The structural basis of the secondary function of PMM1 in the presence of IMP

Abstract

The human phosphomannomutases PMM1 and PMM2 catalyze the interconversion of hexose 6‐phosphates and hexose 1‐phosphates. The two isoforms share 66% sequence identity and similar kinetic properties but appear to have different functional roles. While PMM2 is expressed in all cell types and provides the mannose 1‐phosphate essential for protein glycosylation, PMM1, is expressed mainly in the brain and the presence of inosine monophosphate (IMP), activates a second function—the hydrolysis of glucose 1,6‐bisphosphate (G16BP) to glucose 6‐phosphate. Mechanistically, both enzymes require activation by G16BP to form the phosphoaspartate nucleophile. Our aim is to understand the mechanism of how IMP converts PMM1 from a mutase to a phosphatase. Herein, we present the structure to 1.93 Å resolution of PMM1 in complex with IMP. Together with the kinetics of binding (IMP Kactivation = 1.42 μM), this structure is consistent with the hypothesis that IMP inhibits mutase activity by competing with the substrate binding site at the cap domain necessary for the formation of the active site. Acquisition of phosphatase activity may be a result of IMP induced cap closure thus allowing water to enter the active site and accept the phosphoryl group from the phosphoaspartate nucleophile. Subsequent G16BP binding affords the phosphorylation of the nucleophile allowing PMM1 to effectively act as a G16BP phosphatase. NIH Grant# GM61099