Protein
Unfolding Case
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Robert Santos
Three years ago, Robert, a 50-year-old farmer, learned he has alpha-gal allergy, an allergy caused by a lone star tick bite that causes a delayed anaphylactic reaction after eating meat. He must avoid beef, pork, lamb, rabbit, venison, and buffalo meat. Prior to the tick bite, his usual weight was 185 pounds (84 kg), which was within healthy range for his height. However, after years of restricting the variety of protein foods he eats, he is now underweight and continues to lose weight. He does not like seafood and is tired of eating poultry.
Check Your Knowledge
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Learning Objectives
Upon completion of this chapter, you will be able to
1 Discuss the functions of protein.
2 Compare complete and incomplete proteins.
3 Explain protein “sparing.”
4 Calculate an individual’s protein requirement.
5 Estimate the amount of protein in a sample meal plan.
6 Give examples of conditions that increase a person’s protein requirement.
7 Select appropriate sources of nutrients that are most likely to be deficient in a vegetarian diet.
8 Describe nitrogen balance and how it is determined.
In Greek, protein means “to take first place,” and truly life could not exist without protein. Protein is a component of every living cell: plant, animal, and microorganism. In the adult, protein accounts for 20% of total weight. Dietary protein seems relatively immune to the controversy over optimal intake that surrounds both carbohydrates and fat.
This chapter discusses the composition of protein, its functions, and how it is handled in the body. Sources, Dietary Reference Intakes, and the role of protein in health promotion are presented.
PROTEIN COMPOSITION AND STRUCTURE
Amino Acids
Amino acids are the basic building blocks of all proteins and the end products of protein digestion. All amino acids have a carbon atom core with four bonding sites, one of which holds a hydrogen atom, one an amino group (NH2), and one an acid group (COOH) (Fig. 3.1). Attached to the fourth bonding site is a side group (R group), which contains the atoms that give each amino acid its own distinct identity. Some side groups contain sulfur, some are acidic, and some are basic. The differences in these side groups account for the differences in size, shape, and electrical charge among amino acids.
 Figure 3.1 ▲ Generic amino acid structure. |
There are 20 common amino acids that make up proteins, 9 of which are classified as essential or indispensable because the body cannot make them so they must be supplied through the diet (Box 3.1). The remaining 11 amino acids are classified as nonessential or dispensable because cells can make them as needed through the process of transamination. Some dispensable amino acids may become indispensable when metabolic need is great and endogenous synthesis is not adequate. Note that the terms essential and nonessential refer to whether or not they must be supplied by the diet, not to their relative importance: All 20 amino acids must be available for the body to make proteins.
Protein Structure
Amino acids are joined end to end with a peptide bond. The types and amounts of amino acids and the unique sequence in which they are joined determine a protein’s primary structure. A dipeptide is a molecule of two joined amino acids. Many amino acids bonded together form a polypeptide. Most proteins are several dozen to several hundred amino acids in length: Just as the 26 letters of the alphabet can be used to form an infinite number of words, so can amino acids be joined in different amounts, proportions, and sequences to form a great variety of proteins.
Proteins also vary in shape and may be straight, folded, coiled along one dimension, or a three-dimensional shape such as a sphere or globe. Larger proteins are created when two or more three-dimensional polypeptides combine. A protein’s shape determines its function.
BOX 3.1 Amino Acids
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FUNCTIONS OF PROTEIN
Protein is the major structural and functional component of every living cell. Except for bile and urine, every tissue and fluid in the body contains some protein. In fact, the body may contain as many as 10,000 to 50,000 different proteins that vary in size, shape, and function. Amino acids or proteins are components of or involved in the following:
Body structure and framework. More than 40% of protein in the body is found in skeletal muscle, and approximately 15% is found in each the skin and the blood. Proteins also form tendons, membranes, organs, and bones.
Enzymes. Enzymes are proteins that facilitate specific chemical reactions in the body without undergoing change themselves. Some enzymes (e.g., digestive enzymes) break down larger molecules into smaller ones; others (e.g., enzymes involved in protein synthesis in which amino acids are combined) combine molecules to form larger compounds.
Other body secretions and fluids. Neurotransmitters (e.g., serotonin, acetylcholine), antibodies, and peptide hormones (e.g., insulin, thyroxine, epinephrine) are made from amino acids, as are breast milk, mucus, sperm, and histamine.
