacetate using SAM (S-adenosyl methionine). Once synthesized, creatine is released into the blood for transport to tissues. About 95% of creatine is in muscle, with the remaining 5% in organs such as the kidneys and brain. In tissues, creatine is found both in free form as creatine and in its phosphorylated form. Phosphocreatine functions as a “storehouse for highenergy phosphate.” Over half of the creatine in muscle at rest is in the form of phosphocreatine. Phosphocreatine replenishes ATP in a muscle that is rapidly contracting. Remember, muscle contraction requires energy. This energy is obtained with the hydrolysis of ATP. Creatine and creatine phosphate do not remain indefinitely in muscle; rather, both slowly but spontaneously cyclize into creatinine leaving the muscle, passing across the glomerulus of the kidneys, and is excreted like other nitrogenous waste products (e.g., urea, ammonia, uric acid) in the urine.
- Carnosine (also called b-alanyl histidine) is made from the amino acid histidine and b-alanine in an energy-dependent reaction catalyzed by carnosine synthetase. In the body, carnosine is synthesized and found largely in the cytosol of skeletal and cardiac muscle, but also in the brain, kidneys, and stomach. Carnosine is also found in foods, primarily meats, and may be digested into histidine and b-alanine in the intestine or possibly absorbed intact by peptide transporters.
- Choline is made in the body primarily in the liver through the methylation (involving S-adenosyl methionine, or SAM) of the phospholipid phosphatidylethanolamine when linked with the catabolism of phosphatidylcholine. The formation of phosphatidylserine from phosphatidylcholine, involving the replacement of choline with serine by phosphatidylserine synthase 1, also releases choline for other use in the body. In foods, choline is found free (unattached) in small amounts but is more commonly found bound as part of phosphatidylcholine (also called lecithin) and sphingomyelin, among other forms. Foods rich in lecithin include eggs, meats (especially liver and other organ meats), shrimp, cod, salmon, wheat germ, and legumes such as soybeans and peanuts. Most choline is used to synthesize phosphatidylcholine and sphingomyelin, major components of cell membranes. Phosphatidylcholine also functions in intracellular signaling and in the secretion of very-low-density lipoproteins from the liver. Sphingomyelin is a component of myelin that functions as a sheath around nerves and is important in nerve conduction. Choline is also used in the formation of platelet aggregating factor and for the neurotransmitter acetylcholine.
- Purine and Pyrimidine Bases Nitrogenous bases, along with a five-carbon sugar and phosphoric acid, are needed for the synthesis of two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), in the body. It is amino acids that provide the source for the nitrogen in these bases. The nitrogenous bases can be divided into two categories: pyrimidines and purines.
Glutamine serves several roles in the intestines. It is degraded extensively by intestinal cells, providing a primary source of energy. It has also been shown to have trophic (growth) effects, stimulating gastrointestinal mucosa cell proliferation. Consequently, glutamine helps to prevent both atrophy of gut mucosa and bacterial translocation. In addition, glutamine has been shown to enhance the synthesis of heat shock proteins. It is also needed in large quantities along with threonine for the synthesis of mucins found in mucus secretions in the GI tract. These roles of glutamine in the GI tract have prompted several companies to enrich enteral and parenteral (intravenous) nutrition products with glutamine.
It is estimated that the human GI tract uses up to 10 g of glutamine per day, and that the cells of the immune system use over 10 g per day. In addition to dietary glutamine, much of the body’s glutamine that is produced by the skeletal muscles (and to lesser extents by the lungs, brain, heart, and adipose tissue) is released and taken up, mostly by the intestinal cells. Glutamine not used for energy production within the intestine may also be partially catabolized to generate ammonia and glutamate.
Glutamine has several major roles in the body, one of which is in ammonia transport. Whereas ammonia arising in the liver from amino acid reactions is typically shuttled into the urea cycle, this is not true in other tissues. In extrahepatic tissues, especially muscle but also the lungs, heart, brain, and adipose, glutamine synthetase catalyzes the utilization of ammonia or ammonium ions with glutamate in an ATP-dependent reaction to form glutamine. It is estimated that the body produces 40–80 g glutamine per day.
Glutamine use by cells increases dramatically with hypercatabolic conditions such as infection and trauma. In these conditions muscle glutamine release increases but cannot meet other cellular demands. Thus, glutamine “stores” can become depleted and some cell functions may become impaired. Remember, glutamine plays several roles that are especially critical with illness/ injury.
The Alanine-Glucose Cycle
In addition to glutamine, the amino acid alanine is also important in the intertissue (between tissues) transfer of amino groups generated from amino acid catabolism. These reactions are known as the alanine-glucose cycle detailed in the diagram below. This cycle allows the body to produce significant amounts of glucose without requiring dietary carbohydrate (gluconeogenesis).
