THE CRYSTAL-STRUCTURE OF THE BIFUNCTIONAL ENZYME 6-PHOSPHOFRUCTO-2-KINASE FRUCTOSE-2,6-BISPHOSPHATASE REVEALS DISTINCT DOMAIN HOMOLOGIES/

Background: Glucose homeostasis is maintained by the processes of glyc olysis and gluconeogenesis. The importance of these pathways is demons trated by the severe and life threatening effects observed in various forms of diabetes. The bifunctional enzyme phosphofructo-2-kinase/fruc tose-2,6-bisphosphatase catalyzes both the synthesis and degradation o f fructose-2,6-bisphosphate, a potent regulator of glycolysis, Thus th is bifunctional enzyme plays an indirect yet key role in the regulatio n of glucose metabolism. Results: We have determined the 2.0 Angstrom crystal structure of the rat testis isozyme of this bifunctional enzym e. The enzyme is a homodimer of 55 kDa subunits arranged in a head-to- head fashion, with each monomer consisting of independent kinase and p hosphatase domains, The location of ATP gamma S and inorganic phosphat e in the kinase and phosphatase domains, respectively, allow us to loc ate and describe the active sites of both domains. Conclusions: The ki nase domain is clearly related to the superfamily of mononucleotide bi nding proteins, with a particularly close relationship to the adenylat e kinases and the nucleotide-binding portion of the G proteins. This i s in disagreement with the broad speculation that this domain would re semble phosphofructokinase. The phosphatase domain is structurally rel ated to a family of proteins which includes the cofactor independent p hosphoglycerate mutases and acid phosphatases.