Fluid balance. Proteins help to regulate fluid balance because they attract water, which creates osmotic pressure. Circulating proteins, such as albumin, maintain the proper balance of fluid among the intravascular, intracellular, and interstitial compartments of the body. A symptom of a low albumin level is edema.
Acid-base balance. Because amino acids contain both an acid (COOH) and a base (NH2), they can act as either acids or bases depending on the pH of the surrounding fluid. The ability to buffer or neutralize excess acids and bases enables proteins to maintain normal blood pH, which protects body proteins from being denatured.
Intravascular within blood vessels.
Intracellular within cells.
Interstitial between cells.
Edema the swelling of body tissues secondary to the accumulation of excessive fluid.
Denatured an irreversible process in which the structure of a protein is disrupted, leading to partial or complete loss of function.
Globular spherical.
Transport molecules. Globular proteins transport other substances through the blood. For instance, lipoproteins transport fats, cholesterol, and fat-soluble vitamins; hemoglobin transports oxygen; and albumin transports free fatty acids and many drugs.
Other compounds. Amino acids are components of numerous body compounds such as opsin, the light-sensitive visual pigment in the eye, and thrombin, a protein necessary for normal blood clotting.
Some amino acids have specific functions within the body. For instance, tryptophan is a precursor of the vitamin niacin and is also a component of serotonin. Tyrosine is the precursor of melanin, the pigment that colors hair and skin and is incorporated into thyroid hormone.
Fueling the body. Like carbohydrates, protein provides 4 cal/g. Although it is not the body’s preferred fuel, protein is a source of energy when it is consumed in excess or when calorie intake from carbohydrates and fat is inadequate.
HOW THE BODY HANDLES PROTEIN
Digestion and Absorption
Chemical digestion of protein begins in the stomach, where hydrochloric acid denatures protein to make the peptide bonds more available to the actions of enzymes (Fig. 3.2). Hydrochloric acid also converts pepsinogen to the active enzyme pepsin, which begins the process of breaking down proteins into smaller polypeptides and some amino acids.
The majority of protein digestion occurs in the small intestine, where pancreatic proteases reduce polypeptides to shorter chains, tripeptides, dipeptides, and amino acids. The enzymes trypsin and chymotrypsin act to break peptide bonds between specific amino acids. Carboxypeptidase breaks off amino acids from the acid (carboxyl) end of polypeptides and dipeptides. Enzymes located on the surface of the cells that line the small intestine complete the digestion: Aminopeptidase splits amino acids from the amino ends of short peptides, and dipeptidase reduces dipeptides to amino acids. Protein digestibility is 90% to 99% for animal proteins, over 90% for soy and legumes, and 70% to 90% for other plant proteins.
Protein Digestibility how well a protein is digested to make amino acids available for protein synthesis.
 Figure 3.2 ▶ Protein digestion. |
Amino acids, and sometimes a few dipeptides or larger peptides, are absorbed through the mucosa of the small intestine by active transport with the aid of vitamin B6. Intestinal cells release amino acids into the bloodstream for transport to the liver via the portal vein.
MetabolismThe liver acts as a clearinghouse for the amino acids it receives: it uses the amino acids it needs, releases those needed elsewhere, and handles the extra. For instance, the liver
Retains amino acids to make liver cells, nonessential amino acids, and plasma proteins such as heparin, prothrombin, and albumin
Regulates the release of amino acids into the bloodstream and removes excess amino acids from the circulation
Synthesizes specific enzymes to degrade excess amino acids
Removes the nitrogen from amino acids so that they can be burned for energy
Converts certain amino acids to glucose, if necessary
Forms urea from the nitrogenous wastes when protein and calories are consumed in excess of need
Converts protein to fatty acids that form triglycerides for storage in adipose tissue
Protein Synthesis
Protein synthesis (anabolism) is a complicated but efficient process that quickly assembles amino acids provided through food or released from the breakdown of existing body proteins into proteins the body needs, such as those needed for growth and development or lost through normal wear and tear. The body prioritizes muscle protein synthesis; cells in the liver, heart, and diaphragm are replenished even during short-term periods of catabolism.
