Periodic Structures in Protein Backbones

The a-helix and /3-pleated sheet are periodic stmctures their features repeat at regular intervals. The a-helix is rodlike and involves only one polypeptide chain. The /3-pleated sheet structure can give a two-dimensional array and can involve one or more polypeptide chains. 

A complete protein is one that provides all essential amino acids (Section 23.5) in appropriate amounts for human survival. These amino acids cannot be synthesized by humans, but they are needed for the biosynthesis of proteins. Lysine and methionine are two essential amino acids that are frequendy in short supply in plant proteins. 

Because grains such as rice and corn are usually poor in lysine, and because beans are usually poor in methionine, vegetarians are at risk for malnutrition unless they eat grains and beans together. This leads to the concept of complementary proteins, mixtures that provide all the essential amino acids—for example, corn and beans in succotash, or a bean burrito made with a corn tortilla. The specific recommended dietary allowances for adult males follow. Adult females who are neither pregnant nor lactat-ing need 20% less than the amounts indicated for adult males. 

The /trotein rfficiency ratio (PER) describes how weU a protein supplies essential amino acids. This parameter is useful for deciding how much of a food you need to eat. Most coUege-age, nonpregnant females require 46 g (or about 1.6 oz) of complete protein, and males require 58 g (or about 2 oz) of complete protein per day. If one chooses only a single source of protein for the 

Sign in at www.thomsonedu.com/login and explore a Biochemistry Interactive tutorial on /3-sheets, one of the principal types of secondary structure in proteins. 

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