Phosphocreatine/creatine shuttle

Energy transport in the cytosol the creatine/phosphocreatine shuttle... 

Figure 9.20 The creatine/phosphocreatine shuttle between subsarcolemmal mitochondria and myosin ATPase in muscle. The distance between the mitochondria that reside just below the plasma membrane (sarcolemma) and the myofibrils in which the myosin ATPase results in contraction, is long in such muscles. The advantage of the position of these mitochondria is ready access to oxygen and fuel from blood. Such mitochondria are common in endurance athletes.

Figure 9.21 The creatine/phosphocreatine shuttle in spermatozoa. This shuttle may not be present in all sperm it will depend upon the distance between the mitochondria and the flagellum. Mitochondria are present in the midpiece just below the head. ATP is required for movement of the flagellum which enables the sperm to swim. Dynein ATPase is the specific motor ATPase, similar to myosin ATPase, that transfers energy from ATP to the flagellum. A deficiency of creatine may explain low sperm motility in some infertile men. CK - creatine kinase. Deficiences of enzymes in the pathway for synthesis of creatine are known to occur (see Appendix 8.3).

The midpiece contains the mitochondria which are wrapped around the proximal part of the flagellum. The beating of the flagellum, and hence the swimming of the sperm involves the motor protein known as dynein, which requires ATP hydrolysis. In some species, the diffusion of energy in the spermatozoa is increased by the presence of the creatine/phosphocreatine shuttle (Chapter 9) that is, phosphocreatine and creatine diffuse throughout the cytosol... 

Tombes, R.M. Shapiro, B.M. Enzyme termini of a phosphocreatine shuttle. Purification and characterization of two creatine kinase isozymes from sea urchin sperm. J. Biol. Chem., 262, 16011-16019 (1987)... 

Phosphocreatine shuttle. A myokinase (adenylate kinase) cascade between oxidative phosphorylation and creatine kinase (CK) has been postulated. A similar myofibrillary cascade may exist at the myofibrillar ATPase site. 

In addition to the processes described above, there still remains one further process which, at least in some cells or tissues, is required prior to the utilisation of ATP in the cytosol that is, the transport of energy within the cytosol, via a shuttle. The transport of ATP out and ADP into the mitochondrion, via the translocase, results in a high ATP/ ADP concentration ratio in the cytosol. However, a high ratio means that the actual concentration of ADP in the cytosol is low, which could result in slow diffusion of ADP from a site of ATP utilisation back to the inner mitochondrial membrane. If sufficiently slow, it could limit the rate of ATP generation. To overcome this, a process exists that transports energy within the cytosol, not by diffusion of ATP and ADP, but by the diffusion of phosphocreatine and creatine, a process known as the phosphocreatine/creatine shuttle. The reactions involved in the shuttle in muscle help to explain the significance of the process. They are ... 

The concentration of phosphocreatine is usually greater than that of ATP and that of creatine greater than that of ADP so that these metabolites diffuse more rapidly. This is because diffusion depends upon the concentrations of participants and the concentration gradient the larger the gradient, the greater is the rate of diffusion. Consequently, the shuttle is important in cells where the distance between the sites of ATP utilisation and the mitochondria is large... 

Bessman, S. and F. Savabi, The role of phosphocreatine energy shuttle in exercise and muscle hypertrophy, in Creatine and Creatine Phosphate Scientific and Clinical Perspectives, M.A. Conway and J.F. Clark, Eds. Academic Press, San Diego, CA,... 

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