creatine phosphate

Meaning: Muscle cells hold only a small pool of ATP, enough for a few contractions. Creatine phosphate (phosphocreatine) is a reserve of phosphate groups. The enzyme creatine kinase moves the phosphate to ADP and forms ATP in a single step, with no oxygen and no long pathway. The transfer is favorable because creatine phosphate has a higher phosphoryl transfer potential than ATP. At rest, when oxidative phosphorylation supplies plenty of ATP, the reaction runs in reverse and rebuilds the store.

Equation: creatine phosphate + ADP ⇌ creatine + ATP, catalyzed by creatine kinase. Each species is a concentration in mol/L.

On the MCAT: Passages on muscle metabolism ask for the order of energy sources in exercise. First comes stored ATP, then creatine phosphate (the first several seconds of a sprint or heavy lift), then anaerobic glycolysis with lactate (up to a couple of minutes), then aerobic respiration for prolonged activity. Expect questions on which source is fastest, which needs no O₂, or what happens if creatine kinase is inhibited (ATP falls faster during intense contraction). Another common prompt asks what a resting muscle does with surplus ATP (it restores creatine phosphate).

Common confusion: Creatine phosphate does not power myosin directly. ATP does, and creatine phosphate only regenerates it. It is also not a long-term fuel, because the store is depleted in seconds. Do not confuse it with creatinine, the waste product excreted in urine.