Domestication of the cardiac mitochondrion for energy conversion.

The control of mitochondria energy conversion by cytosolic processes is reviewed. The nature of the cytosolic and mitochondrial potential energy homeostasis over wide ranges of energy utilization is reviewed and the consequences of this homeostasis in the control network are discussed. An analysis of the major candidate cytosolic signaling molecules ADP, Pi and Ca(2+) are reviewed based on the magnitude and source of the cytosolic concentration changes as well as the potential targets of action within the mitochondrial energy conversion system. Based on this analysis, Ca(2+) is the best candidate as a cytosolic signaling molecule for this process based on its ability to act as both a feedforward and feedback indicator of ATP hydrolysis and numerous targets within the matrix to provide a balanced activation of ATP production. These targets include numerous dehydrogenases and the F1-F0-ATPase. Pi is also a good candidate since it is an early signal of a mismatch between cytosolic ATP production and ATP synthesis in the presence of creatine kinase and has multiple targets within oxidative phosphorylation including NADH generation, electron flux in the cytochrome chain and a substrate for the F1-F0-ATPase. The mechanism of the coordinated activation of oxidative phosphorylation by these signaling molecules is discussed in light of the recent discoveries of extensive protein phosphorylation sites and other post-translational modifications. From this review it is clear that the control network associated with the maintenance of the cytosolic potential energy homeostasis is extremely complex with multiple pathways orchestrated to balance the sinks and sources in this system. New tools are needed to image and monitor metabolites within sub-cellular compartments to resolve many of these issues as well as the functional characterization of the numerous matrix post-translational events being discovered along with the enzymatic processes generating and removing these protein modifications.