Evolution has left our species without the ability to synthesize almost half the amino acids that are essential building blocks and precursors of a variety of critical molecules
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| Human beings use 20 amino acids to build peptides and proteins that are essential to the many functions of their cells. Biosynthesis of the amino acids involves synthesis of the carbon skeletons for the corresponding α-keto acids, followed by addition of the amino group via transamination. However, humans are capable of carrying out the biosynthesis of the carbon skeletons of only about half of those α-keto acids. Amino acids that we cannot synthesize are termed essential amino acids, and are required in the diet. While almost all of the amino acids can be classified as clearly essential or nonessential, a few require further qualification. For example, although cysteine is not generally considered an essential amino acid because it can be derived from the nonessential amino acid, serine, its sulfur must come from the required or essential amino acid, methionine. Similarly, the amino acid, tyrosine, is not required in the diet, since it can be derived from the essential amino acid, phenylalanine. This relationship between phenylalanine and tyrosine will be discussed further in considering the inherited disease, phenylketonuria (PKU). Tables 18.4 and 18.5 list the nonessential and essential amino acids, and the source of the carbon skeleton in the case of those not required in the diet.
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