Major phosphoglucomutase isozyme that catalyzes the reversible interconversion of glucose 1-phosphate and glucose 6-phosphate (PubMed:5784209). Constitutes about 80-90% of the phosphoglucomutase activity in the cell (PubMed:14264884, PubMed:5231755). Key enzyme in hexose metabolism. The forward reaction is an essential step in the energy metabolism of galactose since the product of the galactose pathway enzymes in yeast is glucose 1-phosphate. The reverse reaction is an essential step for biosynthesis when carbon sources other than galactose are the energy source because glucose 1-phosphate is the starting point for the synthesis of UDP-glucose, which acts as a precursor for the synthesis of oligosaccharides and trehalose (PubMed:14264884). alpha-D-glucose 1-phosphate = alpha-D-glucose 6-phosphate Binds 1 magnesium ion per subunit. Can also use Zn(2+) as cofactor. Monomer. Induced in response to galactose and severely repressed in response to glucose. O-glycosylated with mannose residues (By similarity). Substrate of UDP-glucose--glycoprotein glucose phosphotransferase, linking glucose in a phosphodiester linkage to O-linked mannose (PubMed:8385141, PubMed:7929458). Blocks galactose utilization, but does not impair growth on glucose. Present with 3790 molecules/cell in log phase SD medium. Belongs to the phosphohexose mutase family.