phosphoglycerate mutase

Meaning: Phosphoglycerate mutase catalyzes step 8 of glycolysis: 3-phosphoglycerate ⇌ 2-phosphoglycerate. A mutase is an isomerase that moves a phosphate group from one position to another within the same molecule. In the usual mechanism, a phosphorylated histidine on the enzyme passes its phosphate to carbon 2, forming 2,3-bisphosphoglycerate, and the enzyme then takes back the phosphate from carbon 3. The reaction is reversible, so the enzyme also works in gluconeogenesis.

On the MCAT: Expect questions that place this step in the pathway, classify the enzyme as an isomerase (not a kinase or lyase), or ask what happens if it is blocked. Blocking it makes 3-phosphoglycerate build up and lowers production of phosphoenolpyruvate (PEP) and pyruvate, so less ATP is made downstream. No ATP or NADH is made or used in this step.

Common confusion: Moving a phosphate does not mean ATP is involved. The phosphate stays on the same molecule, so this is not a kinase reaction. The energy payoff comes next: enolase dehydrates 2-phosphoglycerate to PEP, and pyruvate kinase then transfers PEP's phosphate to ADP. Do not mix it up with phosphoglucomutase, which interconverts glucose 1-phosphate and glucose 6-phosphate.