Phosphocreatine represents a thermodynamic and functional improvement over other muscle phosphagens.

In vertebrate tissues, the only phosphagen is phosphocreatine (PC), and the corresponding phosphotransferase is creatine phosphokinase (CPK). Among invertebrates, a variety of phosphotransferase reactions are found in addition to CPK, including arginine phosphokinase (APK), glycocyamine phosphokinase (GPK), taurocyamine phosphokinase (TPK) and lombricine phosphokinase (LPK). Although there is some uncertainty about the exact value, the apparent equilibrium constant for the CPK reaction (K'cpk = [creatine][ATP]/[PC][ADP]), under physiological conditions similar to those of vertebrate muscle, ranges from 100 to 160. The corresponding K' value for the APK reaction is somewhat controversial, and K' values for the GPK. TPK and LPK reactions are not known. In this study, conventional and 31P-NMR methods were used to evaluate the equilibrium constants for the APK, GPK, TPK and LPK reactions relative to that of CPK. The corresponding K' values for the APK, GPK, TPK and LPK reactions, expressed as a percentage of K'cpk, are 13, 29, 29 and 32%, respectively. The exclusively invertebrate phosphagens exist as a cohort of thermodynamically more stable compounds. Thus, PC constitutes a thermodynamic (and functional) improvement, in that the CPK reaction is able to buffer ATP at much higher ATP/ADP ratios than are other phosphagens. However, possession of a phosphagen system with a lower K' value may be advantageous under certain specific physiological conditions such as intracellular acidosis.