D-3-phosphoglycerate dehydrogenase. Further studies on the enzyme isolated from chicken liver

L-serine synthesis in the central nervous system: a review on serine deficiency disorders

The de novo synthesis of the amino acid L-serine plays an essential role in the development and functioning of the central nervous system (CNS). L-serine displays many metabolic functions during different developmental stages; among its functions providing precursors for amino acids, protein synthesis, nucleotide synthesis, neurotransmitter synthesis and L-serine derived lipids. Patients with congenital defects in the L-serine synthesizing enzymes present with severe neurological abnormalities and underscore the importance of this synthetic pathway. In this review, we will discuss the cellular functions of the L-serine pathway, structure and enzymatic properties of the enzymes involved and genetic defects associated with this pathway.

A novel alpha-ketoglutarate reductase activity of the serA-encoded 3-phosphoglycerate dehydrogenase of Escherichia coli K-12 and its possible implications for human 2-hydroxyglutaric aciduria

Escherichia coli serA-encoded 3-phosphoglycerate (3PG) dehydrogenase catalyzes the first step of the major phosphorylated pathway of L-serine (Ser) biosynthesis. The SerA enzyme is evolutionarily related to the pdxB gene product, 4-phosphoerythronate dehydrogenase, which catalyzes the second step in one branch of pyridoxal 5'-phosphate coenzyme biosynthesis. Both the Ser and pyridoxal 5'-phosphate biosynthetic pathways use the serC(pdxF)-encoded transaminase in their next steps. In an analysis of these parallel pathways, we attempted to couple the transaminase and dehydrogenase reactions in the reverse direction. Unexpectedly, we found that the SerA enzyme catalyzes a previously undetected reduction of alpha-ketoglutarate (alpha KG) to 2-hydroxyglutaric acid (HGA). Numerous criteria ruled out the possibility that this SerA alpha KG reductase activity was caused by contamination in the substrate or purified enzyme preparations. HGA was confirmed as the product of the SerA alpha KG reductase reaction by thin-layer chromatography and by enzyme assays showing that both the D- and L-isomers of HGA were substrates for the reverse (dehydrogenase) reaction. Detailed steady-state kinetic analyses showed that alpha KG reduction (apparent Michaelis-Menten constant [Km(app)] = 88 microM; apparent catalytic constant [kcat(app)] = 33.3 s-1) and 3-phosphohydroxypyruvate reduction (Km(app) = 3.2 microM; kcatapp = 27.8 s-1), which is the reverse reaction of 3PG oxidation, were the major in vitro activities of the SerA enzyme. The SerA alpha KG reductase was inhibited by Ser, D-HGA, 3PG, and glycine (Gly), whereas the D-HGA dehydrogenase was inhibited by Ser, alpha KG, 3-phosphohydroxypyruvate, and Gly. The implications of these findings for the regulation of Ser biosynthesis, the recycling of NADH, and the enzymology of 2-hydroxyacid dehydrogenases are discussed. Since the same pathway of Ser biosynthesis seems to be present in all organisms, these results suggest that a mutation in the human SerA homolog may contribute to the neurometabolic diseases D- and L-2-hydroxyglutaric aciduria, which lead to the accumulation of D-HGA and L-HGA, respectively.