In addition to

cycling at the inner mitochondrial membrane, other types of cycles occur in metabolism. Especially in skeletal muscle cells, the phosphofructokinase (PFK) reaction in conjunction with the fructose-1,6-biphosphatase (FBPase) operating anti-parallel represent a substrate cycle, which may control the pathway of glycolysis (GLY) more Inhibitors,research,lifescience,medical sensitively than would be possible by PFK alone. In this cycle, fructose-1,6-biphosphate (FBP), which is produced by ATP-coupled formation from fructose-6-phosphate (F6P), is cycled back via FBPase to F6P. However, usually both fluxes are not equal. Also to demonstrate the opposite equality of partial conductances Inhibitors,research,lifescience,medical for this kind of cycle, only equal fluxes can be used for this purpose. As a further example, the phosphocreatine shuttle will be considered. The creatine kinase (CK) reaction can also be regarded as a coupled reaction. Here, ATP splitting powers phosphocreatine (PCr) formation from creatine (Cr), which may proceed near equilibrium. As described in detail in reference [1], ATP is shuttled between locations of ATP formation (for

instance in the inter-membrane space in mitochondria) and locations of high ATP demand like Inhibitors,research,lifescience,medical myofibrils. By analogy to an electric Neratinib concentration circuit built by two in series batteries with an outer Inhibitors,research,lifescience,medical circuit

conductance (see Appendix (A4)), the output affinity of PCr formation in the inter-membrane space of mitochondria corresponds to A1I with associated LIc1, whereas the affinity of the reverse reaction in myofibrils corresponds to A1II(with LIIc2). To ensure diffusional flow of PCr and Cr between both locations, an additional driving force (corresponding to Ue; see (A4)) with associated conductance must be present. Under such conditions partial conductances do not match. Only when the additional Inhibitors,research,lifescience,medical conductance corresponding to the diffusional process (Le) is added to LIc1 does this sum become opposite and equal to LIIc2, as is shown in (A4). Le depends Rolziracetam greatly on structural features. So, to achieve a high diffusional conductance, diffusional paths must be as short as possible, which in turn requires a high grade of structural organization [26,27,28]. It seems worth mentioning that coupled systems like pump and leak cycles are often not in a steady state. For instance, steady state cycling through sarco/endoplasmatic reticulum Ca2+ ATPases (SERCA) and Ca2+ release channels of the sarcoplasmatic reticulum (SR) breaks off during activation of contraction. There is an enormous Ca2+ efflux through release channels; meanwhile the pumping rate of SERCAs may be low.