Mechanism
MPI must convert an aldose (mannose) to a ketose (fructose), in addition to opening and closing the rings for these sugars. In humans a mechanism has been suggested which involves a hydrogen transfer between C1 and C2, mediated by Tyr278, and the movement of a proton from O1 and O2 mediated by the associated Zn2+ ion. The ring opening step may be catalyzed by His99 and Asp270, and isomerization is likely a cis-enediol mechanism. A detailed description of the mechanism of phosphomannose isomerase may be found in the 1968 Journal of Biological Chemistry paper written by Robert W. Gracy and Ernst A Noltmann titled "Studies on Phosphomannose Isomerase: A Mechanism for Catalysis and for the Role of Zinc in the Enzymatic and the Nonenzymatic Isomerization."
PMI shows a high degree of selectivity for the beta anomer of M6P, and the alpha anomer has no activity, and may in fact act as an inhibitor. Phosphoglucose Isomerase (PGI) has a very similar function to PMI, (as it catalyzes the interconversion of Glucose 6-Phosphate and F6P) however PGI can anomerize alpha and beta G6P, and may also catalyze the conversion of alpha M6P to beta M6P, while PMI may not anomerize M6P. It is likely the cis-enediol intermediate formed by PMI is the same intermediate formed by PGI.
Read more about this topic: Mannose Phosphate Isomerase
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