Subunit Interactions in Enzyme Catalysis

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Abstract
By using the principles of structural kinetics previously formulated, the effects of subunit interactions and conformation changes on transient rate of catalysis have been investigated for one‐substrate dimeric enzymes. The analytical solutions of transient rate equations and the expression of the induction time, τ have been obtained for the following models of protomer interactions: simple sequential, partially‐concerted, fully‐concerted, and exclusive allosteric.The progress curve of the reaction during the pre steady‐state phase, as well as the curve expressing the variation of τ−1 against substrate concentration, [S], have been simulated by computer analysis. When the protomers do not exhibit any interaction in the simple sequential mechanism, the plot τ−1versus [S] is a straight line and the induction time is always positive. If subunit interaction occurs but does not change during substrate binding, the plot of τ−1 against [S] is still linear but the length of the induction time depends on the value of the interaction coefficient. If this value is much lower than one, the enzyme, although Michaelian, behaves as “hysteretic” when rapidly mixed with its substrate. In the same way, the exclusive allosteric model also predicts positive z values, but the plot of τ−1 against [S] is a curve reaching a plateau for high concentrations of the substrate. The sequential models (simple, partially‐concerted, fully‐concerted) can give rise to either positive or negative induction time. Moreover, the asymptote of the curve τ−1 against [S] is oblique. It is thus possible, at least in theory, from pre‐steady‐state kinetics alone, to rule out either the exclusive allosteric or the sequential models.The above theoretical developments have been applied to the early kinetics of l‐phenylalanine ammonia‐lyase reaction. The induction time, that can be calculated from the progress curve, is negative for a large range of substrate concentrations. This result allows one to rule out, for that enzyme, the allosteric transition as a possible explanation of cooperativity. Sequential binding of substrate and subunit interactions can thus generate a burst, that is a transient activation of the reaction in the absence of any activator. It is tempting to assume this transient activation to be of regulatory significance.

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