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J. Bacteriol., 01 1996, 232-239, Vol 178, No. 1
G Zhao and ME Winkler
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.
Copyright © 1996, American Society for Microbiology
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
Department of Microbiology and Molecular Genetics, University of Texas Houston Medical School 77030-1501, USA.
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