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17.3: Proteins: Muscle and Much More

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  • Learning Objectives

    • Describe the metabolism of proteins.
    • Know the importance of essential amino acids.
    • Know the sources and function of common proteins in the diet.

    Early nutritional scientists such as the German believed that protein was the most important nutrient for maintaining the structure of the body, because it was generally believed that "flesh makes flesh."

    Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: , , and . Almost all globular proteins are and many are enzymes. Fibrous proteins are often structural, such as , the major component of connective tissue, or , the protein component of hair and nails. Membrane proteins often serve as or provide channels for polar or charged molecules to pass through the . No doubt there is no question on the importance of enzymes, collagen, keratin, enzymes, and chemical receptors for regular bodily functions. Thus proteins are very important in our diets. Several dietary sources of proteins include nuts, beans/legumes, skim milk, egg whites, and meat.

    Protein Metabolism: Essential Amino Acids

    Protein digestion begins in the stomach (Figure \(\PageIndex{3}\)), where the action of gastric juice hydrolyzes about 10% of the peptide bonds. Gastric juice is a mixture of water (more than 99%), inorganic ions, hydrochloric acid, and various enzymes and other proteins.

    The pain of a gastric ulcer is at least partially due to irritation of the ulcerated tissue by acidic gastric juice.

    Figure \(\PageIndex{1}\) The principal events and Sites of Protein Digestion

    The hydrochloric acid (HCl) in gastric juice is secreted by glands in the stomach lining. The pH of freshly secreted gastric juice is about 1.0, but the contents of the stomach may raise the pH to between 1.5 and 2.5. HCl helps to denature food proteins; that is, it unfolds the protein molecules to expose their chains to more efficient enzyme action. The principal digestive component of gastric juice is pepsinogen, an inactive enzyme produced in cells located in the stomach wall. When food enters the stomach after a period of fasting, pepsinogen is converted to its active form—pepsin—in a series of steps initiated by the drop in pH. Pepsin catalyzes the hydrolysis of peptide linkages within protein molecules. It has a fairly broad specificity but acts preferentially on linkages involving the aromatic amino acids tryptophan, tyrosine, and phenylalanine, as well as methionine and leucine.

    Protein digestion is completed in the small intestine.

    Amino Acids

    The human body uses amino acids to make proteins to help the body:

    • Break down food
    • Grow
    • Repair body tissue
    • Perform many other body functions

    Amino acids can also be used as a source of energy by the body.

    Amino acids are classified into three groups namely: essential amino acids, nonessential amino acids, and conditional amino acids.


    • Essential amino acids cannot be made by the body. As a result, they must come from food.
    • The 9 essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.


    Nonessential means that our bodies produce an amino acid, even if we do not get it from the food we eat. Nonessential amino acids include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.


    • Conditional amino acids are usually not essential, except in times of illness and stress.
    • Conditional amino acids include: arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine.

    Complete and Incomplete Proteins

    Proteins can be classified as either complete or incomplete. Complete proteins provide adequate amounts of all nine essential amino acids. Animal proteins such as clipboard_e083fcabc41b575b4e246e69cc5fbdd40.pngmeat, fish, milk, and eggs are good examples of complete proteins. Incomplete proteins do not contain adequate amounts of one or more of the essential amino acids. For example, if a protein doesn't provide enough of the essential amino acid leucine it would be considered incomplete. Leucine would be referred to as the limiting amino acid, because there is not enough of it for the protein to be complete. Most plant foods are incomplete proteins, with a few exceptions such as soy. Table \(\PageIndex{1}\) shows the limiting amino acids in some plant foods.

    Table \(\PageIndex{1}\) Limiting Amino Acids in Some Common Plant Foods.

    Complementary Proteins

    clipboard_e69fbe4fa9ae163220dff71355f2e6d28.png containing incomplete proteins with different limiting amino acids to provide adequate amounts of the essential amino acids. These two proteins are called complementary proteins, because they supply the amino acid(s) missing in the other protein. A simple analogy would be that of a 4 piece puzzle. If one person has 2 pieces of a puzzle, and another person has 2 remaining pieces, neither of them have a complete puzzle. But when they are combined, the two individuals create a complete puzzle.
    Figure \(\PageIndex{2}\)

    Complementary proteins are kind of like puzzle pieces.

    Two examples of complementary proteins are shown below.


    Figure \(\PageIndex{3}\)

    Two complementary protein examples

    It should be noted that complementary proteins do not need to be consumed at the same time or meal. It is currently recommended that essential amino acids be met on a daily basis, meaning that if a grain is consumed at one meal, a legume could be consumed at a later meal, and the proteins would still complement one another.

    Protein Deficiency in Young and Old

    clipboard_e6aff617d9cb878dbe722b8831b14acbb.pngmalnutrition. They differ in the severity of energy deficiency as shown in Figure \(\PageIndex{4}\).
    Figure \(\PageIndex{4}\) The two types of protein-energy malnutrition


    clipboard_ee18533dae5090d0217354a8639db0057.pngKwashiorkor is a Ghanaian word that means "the disease that the first child gets when the new child comes." The characteristic symptom of kwashiorkor is a swollen abdomen (Figure \(\PageIndex{4}\)). Energy intake could be adequate, but protein consumption is too low.
    Figure \(\PageIndex{5}\) A child suffering from kwashiorkor


    The video does a nice job showing the symptoms of the condition.


    clipboard_e7866f0a23f12221feb55d4fc341b79de.pngMarasmus means "to waste away" or "dying away", and thus occurs in individuals who have severely limited energy intakes.
    Figure \(\PageIndex{6}\) Two individuals suffering from marasmus


    • Protein digestion begins in the stomach where hydrolysis of the protein linkages occurs with the action of gastric juices (mainly HCl ) and the active enzyme pepsin. Protein digestion is completed in the small intestine wherein other protein digesting enzymes are involved.
    • Essential amino acids cannot be made by the body and must come from food.
    • Complete proteins provide adequate amounts of all nine essential amino acids.
    • Complementary proteins are made up of two proteins wherein one protein supply the amino acid(s) missing in the other protein.
    • Kwashiorkor and marasmus are two forms of protein energy malnutrition that are not common in the U.S., but is more prevalent in less developed countries