Structure of 6-phosphogluconate dehydrogenase refined at 2 A resolution

The X-ray unliganded structure of 6-phosphogluconate dehydrogenase (E.C. 1.1.1.44) (6-PGDH) from sheep liver has been determined at 2A resolution and refined to a final R-factor of 19.8% for 35 031 unique reflections. The enzyme is dimeric, each subunit being comprised of an N-terminal coenzyme-binding domain with a Rossmann fold, a large all-helical domain and a small C-terminal tail. The model contains 473 residues, three sulfate ions and 346 water molecules; the two best defined sulfates are found in the active site. This structure, based on improved diffraction data, is an extension of the 2.5 A, resolution model reported earlier. It has good geometry with 92% of the residues falling in the most favoured areas of the Ramachandran plot. Several unusual features are discussed: the incorporation of an alanine in place of the second conserved glycine of the dinucleotide-binding fingerprint; a duplicated five-helix motif which is unique to this enzyme; an extended water network at the dimer interface and a C-terminal tail which is incorporated within the second subunit, forming not only a major part of the dimer interface but also part of the active site.