Substrate Form of D-Fructose 1,6-Bisphosphate Utilized by Fructose 1,6-Bisphosphatase

Rapid quench kinetic experiments on fructose 1,6-bisphosphatase demonstrate a stereospecificity for the α anomer of fructose 1,6-bisphosphate relative to the β configuration. The β anomer is only utilized after mutarotation to the α 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 α 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.