STRUCTURE AND CATALYTIC MECHANISM OF GLUCOSAMINE 6-PHOSPHATE DEAMINASE FROM ESCHERICHIA-COLI AT 2.1-ANGSTROM RESOLUTION

Background: Glucosamine 6-phosphate deaminase from Escherichia coli is an allosteric hexameric enzyme which catalyzes the reversible convers ion of D-glucosamine 6-phosphate into D-fructose 6-phosphate and ammon ium ion and is activated by N-acetyl-D-glucosamine 6-phosphate, Mechan istically, it belongs to the group of aldose-ketose isomerases, but it s reaction also accomplishes a simultaneous amination/deamination. The determination of the structure of this protein provides fundamental k nowledge for understanding its mode of action and the nature of allost eric conformational changes that regulate its function. Results: The c rystal structure of glucosamine 6-phosphate deaminase with bound phosp hate ions is presented at 2.1 Angstrom resolution together with the re fined structures of the enzyme in complexes with its allosteric activa tor and with a competitive inhibitor. The protein fold can be describe d as a modified NAD-binding domain. Conclusions: From the similarities between the three presented structures, it is concluded that these re present the enzymatically active R state conformer. A mechanism for th e deaminase reaction is proposed. It comprises steps to open the pyran ose ring of the substrate and a sequence of general base-catalyzed rea ctions to bring about isomerization and deamination, with Asp72 playin g a key role as a proton exchanger.