Creatine phosphate phosphoryl-transfer potential

This reaction is endergonic (AG = +12 kcal/mol) under standard conditions but, because ATP levels are very high within mitochondria, and creatine phosphate levels are relatively low, the reaction is exergonic as written and proceeds to the right. Thus, the cellular concentration of metabolites has a drastic effect on the free energy for a reaction, enabling ATP to create phosphorylated compounds with higher phosphoryl transfer potential than itself under standard conditions. 

Creatine (N-methylguanidoacetic acid) and its phosphorylated form creatine phosphate (a guanidophosphate) serve as an ATP buffer in muscle metabolism. In creatine phosphate, the phosphate residue is at a similarly high chemical potential as in ATP and is therefore easily transferred to ADP. Conversely, when there is an excess of ATP, creatine phosphate can arise from ATP and creatine. Both processes are catalyzed by creatine kinase [5]. 

Creatine phosphate in vertebrate muscle serves as a reservoir of high-potential phosphoryl groups that can be readily transferred to ATP. Indeed, we use creatine phosphate to regenerate ATP from ADP every time we exercise strenuously. 

As its high phosphate transfer potential suggests (see Figure 3.7), this compound is capable of phosphorylating ADP very efficiently. The reaction is catalyzed by the enzyme creatine kinase as follows ... 

Creatine- and arginine phosphate (7 and 9) play important roles in the storage of phosphorylating potential in vertebrates and invertebrates, respectively[25, 26f In living cells, these N-phosphoguanidine derivatives are formed by phosphoryl group transfer from ATP, and in the reverse reaction ADP is the only acceptor for 7 and 9. 

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