This enzyme participates in both the breakdown and synthesis of glucose (Probable). Factor that affects seed oil content (PubMed:10759515). Accumulated starch in young embryos may play an important role in providing carbon resources for seed storage lipid biosynthesis in oilseed plants (Probable). alpha-D-glucose 1-phosphate = alpha-D-glucose 6-phosphate Binds 1 Mg(2+) ion per subunit. Expressed in flowers, siliques and germinating seeds. Reduced oil content in seeds. Belongs to the phosphohexose mutase family.