Phophocreatine (PCr), otherwise known as creatine phosphate, is a high energy compound used to synthesise ATP from ADP in muscle cells. It is used under anaerobic conditions to maintain the ATP concentration when there is a high energy demand in the cell. 
PCr is made by phosphorylation of a creatine molecule using ATP. The reaction is catalysed by creatine kinase to yield phosphocreatine and ADP. The production of PCr in this way is completely reversible. 
The breakdown of PCr into creatine and inorganic phosphate is an exergonic reaction. By again using the creatine kinase enzyme, the cells can use the energy and the inorganic phosphate from this reaction to synthesise ATP from ADP. By coupling these reactions, PCr is able to transfer its phosphate to ADP, thanks to its high phosphoryl transfer potential. Phosphocreatine is sometimes referred to as a phosphoryl buffer for this reason.
PCr is used primarily in muscle, but also in the brain and other cells or tissues which go through rapid ATP consumption. In fact 98% of the creatine in the human body is found in muscle. PCr is critical to the function of muscle particularly when undergoing maximal effort exertion. The normal amount of ATP in the muscle cells lasts only for a second, but PCr allows maximum effort to be made for 6-7 seconds or even longer, depending on individual conditioning.
In neutral pH, the standard free energy of phosphocreatine hydrolysis equals -43.1 kJmol-1. For ATP hydrolysis the free energy is -30.5 kJmol-1. So the overal change in standard free energy is -12.6 kJmol-1. Therefore the transfer of phosphate from PCr is more spontaneous than that of phosphate from ATP.
Muscle cells at rest contain the metabolites essential for PCr synthesis in the following concentrations. ATP= 4mM, ADP= 0.013mM, PCr =25mM and Creatine = 13mM.
- ↑ Berg, J.M., Tymoczko, J. L. and Stryker, L. (2012) Biochemistry, 7th Edition, United States of America: W. H. Freeman and Company, pp. 450.
- ↑ MacAuley, D. (2013) Oxford Handbook of: Sport and Exercise Medicine, 2nd Edition, China: Oxford University Press, pp.126.
- ↑ Marcovitch, H. (2007) Student Medical Dictionary, 2nd Edition, London: A & C Black, pp. 169.
- ↑ Berg, J. M., Tymoczko, J. L. and Stryker, L. (2012) Biochemistry, 7th Edition, United States of America: W. H. Freeman and Company, pp. 244.