Why do many Michaelian enzymes exhibit an equilibrium constant close to unity for the interconversion of enzyme‐bound substrate and product?

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Abstract
1. The idea is advanced that an evolutionary pressure in the direction of increased metabolic fluxes should lead to the selection at any evolutionary stage of enzymes which are optimally efficient considering the total evolutionary effort which has been spent on their improvement as catalysts. Relationships are derived which may be used for determination of the optimal state of operation of a Michaelian enzyme at a given total evolutionary effort as measured, for instance, by the mean magnitude of variable rate constants in the reaction mechanism. These relationships are applied to characterize the dependence on the total evolutionary effort of the optimal value of the equilibrium constant (Kint) for the conversion of enzyme-bound substrate into enzyme-bound product. 2. The results show that the optimal value of Kint reflects primarily the magnitude of the overall equilibrium constant for the catalysed reaction, whether or not the enzyme operates close to equilibrium. When the total evolutionary effort tends towards infinitely high values, Kint approaches unity, regardless of the magnitude of the overall equilibrium constant and of the concentrations of substrate and product. Concomitantly, the enzyme approaches its ultimate state of catalytic perfection where all variable rate constants in the mechanism tend towards infinitely high values. 3. It is concluded from previously reported kinetic data that triosephosphate isomerase has been subjected to such a low total evolutionary effort that the enzyme cannot possibly have reached (or even significantly approached) the ultimate state of perfection where the Kint value necessarily becomes close to unity. The reason why this enzyme exhibits a Kint value relatively close to unity is that it catalyses a reaction with an equilibrium constant relatively close to unity. Evidence is presented to show that similar considerations may apply for other enzymes operating by the examined reaction mechanism. 4. Analytical data are reported which indicate that Kint ultimately approaches unity because forward and reverse rate constants for the interconversion of enzyme-bound substrate and product are equivalent with regard to the evolutionary effort required to increase their magnitude.