Thermodynamic limits for mechanisms

Any considered mechanism must, first of all, be consistent with the thermodynamic constraints of the system. Such limits are set by the span of oxidoreduction potentials in the respiratory chain and by the protonmotive force that opposes the proton movement. The relative magnitudes of these two forces set an absolute upper limit for H /e stoicheiometry of proton translocation. The stoicheiometry in turn, puts limits on the underlying mechanisms. Analogous limits for the H /ATP stoicheiometry of ATP synthesis are obtained from the relative magnitudes of phosphorylation potential and pmf. An elementary thermodynamic analysis of the system can therefore be helpful in defining the degree of freedom in discussions of chemical mechanisms (see Ref. 8). 

Mitochondrial respiration has certain characteristic states, defined originally by the classical work of Chance and Williams [23]. States 3 and 4 are of particular relevance here. In State 4 the rate of respiration is minimal ( resting state ) due to a maximal back-pressure from the pmf (or from the phosphorylation potential via the pmf). This is a state termed static head in the theory of thermodynamics of irreversible processes (see, e.g., Refs. 24, 25). 

State 3 ( active , phosphorylating state) is characterised by high respiratory activity due to a lowered phosphorylation potential (addition of ADP). However, respiration is usually not at its maximum, and the measured pmf is only little 

The maximum pmf generated in State 4 has been measured by various groups using several techniques (see Refs. 8,26,27). The results vary between about 160 and 240 mV. However, from the observed shifts of the redox equilibria in the respiratory chain imposed by the pmf (or by ATP via the pmf), it can be concluded that the functionally relevant pmf must be at least 200 mV in State 4 of well-coupled mitochondria [8]. (This need not be equivalent with the pmf between the bulk aqueous C and M phases). This limits the H /e ratio in State 4 to maximal values of 5.6 and 3.8 for the spans from NADH and ubiquinol to oxygen, respectively [8]. These are indeed upper limit stoicheiometries, attainable, of course, only at thermodynamic equilibrium between respiration and proton translocation. The true values must therefore be lower, particularly as the terminal step in the chain, i.e., oxidation of cytochrome c by Oj is irreversible (see Refs. 28, 29 and below). 

The above estimates of the minimum pmf in State 4 are important also insofar as they are independent of the present uncertainty of whether protonic coupling is localised [15,19-22,35-38] with direct protonic circuits between respiratory complexes and ATP-synthase, or entirely delocalised between the bulk aqueous compartments on each side of the membrane. Clearly, any reUable measurement of a bulk phase pmf in State 4 that yields values lower than 200 mV is indicative of localised coupling (as may be the case above). 

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