Serine phosphohydroxypyruvate

The amino acid serine is biosynthesized by a route that involves reaction of 3-phosphohydroxypyruvate with glutamate. Propose a mechanism. 

One excellent example of a Emax-type allosteric enzyme is Escherichia coli phosphoglycerate dehydrogenase (PGDH), a tetramer of identical subunits that catalyzes the formation of D-3-phosphohydroxypyruvate from D-3-phosphoglycerate in a reaction that uses NAD+ as a redox cofactor. This regulatory enzyme is allosteri-cally controlled by serine. All available information suggests that the effects on the for substrate are minor... 

Serine is synthesized from 3-phosphoglycerate, an intermediate in glycolysis. The first step is an oxidation to 3-phosphohydroxypyruvate. This a-ketoacid is transaminated to 3-phosphoserine, which is then hydrolyzed to serine. 

Phosphoglycerate dehydrogenase catalyzes the conversion of 3-phosphoglycerate to 3-phosphohydroxypyruvate, an intermediate in the biosynthesis of serine. 

One advantage of this pathway over serine dehydratase is that in transamination free ammonia is not released and the potential difficulty of ammonia toxicity is minimized. Serine in people is not an essential amino acid and can be synthesized in the liver from the glycolytic intermediate 3-phosphoglycerate, which can be oxidized to phosphohydroxypyruvate and then transaminated with glutamate to form phosphoserine. The... 

In the biosynthesis of serine from glucose, 3-phosphoglycerate is first oxidized to a 2-keto compound (3-phosphohydroxypyruvate), which is then transaminated to form phosphoserine (Fig. 39.5). Phosphoserine phosphatase removes the phosphate, forming serine. The major sites of serine synthesis are the liver and kidney. 

Minor Pathways. Dihydroxyacetone phosphate can be transformed to glycerol phosphate, which is used for the synthesis of fats and glycerophosphatides. Phosphoglycerate can be converted to 3-phosphohydroxypyruvate and further to hydroxypyruvate. This last compound is possibly related to glycolate (by oxidative decarboxylation) and to glyoxylate, and hence to glycine. Pyruvate is related to alanine by transamination, and hydroxypyruvate, to serine. 

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