Phosphohydroxypyruvate, phosphoserine

This byproduct reacts with 3-phosphohydroxypyruvate to give 3-phosphoserine plus regenerated pyridoxal phosphate. 

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. 

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. 

Hanford and Davies (1958) showed that a partly purilied extract from pea epicotyls converted phosphoglycerate to phosphoserine. The reaction was dependent on NAD, glutamate, and pyridoxal phosphate. The first step was presumably catalyzed by phosphoglycerate dehydrogenase and the second by glutamate phosphohydroxypyruvate aminotransferase. Such a phosphoserine aminotransferase was present in extracts of pea seeds, leaves, and apical meristems (Cheung et al., 1968). Little is known of phosphoserine phosphatase in plants, but its presence in spinach leaves is reported by Larsson and Albertsson (1979). 

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

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