ATP utilization reaction

A quantitative description of oxidative phosphorylation within the cellular environment can be obtained on the basis of nonequilibrium thermodynamics. For this we consider the simple and purely phenomenological scheme depicted in Fig. 1. The input potential X0 applied to the converter is the redox potential of the respiratory substrates produced in intermediary metabolism. The input flow J0 conjugate to the input force X0 is the net rate of oxygen consumption. The input potential is converted into the output potential Xp which is the phosphate potential Xp = -[AG hoS -I- RT ln(ATP/ADP P,)]. The output flow Jp conjugate to the output force Xp is the net rate of ATP synthesis. The ATP produced by the converter is used to drive the ATP-utilizing reactions in the cell which are summarized by the load conductance L,. Since the net flows of ATP are large in comparison to the total adenine nucleotide pool to be turned over in the cell, the flow Jp is essentially conservative. 

The control of the respiration process and ATP synthesis shifts as the metabolic state of the mitochondria changes. In an isolated mitochondrion, control over the respiration process in state 4 is mainly due to the proton leak through the mitochondrial inner membrane. This type of control decreases from state 4 to state 3, while the control by the adenine nucleotide and the dicarboxylate carriers, cytochrome oxidase, increases. ATP utilizing reactions and transport activities also increase. Therefore, in state 3, most of the control is due to respiratory chain and substrate transport. 

Although it is conventional to represent the reaction of hexokinase, and of many other enzymes, using a unidirectional arrow, this convention is not scientifically accurate. The unidirectional arrow is used only to make it convenient to remember that the equilibrium lies far in the direction of product formation (glucose-6-P formation), In general, most ATP-utilizing reactions are driven in the direction of product formation, and arc therefore said not to be freely reversible. 

The mitochondrial inner membrane acts as a barrier to the free movement of ATP and ADP [see Klingenberg (1972) for a review]. Since ATP is formed on the inside of the inner mitochondrial membrane, whereas most of the ATP-utilizing reactions are in the cytosol, the mechanism for ATP translocation is of considerable importance. Klingenberg and his associates (1966, 1972) were the first to clearly describe the transport process and to point out the nature of the specific carrier... 

TABLE 2. Effect of limited GA modification on kinetic param eters for the ATP utilizing reactions. 

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