This enzyme participates in both the breakdown and synthesis of glucose. alpha-D-glucose 1-phosphate = alpha-D-glucose 6-phosphate Binds 1 Mg(2+) ion per subunit. Optimum pH is 7.4-7.6. Pgm-A is more thermostable than Pgm-B. Localized primarily to fat bodies in third instar larvae. The polymorphisms show clinal variations. There are 2 common electrophoretic variants, Pgm-A and Pgm-B, which differ in their kinetic and stability parameters. Variations in Pgm are associated with differences in enzyme activity and glycogen content. Belongs to the phosphohexose mutase family.