THE 3-DIMENSIONAL STRUCTURE OF GLUCOSE-6-PHOSPHATE-DEHYDROGENASE FROMLEUCONOSTOC-MESENTEROIDES REFINED AT 2.0-ANGSTROM RESOLUTION

Background: Glucose 6-phosphate dehydrogenase (G6PD) is the first enzy me of the pentose phosphate pathway. Normally the pathway is synthetic and NADP-dependent, but the Gram-positive bacterium leuconostoc mesen teroides, which does not have a complete glycolytic pathway, also uses the oxidative enzymes of the pentose phosphate pathway for catabolic reactions, and selects either NAD or NADP depending on the demands for catabolic or anabolic metabolism. Results: The structure of G6PD has been determined and refined to 2.0 Angstrom resolution. The enzyme is a dimer, each subunit consisting of two domains. The smaller domain is a classic dinucleotide-binding fold, while the larger one is a new be ta+alpha fold, not previously seen, with a predominantly antiparallel nine-stranded beta-sheet. There are significant structural differences in the coenzyme-binding domains of the two subunits, caused by Pro 14 9 which is cis in one subunit and trans in the other. Conclusions: The structure has allowed us to propose the location of the active site a nd the coenzyme-binding site, and suggests the role of many of the res idues conserved between species. We propose that the conserved Arg46 w ould interact with both the adenine ring and the 2'-phosphate of NADP. Gln47, which is not conserved, may contribute to the change from NADP to dual coenzyme specificity. His178, in a nine-residue peptide conse rved for all known sequences, binds a phosphate in the active site poc ket. His240 is the most likely candidate for the base to oxidize the 1 -hydroxyl group of the glucose 6-phosphate substrate.