Cystathionine synthase is an enzyme in the metabolism of sulfur-containing amino acids that catalyzes the reaction that follows (Figure 21.8):

Homocysteine + Serine <=> Cystathionine + H2O

In humans, deficiency of cystathionine synthase leads to a condition called homocystinuria, in which homocysteine overaccumulates. The condition results in severe mental retardation and dislocation of the lens of the eye.

See also: Metabolism of Sulfur-Containing Amino Acids, Homocysteine, Figure 21.9