In the 1990s, researchers discovered that the essential amino acid leucine has the ability to stimulate muscle protein synthesis. It appears that the amount of protein in a meal necessary to promote protein synthesis may be dependent on its leucine content (Layman et al., 2015). In adults, an intake of 25 to 30 g protein per meal with 2.2 g or more of leucine may be optimal for muscle protein synthesis. A rich source of leucine is whey, a protein in milk.
Part of what makes every individual unique is the minute differences in body proteins, which are caused by variations in the sequencing of amino acids determined by genetics. Genetic codes created at conception hold the instructions for making all of the body’s proteins. Cell function and life itself depend on the precise replication of these codes. Some important concepts related to protein synthesis are protein turnover, metabolic pool, and nitrogen balance.
Protein Turnover
Protein turnover is a continuous process that occurs within each cell as proteins are broken down from normal wear and tear and replenished. Body proteins vary in their rate of turnover. For example, red blood cells are replaced every 60 to 90 days, gastrointestinal cells are replaced every 2 to 3 days, and enzymes used in the digestion of food are continuously replenished.
Metabolic Pool
Although protein is not truly stored in the body as are glucose and fat, a supply of each amino acid exists in a metabolic pool of free amino acids within cells and circulating in blood. This pool consists of recycled amino acids from body proteins that have broken down and also amino acids from food. Because the pool accepts and donates amino acids as they become available or are needed, it is in a constant state of flux.
Nitrogen Balance
Nitrogen balance reflects the state of balance between protein breakdown (catabolism) and protein synthesis (anabolism). It is determined by comparing nitrogen intake with nitrogen excretion over a specific period of time, usually 24 hours. To calculate nitrogen intake, protein intake is measured (in grams) over a 24-hour period and divided by 6.25 because protein is 16% nitrogen. The result represents total nitrogen intake for that 24-hour period. Nitrogen excretion is computed by analyzing a 24-hour urine sample for the amount (grams) of urinary urea nitrogen it contains and adding a coefficient of 4 to this number to account for the estimated daily nitrogen loss in feces, hair, nails, and skin. Comparing grams of nitrogen excretion to grams of nitrogen intake will reveal the state of nitrogen balance, as illustrated in Box 3.2.
A neutral nitrogen balance, or state of equilibrium, exists when nitrogen intake equals nitrogen excretion, indicating protein synthesis is occurring at the same rate as protein breakdown. Healthy adults are in neutral nitrogen balance. When protein synthesis exceeds protein breakdown, as is the case during growth, pregnancy, or recovery from injury, nitrogen balance is positive. A negative nitrogen balance indicates that protein catabolism is occurring at a faster rate than protein synthesis, which occurs during starvation or the catabolic phase after injury.
Protein Catabolism for Energy
Using protein for energy is a physiologic and economic waste because amino acids used for energy are not available to be used for protein synthesis, a function unique to amino acids. Normally, the body uses very little protein for energy as long as intake and storage of carbohydrate and fat are adequate. If insufficient carbohydrate and fat are available for energy use (e.g., when calorie intake is inadequate), dietary and body proteins are sacrificed to provide amino acids that can be burned for energy. Over time, loss of lean body tissue occurs and, if severe, can lead to decreased muscle
strength, altered immune function, altered organ function, and ultimately death. To “spare” protein—both dietary and body—from being burned for calories, an adequate supply of energy from carbohydrate and fat is needed.
strength, altered immune function, altered organ function, and ultimately death. To “spare” protein—both dietary and body—from being burned for calories, an adequate supply of energy from carbohydrate and fat is needed.
BOX 3.2 Calculating Nitrogen Balance
Mary is a 25-year-old woman who was admitted to the hospital with multiple fractures and traumatic injuries from a car accident. A nutritional intake study indicated a 24-hour protein intake of 64 g. A 24-hour urinary urea nitrogen (UUN) collection result was 19.8 g.
Determine nitrogen intake by dividing protein intake by 6.25:
64 g ÷ 6.25 = 10.24 g of nitrogen
Determine total nitrogen output by adding a coefficient of 4 to the UUN:
19.8 + 4 g = 23.8 g of nitrogen
Calculate nitrogen balance by subtracting nitrogen output from nitrogen intake:
10.24 – 23.8 g = – 13.56 g in 24 hours
Interpret the results.