About 40% of the body’s protein is found in muscle, and skeletal muscle mass represents up to about 43% of the body’s mass. Uptake of amino acids by the skeletal muscles readily occurs following ingestion of food, especially a mixed meal rich in protein. Exercise further encourages amino acid uptake by muscles. After eating, skeletal muscles exhibit net protein synthesis (i.e., protein synthesis is greater than protein degradation).
In a postabsorptive state such as between meals or in a fasting situation, the reverse is true. Protein degradation predominates over synthesis, and amino acids may be released into the blood for use by other tissues. Muscle protein degradation is also associated with exercise. Cortisol, secreted by the adrenal glands, in response to exercise-induced stress, promotes, in part, this muscle and amino acid catabolism.
Like other tissues, muscles preferentially catabolize some amino acids more than others; six amino acids (aspartate, asparagine, glutamate, leucine, isoleucine, and valine) appear to be catabolized to greater extents in the skeletal muscle than other tissues. This use of amino acids by muscle as well as leucine’s role in promoting protein synthesis has prompted the consumption of branchedchain amino acid supplements by some athletes.
While proteolysis (protein breakdown) of muscle generates amino acids that are released into the plasma for circulation to other tissues, changes in plasma amino acid concentrations do not reflect changes in muscle mass. Instead, it’s creatinine and 3-methylhistidine, that are used as indicators of existing muscle mass and muscle degradation, respectively.
Strength trained individuals often have elevated creatinine levels. Since creatinine levels correlate roughly with muscle mass, many doctors mistakenly interpret this as a sign of inflammation, kidney malfunction or muscle trauma.
How Much Protein Do You Need?
Now that we’re done with protein biochemistry it’s time to get into more practical matters.
You’ve probably heard from bodybuilders that 1g/lb (2.2g/kg ) is the end all be all number for the amount of protein that you need to eat to build muscle, lose fat and all that jazz. It was so commonly suggested until a couple of years ago and people actually believed that if they ate a gram or two less than that magical number all their gains would magically dissapear.
Despite decades of research , there is still argument among researchers over the true human requirements for protein, both for the average non-training person as well as the athlete. With research existing that supports both sides of the argument, this only further contributes to the controversy, confusion and disagreement.
Keep in mind that I’m going to talk about protein requirements for athletes. I’m not going to talk about protein requirements for the elderly or for people that don’t lift weights. For older people there is an anabolic resistance that occurs with aging, making their protein requirements systematically higher than younger individuals.
When the IIFYM craze hit the fitness industry back in the early 2010s, that magical 1g/lb number decreased to 0.82g/lb, and depending on the study that was cited, went down as low as 0.67g/lb. But what is the truth? Should you gulp down scoop after scoop of whey protein powders or eat chicken breast after chicken breast or should you not care at all since protein needs are over-exaggerated anyways?
Studies show that there are no additional benefits for muscle gain when going higher than 1g/lb in non dieting conditions, assuming adequate calorie intake. When dieting however, protein requirements go up and can be especially high for lean athletes.
Looking at weight and fat loss in general, eating insufficient dietary protein may cause people to over eat in general (presumably the body increases hunger in an attempt to get sufficient protein), contributing to obesity in the modern world.
After a fat loss diet has ended, higher protein intakes have been shown to not only help prevent weight regain but, especially when resistance training is being done, to cause more of any weight that is regained to be from LBM. Similarly, a high protein intake is absolutely crucial for the optimization of muscle mass retention during weight loss diets.
The problem with most studies is that generally they have not examined performance as an endpoint. They only address issues of protein requirements. Coaches and athletes are ultimately less interested in scientific arguments and more interested in what will optimize athletic performance.
Also, the definition “requirement” is context specific. The requirements of a strength athlete trying to make weight versus a strength/power athlete wanting to gain strength and mass or an endurance athlete that wants to avoid muscle mass gains while supporting performance adaptations will all be completely different. Answering the question of “How much protein is required?” depends entirely on the context.
Additionally, while small changes in either muscle mass or performance may not be statistically significant in research terms, those same small changes may be crucially important in high level sports. While a 1% difference in gains means nothing in a statistical or scientific sense, it can make the difference between first and last place in the real world of high-level sports.
This study, took all the points I mentioned above, examined both sides of the low protein and high protein arguments in some
detail and concluded that a daily protein intake of 2.5-3.0 g/kg (1.1-1.4 g/lb) for strength/power athletes is not harmful, may give small but important performance improvements over the long-term, more than covering any needs for protein synthesis, and any excess will simply be oxidized off in the first place.
Protein has other additional and important qualities like its superior effect on satiety over carbs and fat, ensuring good diet compliance, since you won’t get hungry and risk overeating. Secondly, TEF (Thermic Effect of Food) is much greater for protein than for both carbs and fat. Due to this, Livesey proposed that protein should be counted as 3.2 kcal and not 4 kcal as the current guidelines state.
This review concluded that when comparing two hypo-energetic diets with the same caloric intake, the diet with the highest protein percentage of total calorie intake will show superior results due to protein’s effects on thermogenesis and satiety. It is precisely this protein induced thermogenesis that helps you lose body weight faster when dieting, while keep you leaner during mass gaining phases. Another added benefit of higher protein intakes is that they make you think about your food choices more, therefore making you cheat less.
