Category: biosynthesis
Creatine + phosphocreatine (PCr) form the high-energy phosphate buffer for cells with rapid ATP turnover (skeletal muscle, brain). Creatine kinase reversibly transfers a phosphate between ATP/ADP and PCr/Cr — under high-energy demand PCr → ATP within milliseconds (faster than glycolysis/oxidative phosphorylation can ramp). Endogenous creatine synthesis: glycine + arginine → guanidinoacetate (AGAT in kidney) → creatine (GAMT in liver, methyl group from SAM — cross-link: methionine_sam_cycle). Daily ~1-2 g endogenous + dietary turnover. Supplemental creatine monohydrate ~3-5 g/d saturates muscle PCr stores in ~30 days (slow-load) or 5-7 days (loading 20 g/d × 5d then maintenance). Documented benefits: anaerobic performance, muscle mass, possibly cognitive function under stress. Phosphocreatine system limits acidosis-driven fatigue at sprint-duration efforts. Cross-link: carnitine + carnitine_shuttle complements creatine for energy systems.