A negative number indicates that protein breakdown is exceeding protein synthesis. Mary is in a catabolic state.
SOURCES OF PROTEIN
The protein group consists of meat, poultry, seafood, eggs, legumes, and nuts and seeds. As a group, protein provides a variety of other nutrients, such as iron, riboflavin, niacin, vitamin B6, vitamin B12, choline, potassium, zinc, copper, vitamin D, vitamin E, and fiber (Phillips et al., 2015). Not all items within the group provide the same array of nutrients. For instance, red meat is the best source of heme iron, poultry provides niacin, vitamin D is found in seafood, legumes provide fiber, and nuts and seeds provide vitamin E. Protein is also found in dairy, grains, and vegetables (Fig. 3.3).
The protein content of individual foods within the protein group varies with what is defined as an “ounce-equivalent.” For instance, for meat, poultry, and seafood, 1 ounce equals an ounce-equivalent. For nuts and seeds, an ounce-equivalent is actually ½ ounce because they are higher in calories than lean meats, poultry, and seafood. At ½ oz as an “ounce-equivalent,” nuts and seeds provide approximately ½ or less the amount of protein as an ounce of animal protein (Berner, Becker, Wise, & Doi, 2013). Table 3.1 shows the MyPlate recommendations for the protein group at various calorie levels.
Protein Quality
Dietary proteins differ in quality based on their content of essential amino acids. For most Americans, protein quality is not important because the amounts of protein and calories consumed over the course of a day are more than adequate. But when protein needs are increased or protein intake is marginal, quality becomes a crucial concern.
Terms that refer to protein quality are complete and incomplete. Complete proteins provide all nine essential amino acids in adequate amounts and proportions needed by the body for protein synthesis. These high-quality proteins include all animal sources of protein plus soy and quinoa. Sources of complete protein include the following:
Meat, poultry, seafood, eggs
Milk, yogurt, cheese
Soybeans, soybean products, quinoa
 Figure 3.3 ▲ Protein content of MyPlate. (Source: U.S. Department of Agriculture, Center for Nutrition Policy and Promotion. [2016]. Available at www.choosemyplate.gov. Accessed 2/7/16.) |
Table 3.1 The Number of Protein Group Equivalents Recommended at Various Calorie Levels in Healthy U.S.-Style Eating Patterns | ||||||||||||||||||||
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Incomplete proteins also provide all the essential amino acids, but one or more are present in insufficient quantities to support protein synthesis. These amino acids are considered “limiting” in that they limit the process of protein synthesis. All plant proteins, with the exception of soy and quinoa, are incomplete proteins, as is gelatin. Sources of incomplete protein include the following:
Grains and products made with grains
Legumes and lentils
Nuts and seeds
Vegetables
Gelatin
Different sources of incomplete proteins differ in their limiting amino acids. For instance, grains are typically low in lysine and isoleucine, and legumes are low in methionine and cysteine. Two incomplete proteins that have different limiting amino acids are known as complementary proteins because together they form the equivalent of a complete protein. Likewise, a complete protein combined with any incomplete protein is complementary. Examples of food that contain complementary proteins appear in Box 3.3. It is not necessary to eat complementary proteins at the same meal; what is important is eating a variety of proteins over the course of a day and consuming adequate calories.
Concept Mastery AlertTwo kinds of beans (such as black beans and kidney beans) do not create a complete, or complementary, protein because they have the same incomplete proteins. Consuming one of those kinds of beans with a different incomplete or complete protein (such as rice or a toasted cheese sandwich) would create a complete protein.
BOX 3.3 Sources of Foods with Complementary Proteins
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DIETARY REFERENCE INTAKES
The Recommended Dietary Allowance (RDA) for protein for healthy adults age 19 years and older is 0.8 g/kg, derived from the absolute minimum requirement needed to maintain nitrogen balance plus an additional factor to account for individual variations and the mixed quality of proteins typically consumed (National Research Council, 2005). This figure also assumes calorie intake is adequate. For the reference adult male who weighs 154 pounds, this translates to 56 g protein/day; the reference female weighing 127 pounds needs 46 g/day. The RDA for protein is higher for people younger than 19 years (in gram per kilogram body weight) and for pregnant or lactating women to support growth and development (Table 3.2).
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