Minor phosphoglucomutase isozyme that catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate (PubMed:5784209). Constitutes about 10-20% 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 Mg(2+) ion per subunit. Monomer. Present with 9820 molecules/cell in log phase SD medium. Belongs to the phosphohexose mutase family.