This pathway describes the synthesis and breakdown of several small amino acids, including glycine, serine, and cysteine. All of these compounds share common intermediates and almost all can be biosynthesized from one another. Serine and glycine are not essential amino acids and can be synthesized from several routes. On the other hand, cysteine is a conditionally essential amino acid, meaning that it can be endogenously synthesized but insufficient quantities may be produced due to certain diseases or conditions. Serine is central to the synthesis and breakdown of the other two amino acids. Serine can be synthesized via glycerate, which can be converted into glycerate 3-phosphate (via glycerate kinase), which in turn is converted into phosphohydroxypyruvate by phosphoglycerate dehydrogenase and then phosphoserine (via phosphoserine transaminase) and finally to serine (via phosphoserine phosphatase). The serine synthesized via this route can be used to create cysteine and glycine through the homocysteine cycle. In the homocysteine cycle, cystathionine beta-synthase catalyzes the condensation of homocysteine and serine to give cystathionine. Cystathionine beta-lyase then converts this double amino acid to cysteine, ammonia, and alpha-ketoglutarate. Glycine is biosynthesized in the body from the amino acid serine. In most organisms, the enzyme serine hydroxymethyltransferase (SHMT) catalyzes this transformation using tetrahydrofolate (THF), leading to methylene THF and glycine. Glycine can be degraded via three pathways. The predominant pathway in animals involves the glycine cleavage system, also known as the glycine decarboxylase complex or GDC. This system is usually triggered in response to high concentrations of glycine. The system is sometimes referred to as glycine synthase when it runs in the reverse direction to produce glycine. The glycine cleavage system consists of four weakly interacting proteins: T, P, L and H-proteins. The glycine cleavage system leads to the degradation of glycine into ammonia and CO2. In the second pathway, glycine is degraded in two steps. The first step in this degradation pathway is the reverse of glycine biosynthesis from serine with serine hydroxymethyltransferase (SHMT). The serine generated via glycine is then converted into pyruvate by the enzyme known as serine dehydratase. In the third route to glycine degradation, glycine is converted into glyoxylate by D-amino acid oxidase. Glyoxylate is then oxidized by hepatic lactate dehydrogenase into oxalate in an NAD+-dependent reaction.

Glycine and Serine Metabolism References