phosphoglycerate kinase

Meaning: Phosphoglycerate kinase (PGK) transfers the high-energy phosphoryl group of 1,3-bisphosphoglycerate to ADP. This is substrate-level phosphorylation: ATP forms by direct transfer from a substrate, not through the electron transport chain. The reaction runs near equilibrium, so it is reversible. In gluconeogenesis it runs backward and uses ATP. Each glucose gives two trioses, so PGK makes 2 ATP, which repays the 2 ATP spent earlier by hexokinase and PFK-1.

Equation: 1,3-bisphosphoglycerate + ADP ⇌ 3-phosphoglycerate + ATP. Each species is an amount in mol (concentration in mol/L).

On the MCAT: Expect questions that count ATP per glucose. PGK and pyruvate kinase each give 2 ATP, so glycolysis nets 2 ATP. You may be asked to name the substrate-level phosphorylation steps. A passage on red blood cells may describe the shunt that converts 1,3-bisphosphoglycerate to 2,3-BPG. That shunt bypasses PGK, so the cell loses the ATP from that step. The 2,3-BPG lowers hemoglobin's O₂ affinity (right shift).

Common confusion: Students expect a kinase to use ATP, and many kinases do. PGK is named for the reverse reaction, and in glycolysis ATP is a product. The direction follows substrate and product concentrations. PGK is not a main regulatory step. PFK-1 is the main control point.