The structure of glucose-fructose oxidoreductase from Zymomonas mobilis: an osmoprotective periplasmic enzyme containing non-dissociable NADP.

BACKGROUND

The organism Zymomonas mobilis occurs naturally in sugar-rich environments. To protect the bacterium against osmotic shock, the periplasmic enzyme glucose-fructose oxidoreductase (GFOR) produces the compatible, solute sorbitol by reduction of fructose, coupled with the oxidation of glucose to gluconolactone. Hence, Z mobilis can tolerate high concentrations of sugars and this property may be useful in the development of an efficient microbial process for ethanol production. Each enzyme subunit contains tightly associated NADP which is not released during the catalytic cycle.

RESULTS

The structure of GFOR was determined by X-ray crystallography at 2.7 A resolution. Each subunit of the tetrameric enzyme comprises two domains, a classical dinucleotide-binding domain, and a C-terminal domain based on a predominantly antiparallel nine-stranded beta sheet. In the tetramer, the subunits associate to form two extended 18-stranded beta sheets, which pack against each other in a face to face fashion, creating an extensive interface at the core of the tetramer. An N-terminal arm from each subunit wraps around the dinucleotide-binding domain of an adjacent subunit, covering the adenine ring of NADP.

CONCLUSIONS

In GFOR, the NADP is found associated with a classical dinucleotide-binding domain in a conventional fashion. The NADP is effectively buried in the protein-subunit interior as a result of interactions with the N-terminal arm from an adjacent subunit in the tetramer, and with a short helix from the C-terminal domain of the protein. This accounts for NADP's inability to dissociate. The N-terminal arm may also contribute to stabilization of the tetramer. The enzyme has an unexpected structural similarity with the cytoplasmic enzyme glucose-6-phosphate dehydrogenase (G6PD). We hypothesize that both enzymes have diverged from a common ancestor. The mechanism of catalysis is still unclear, but we have identified a conserved structural motif (Glu-Lys-Pro) in the active site of GFOR and G6PD that may be important for catalysis.