5.7: Protein

Protein is made of amino acids linked together. Amino acids have an amino part (-NH₂) and an acid part (-COOH). The amino part of one amino acid combines with the acid part of the adjoining amino acid to form the chain that makes up the protein (Figure 5.6).

Like carbohydrate and fat, protein is made mostly of carbon, hydrogen, and oxygen. But protein differs in that it always has nitrogen (N) due to its amino part (-NH₂). Most of the nitrogen in the body (and food) is in protein. Protein has 4 calories per gram.

Twenty kinds of amino acids are needed to make protein; the body can make all but nine of these. These nine must come from the diet. Virtually all proteins have all of the 20 kinds of amino acids, although they have different amounts of each kind. Dietary protein is the topic of Chapter 11.

The amino acids are linked in a precise order to form a single chain. The number of amino acids in the chain can vary from just a few to more than a thousand. The sequence in which the amino acids are linked is unique for each protein.† The number of amino acids and the precise order in which they are linked are coded in our genes. There’s a gene for each and every protein the body makes. (Likewise, every plant has a gene for each of its proteins.) Genes and protein synthesis are discussed in Chapter 10.

The chain of amino acids automatically folds upon itself to form a distinctive shape, based on the precise number and the unique order of amino acids in the chain. A protein’s shape determines its function. The wide variety of proteins have a wide variety of uses, e.g., practically all of the enzymes that catalyze chemical reactions are proteins,* some hormones (e.g., insulin) are proteins, antibodies are proteins, and so are the ones in muscle that enable us to move.

Figure 5.6: Protein is a Chain of Amino Acids

Protein is said to be denatured when its shape is altered permanently.† Cooks (and food companies) usually blanch (heat quickly and briefly) vegetables before freezing them. This denatures—and thereby inactivates—the vegetable’s enzymes (proteins). Vegetables can be blanched by a short immersion into boiling water—long enough to denature the enzymes, but brief enough to minimize changes in the vegetable’s flavor, texture, and nutrients. Putting these enzymes out of commission slows a vegetable’s deterioration.

Corn loses its sweetness as it ages because its sugar turns to starch. This making of starch from sugar is catalyzed by enzymes. “Killing” these enzymes by blanching helps preserve the corn’s sweetness.

If you add fresh pineapple to a gelatin concoction (e.g., Jell-O) it doesn’t gel. You avoid this mishap with canned pineapple, because the pineapple was heated during processing. The heat kills, by denaturation, an enzyme in fresh pineapple that breaks down gelatin.

Cooks don’t always have “killing” in mind when they denature protein by heating, beating, etc. Egg white is protein and changes dramatically when denatured. Raw egg white is liquid and clear, but becomes white and fluffy when beaten/ denatured to top a lemon meringue pie, and solid and white when heated/denatured to make a hard-boiled egg. The amino acids don’t change when egg white protein is denatured by beating or boiling; only the protein’s shape changes.

Protein is also denatured by acidifying the surrounding liquid. In the Latin American dish ceviche, raw seafood is marinated in seasoned lime juice and then served. It looks cooked because the acid in the lime juice denatures the seafood’s protein.** We also see this acid effect when milk-protein curdles as milk “goes sour,” or when we marinate beef in vinegar to make sauerbraten.

Proteins in the body can denature by changes in acidity as well. As discussed in Chapter 3, enzymes in blood and tissue work best only when the pH is 7.35 to 7.45 (slightly basic). This sensitivity is expected—a change in pH alters the shape of enzymes and thereby alters their function.

When athletes feel sudden fatigue (hit the wall) in endurance events, one explanation is that local acidity has impaired the function of muscle proteins (including muscle enzymes). Muscle fluids become more acid during intense and prolonged muscle action because lactic acid accumulates (Chap. 9).

Note that denaturation changes a protein’s shape, not its amino acid content. So denaturing protein by cooking†† doesn’t ordinarily lower its nutritional value. Cooking can, in fact, make a food more nutritious—particularly plant foods—by making it more digestible. Cooking improves the nutritional value of rice—uncooked rice is quite indigestible. Making plants more digestible is especially important for people in developing countries; plants are their major source of both the calories and protein that are often deficient in their diet.

^{†Frederick Sanger in 1953 determined the amino acid sequence of a protein (insulin’s 30 amino acids), showing for the first time that a protein’s amino acids are linked in a precise order; he won a 1958 Nobel Prize. Christian Anfinsen showed in 1960 that a protein’s amino acid sequence determines its shape; he won a 1972 Nobel Prize.
*All enzymes were thought to be proteins until Sidney Altman and Thomas Cech found that RNA (Chap. 10) can act as an enzyme; they won a 1989 Nobel Prize.
†Prions are denatured proteins thought to cause the fatal brain disease “mad cow disease” in cows and Creutzfelt-Jacob disease in humans. Stanley Prusiner won a 1997 Nobel Prize for discovering prions.
**“Cooking” this way, instead of using heat, can be a problem. Peru had a deadly epidemic of cholera in 1991 when cholera-contaminated sewage tainted the seafood. Ceviche is a popular “fast food” sold at stands along the Peruvian coast. Lime juice’s mild acidity doesn’t kill cholera organisms; thorough cooking does.
††Some amino acids can be altered by cooking, e.g., some combine with carbohydrate on the surface of foods like bread and cake when baked, making the surface brown}