As an athlete you have to make sure that you provide your body with a sufficient protein intake. Eating 1-1.4 g/lb (2.2-3.0 g/kg) of body weight will help you reap all the benefits of higher protein intakes that were discussed previously, while optimizing your performance to supreme levels.
Myths About Protein
While individuals with compromised kidney function may have to reduce their protein intake and there’s no evidence suggesting that protein harms the kidneys.
In fact, increased dietary protein intake may actually improve kidney function and there is no evidence that a high-protein intake has any effect unless there is a pre-existing condition. There has never been a case of kidney damage linked to protein intake and studies using high protein intakes to athletes show no negative effects.
It’s also often stated that a high protein intake is harmful to bone health but this is only true if insufficient calcium/Vitamin D along with fruits and vegetables are being consumed. When those nutrients are consumed in adequate amounts, a higher protein intake actually improves bone density.
In the modern Western diet, high meat intakes are typically associated with a low fruit and vegetable intake and it is the lack of fruits and vegetables that are more to blame than the presence of meat itself.
Lean red meat has been shown to improve health, is inversely associated with mortality, contains many cancer preventing nutrients, and does NOT influence cardiovascular disease risk factors.
Regardless of potential health benefits related to meal frequency, there are practical reasons to eat more or less frequently and ultimately it will depend on individual preferences, tolerance, lifestyle and total caloric intake. For athletes with high caloric requirements, eating multiple smaller meals may make consuming sufficient calories (and especially carbohydrates) easier to accomplish compared to eating a few larger meals.
Taking a caloric intake of 4000 kcals for example it may be easier to divide into 4 meals of 1000 kcals each instead of 2 large 2000 kcal meals. Similarly a demale that has to eat 1200-1500 kcals will most likely find it easier to adhere to fewer larger meals over 200 kcal “meals”. Another issue has to do with the amount of training being done. An athlete that trains for 3-4 hours per day, for example a cyclist, or an athlete that trains twice per day, have a limited amount of time to consume a large number of calories, thus eating fewer larger meals may be the only realistic way to consume sufficient calories to support a heavy training schedule.
Work schedules may or may not allow athletes to fit in multiple small meals per day. So even if such an eating schedule is ideal, it may not be practically possible. From an evolutionary stand point, “Our ancestors consumed food much less frequently and often had to subsist on one large meal per day, and thus from an evolutionary perspective, human beings were adapted to intermittent feeding rather than to grazing”, so that should also be kept into account.
Tldr; Proteins are formed by amino acids, which vary in their chemical structure. The chemical structure of an amino acid determines its function in the body. Dietary protein is your body’s principle source of amino acids. Protein is generally digested in the stomach, subsequently absorbed in the intestine and then transported in the blood to the liver.
Protein synthesis as well as catabolism occur in the body at various locations across the entire day. Net protein balance determines whether there is net tissue build-up or breakdown. Which tissues are built on and which are broken down is determined by many factors, notably hormone levels.
Contractile proteins allow you to flex your muscles. Fibrous proteins give strength to your connective tissue.
Enzymes are proteins that speed up or slow down the chemical reactions and processes in the body.
Certain proteins are hormones that act as messengers in the body activating or deactivating other processes. Some proteins act as buffers that regulate acidity levels, while other proteins act as fluid balancers regulating water levels, or as transporters, sending nutrients, hormones or drugs to and within the body. There are also some proteins that are anti-bodies in your immune system preventing you from getting sick, while other proteins have similar functions to keep you healthy when you do get unwell.
Glutamine is particularly important for intestinal health and your immune system.
Creatinine levels correlate roughly with muscle mass whereas 3-methylhistidine levels in your urine are a rough marker of protein catabolism in your body. Strength trained individuals often have elevated creatinine levels. Since creatinine levels correlate roughly with muscle mass, many doctors mistakenly interpret this as a sign of inflammation, kidney malfunction or muscle trauma.
Protein requirements vary. During weight loss diets, higher protein intakes are required in order to retain Lean Body Mass (LBM). Protein has additional benefits including improved body composition and satiety, higher Thermic Effect of Food (TEF), as well as help prevent weight regain. A higher protein intake will also make you think about your food choices more, making you cheat less. As an athlete you have to make sure that you provide your body with a sufficient protein intake. Eating 1-1.4 g/lb (2.2-3.0 g/kg) of body weight will help you reap all the benefits of higher protein intakes that were discussed previously, while optimizing your performance to supreme levels.
Protein is NOT harmful to your kidneys, bone health and density. In fact, it’s quite the opposite. You need protein in order to be healthy.
Meal frequency is completely individualistic, based on the athlete’s work schedule, training duration and frequency, as well as preferences and ability to tolerate and digest larger meals during higher caloric intakes. Fewer larger meals may be more beneficial for lower caloric intakes.