Regulation of oxidative phosphorylation in the mammalian cell

The cell is capable of maintaining a steady-state flux of energy from mitochondrial oxidative phosphorylation, producing ATP, to the cytosolic adenosinetriphosphatases (ATPases), performing work. Considerable effort has been devoted to investigating the individual mechanisms involved in these two processes. However, less effort has been directed toward learning how these reactions of energy metabolism interact through the cytosol to maintain the observed steady state in the intact cell. The "classical" model for the cytosolic interaction of these two processes involves the feedback of ATP hydrolysis products, ADP and Pi, from the ATPases to oxidative phosphorylation. This model is based on data from isolated mitochondria in which the rate of oxidative phosphorylation is controlled by the concentration of ADP and Pi. Yet, recent data from intact tissues with high oxidative phosphorylation capacities (i.e., heart, brain, and kidney) indicate that the cytosolic concentration of ADP and Pi do not change significantly with work. These data imply that this simple feedback model is not adequate to explain the regulation of energy metabolism in these tissues. Other sites within the oxidative phosphorylation process must be playing a regulatory role or the kinetics of ATP synthesis must be very different than currently believed to establish the steady state. This review covers the potential sites within oxidative phosphorylation which may be regulated through cytosolic transducers to result in the necessary feedback network regulating the steady-state flow of energy in the cell. These sites will include substrate delivery to the cytochrome chain, the processes involved in the phosphorylation of ADP to ATP, and the delivery of oxygen.