Mannose 6-Phosphate: Anomeric Form Used by Phosphomannose Isomerase and Its l-Epimerization by Phosphoglucose Isomerase

Phosphomannose isomerase is highly specific for the β anomer of mannose 6-phosphate. The α anomer has no activity as a substrate and is, at best, a poor inhibitor. This specificity agrees with previous predictions based on stereo-chemical arguments. In addition, phosphomannose isomerase differs from phosphoglucose isomerase, which can also catalyze the function of anomerization of its substrates. Using the specificity of phosphomannose isomerase as a test system, it is shown that yeast phosphoglucose isomerase also catalyzes the conversion of α-mannose 6-P to β-mannose 6-P.