Catalyzes the interconversion of 2-phosphoglycerate (2-PGA) and 3-phosphoglycerate (3-PGA). (2R)-2-phosphoglycerate = (2R)-3-phosphoglycerate Binds 2 manganese ions per subunit. Insensitive to vanadate. kcat is 22 sec(-1) for mutase with 3-PGA as substrate (at pH 7 and 30 degrees Celsius). kcat is 10 sec(-1) for mutase with 2-PGA as substrate (at pH 7 and 30 degrees Celsius). Carbohydrate degradation; glycolysis; pyruvate from D-glyceraldehyde 3-phosphate: step 3/5. Monomer. Expressed most strongly in early exponential growth, with levels falling off as cells reached late log phase and stationary phase. Inhibition by vanadate is a diagnostic test for discrimination between the cofactor-dependent (GpmA) and -independent (GpmI) phosphoglycerate mutases. Belongs to the BPG-independent phosphoglycerate mutase family.