Substrate form of D-frutose 1,6-bisphosphate utilized by fructose 1,6-bisphosphatase

Rapid quench kinetic experiments on fructose 1,6-bisphosphatase demonstrate a stereospecificity for the alpha anomer of fructose 1,6-bisphosphate relative to the beta configuration. The beta anomer is only utilized after mutarotation to the alpha form in a process that is not enzyme catalyzed. Studies employing analogues of the acyclic keto configuration indicate that the keto form is utilized at a rate less than 5% that of the alpha anomer, a finding also confirmed by computer simulation of the rapid quench data. Chemical trapping experiments of the keto analogue, xylulose 1,5-bisphosphate, and the normal substrate suggest that interconversion of the acyclic and anomeric configurations is retarded by their binding to the enzyme. A hypothesis is advanced attributing substrate inhibition of fructose 1,6-bisphosphatase to possible binding of the keto species.