Determinants of substrate specificity in D-3-phosphoglycerate dehydrogenase. Conversion of the M. tuberculosis enzyme from one that does not use alpha-ketoglutarate as a substrate to one that does

摘要

D-3-Phosphoglycerate dehydrogenase (PGDH) converts D-3-phosphoglycerate (PGA) to phosphohydroxypyruvate (PHP) in the first step of L-serine biosynthesis. This reaction is reversible, and some PGDHs are capable of using alpha-ketoglutarate (alpha KG) instead of PHP in the reverse direction to produce alpha-hydroxyglutarate. The enzymes so far shown to have this ability are Type II PGDHs, suggesting that this may be a common feature of the Type II enzymes. Type I PGDHs examined so far do not share this feature. Inspection of PGDH sequences shows that a GCFCI ... WXKX motif is commonly found in Type II PGDHs while a GRAGT ... WXRX motif is commonly associated with Type I PGDHs. The removal of the cationic side chain at the first position shown above in the Type I PGDH from Mycobacterium tuberculosis converts it to an enzyme capable of using aKG where the native enzyme is not. It also produces an enzyme that regenerates NAD+ in the forward reaction when coupled to phosphoserine aminotransferase, as was previously shown for E. coli PGDH. Substitution of an arginyl residue for a lysyl residue at the second position of ecPGDH, decreases the k(cat)/K-m of the enzyme by approximately 50-fold when using aKG, but only approximately 3-fold when using PHP. This suggests that a PGDH dependent cycle that conserves NAD(+) in E. coli may be operative in many other organisms expressing the GCFCI ... WXKX motif.