Estimation of the turnover number of bovine heart FoF1 complexes for ATP synthesis

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Abstract
In mitochondria and submitochondrial particles (SMP), the rate of ATP synthesis is restricted by the rate of energy production by the respiratory chain. Fractional inactivation of the ATP synthase complex (F0F1) of bovine heart SMP by covalent modifiers increased the rate of ATP synthesis per mole of active F0F1. Thus, by use of SMP containing fractionally inactivated F0F1 complexes, a synthetic rate of 420 mol of ATP (mol of F0F1.cntdot.s)-1 was measured, which extrapolated to a Vmax of 440 s-1. At this extrapolated point, the turnover rate of F0F1 complexes was independent of the rate of energy production by the respiratory chain. These results have been discussed in relation to the effect of fractional inactivation of the F0F1 complexes of SMP on the steady-state free energy of the system. The above rate of ATP synthesis is comparable to the rate of ATP hydrolysis by SMP (400-520 s-1) in the absence of energy coupling constraints and control by the ATPase inhibitor protein. More interestingly, this rate is also comparable to the rate of ATP synthesis by chloroplast F0F1 under high light intensity (.apprx.420 s-1). Under the conditions specified, bovine heart SMP and chloroplast show similar apparent Km values for ADP. Thus, it appears that the mammalian and chloroplast ATP synthase complexes are similar not only in structure but also in catalytic efficiency for ATP synthesis.

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