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In animals, the sulfur-containing amino acid cysteine is synthesized from homocysteine, which is itself derived from metabolism of the amino acid methionine. Methionine is an essential amino acid for animals, making cysteine indirectly an essential amino acid as well. Homocysteine condenses with serine to form cystathionine, which is deaminated and hydrolyzed to form cysteine and alpha-ketobutyrate. While the sulfur in cysteine comes from homocysteine, the rest of the molecule comes from the initial serine residue. Cysteine biosynthesis occurs by a distinct mechanism in plants and prokaryotes. Cysteine is an important amino acid for all organisms due to the role it plays in protein folding. The formation of disulfide linkages between cysteine residues helps to stabilize the tertiary and quaternary structure of proteins. Disulfide linkages are most common in secreted proteins, where proteins are exposed to more oxidizing conditions that are found in the cellular interior. Elevated blood levels of homocysteine are a risk factor for the development of cardiovascular disease. One of the causes of elevated homocysteine is a genetic lack of cystathionine beta-synthase and excess methionine intake may be another explanation. Control of methionine intake and supplementing folic acid and vitamin B12 in the diet are used to lower homocysteine levels. (This definition may be outdated - see the DesignNote.)
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