The biosynthesis of threonine starts with L-aspartic acid being phosphorylated by an ATP-driven aspartate kinase resulting in a release of an ADP and an L-aspartyl-4-phosphate. This compound interacts with a hydrogen ion through an NADPH-driven aspartate semialdehyde dehydrogenase resulting in the release of a phosphate, an NADP, and an L-aspartate-semialdehyde. The latter compound interacts with a hydrogen ion through an NADPH-driven aspartate kinase / homoserine dehydrogenase resulting in the release of an NADP and an L-homoserine. L-Homoserine is phosphorylated through an ATP driven homoserine kinase resulting in the release of an ADP, a hydrogen ion, and an O-phosphohomoserine. The latter compound then interacts with a water molecule threonine synthase resulting in the release of a phosphate and an L-threonine. L-threonine is degraded into glycine and acetaldehyde by reacting with a threonine aldolase. Acetaldehyde can then be integrated into the mitochondria or stay in the cytosol. It is then degraded into acetyl-CoA through an aldehyde dehydrogenase.

Threonine Metabolism References