Amino acid synthesis encompasses the metabolic pathways by which cells produce amino acids. Humans can synthesize eleven of the twenty standard amino acids, while the remaining nine — called essential amino acids — must be obtained from the diet.

Essential vs. Non-Essential Amino Acids

Essential Amino Acids

Humans cannot synthesize histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These must be supplied by the diet. The lack of any essential amino acid leads to protein deficiency, as protein synthesis requires all twenty amino acids simultaneously.

Non-Essential Amino Acids

The remaining eleven amino acids can be synthesized by the human body. They are produced from common metabolic intermediates such as alpha-keto acids, which are derived from glycolysis and the citric acid cycle.

Biosynthetic Pathways

Transamination

The first step in synthesizing many amino acids is transamination. An alpha-keto acid accepts an amino group from another amino acid (usually glutamate), catalyzed by aminotransferases. This reaction converts the keto acid into an amino acid.

Families of Amino Acids

Amino acids are grouped into families based on their biosynthetic precursors: from alpha-ketoglutarate come glutamate, glutamine, proline, and arginine; from oxaloacetate come aspartate, asparagine, methionine, threonine, and lysine; from pyruvate come alanine, valine, leucine, and isoleucine; from 3-phosphoglycerate come serine, glycine, and cysteine; from shikimate come phenylalanine, tyrosine, and tryptophan; and from ribose-5-phosphate comes histidine.

Regulation

Amino acid synthesis is tightly regulated by feedback inhibition. The end product of a pathway inhibits the first committed enzyme, preventing overproduction. This allows cells to match amino acid production to their needs.