W0382

Phosphoglycerate Dehydrogenase (PGDH) Structure Without the Inhibitor Serine. J. Thompson, G. Grant^#, and L.J. Banaszak, Univ. of Minnesota, Minneapolis, MN and ^#Washington Univ., St Louis, MO, USA.

The completion of refinement of the E. coli Phosphoglycerate dehydrogenase structure to 2.2 Å resolution is anticipated. PGDH catalyzes the first step in serine biosynthesis by oxidizing 3-phosphoglycerate to 3-phosphohydroxypyruvate with concomitant reduction of NAD⁺. An unusual allosteric regulation occurs along with cooperative serine binding to tetrameric PGDH, which modulates V_max rather than K_m. A crystal structure of PGDH in complex with serine and NAD revealed the conformation of the inhibited enzyme [Schuller et. al, Nat. Struct. Biol. (1995) 2, 69-76]. Presented is serine-free PGDH, which in comparison provides discussion for the allosteric effects of serine and its cooperativity in binding.

The PGDH monomer includes three distinct domains, regulatory, substrate and nucleotide binding; the homotetramer involves two unique subunit interfaces. In the absence of bound serine, the regulatory domain adopts a different conformation at one of these subunit interfaces. The result appears to be a structural change involving the overall tetramer. In conflict with our hypotheses, the "catalytic site" clefts between the substrate and nucleotide binding domains remain solvent exposed and differ little from those in the inhibited enzyme. Electron density for the four NAD molecules is present along with disordered density that may belong to co-crystallized alpha-keto glutarate molecules. (Supported by the NIH GM13925, GM 56676 & Minnesota Supercomputing Institute)