SUMMARY

• 3.1 Proteins Are Built from a Repertoire of 20 Amino Acids

  Proteins are linear polymers of amino acids. Each amino acid consists of a central tetrahedral carbon atom that is bonded to an amino group, a carboxylic acid group, a distinctive side chain, and a hydrogen atom. These tetrahedral centers, with the exception of that of glycine, are chiral; only the l isomer exists in natural proteins. All natural proteins are constructed from the same set of 20 amino acids.

• 3.2 Amino Acids Contain a Wide Array of Functional Groups

  The side chains of these 20 building blocks vary tremendously in size, shape, and the presence of functional groups. They can be grouped as follows on the basis of the chemical properties of the side chains: (1) hydrophobic side chains—glycine, alanine, valine, leucine, isoleucine, methionine, proline, and the aromatic amino acids phenylalanine and tryptophan; (2) polar amino acids—serine, threonine, tyrosine, asparagine, and glutamine; (3) positively charged amino acids—lysine, arginine, and histidine; and (4) negatively charged amino acids—aspartic acid and glutamic acid.

• 3.3 Essential Amino Acids Must Be Obtained from the Diet

  Most microorganisms are capable of making all 20 of the amino acids from simpler molecules. Although human beings can make 11 amino acids, 9 must be acquired from the diet. These 9 amino acids are called essential amino acids because they are required for healthy growth and development.