Control of kinetics by cooperative interactions

Abstract

SummaryCooperative effects in ligand binding and dissociation kinetics are much less investigated than steady state kinetics or equilibrium binding. Nevertheless, cooperativity in ligand binding leads necessarily to characteristic properties with respect to kinetic properties of the system. In case of positive cooperativity as found in oxygen binding proteins, a typical property is an autocatalytic ligand dissociation behavior leading to a time dependent, apparent ligand dissociation rate. To follow systematically the influence of the various potentially involved parameters on this characteristic property, simulations based on the simple MWC model were performed which should be relevant for all types of models based on the concept of an allosteric unit. In cases where the initial conformational distribution is very much dominated by the R-state, the intrinsic kinetic properties of the T-state are of minor influence for the observed ligand dissociation rate. Even for fast conformational transition rates, the R-state properties together with the size of the allosteric unit and the allosteric equilibrium constant define the shape of the curve. In such a case, a simplified model of the MWCscheme (the irreversible n-chain model) is a good approximation of the full scheme. However, if in the starting conformational distribution some liganded T-molecules are present (a few percent is enough), the average off-rates can be significantly altered. Thus, the assignment of the initial rates to R-state properties has to be done with great care. However, if the Rstate strongly dominates initially it is even possible to get an estimation of the lower limit for the number of interacting subunits from kinetic data: similar to the Hill-coefficient for equilibrium conditions, a measure for ''kinetic cooperativity'' can be derived by comparing initial and final ligand dissociation rates. RT and l i TR , rate of conformational transition from R to T and T to R at ligation level i; MWCmodel, Monod-Wyman-Changeux-model.

Entrapment and characterization of functional allosteric conformers of hemocyanin in sol–gel matrices

“…In contrast, at pH 7.8 (and even more so at pH 8.3 due to the Bohr-effect) under oxygenated conditions hemocyanin is present mainly in conformation rT, and addition of L-lactate shis the distribution further into this direction. It has been shown before that concomitantly occurring conformational changes can alter the apparent stoichiometry of binding in an ITC experiment, 45 which could be the reason for the better t of a model with two different binding sites in case of pH 6.5. 22 Based on the same considerations the results of ash-photolysis experiments at pH 6.5 and pH 7.8 22 can be rationalized: at pH 6.5 the rate of oxygen-rebinding increases from 70 s À1 to about 82 s À1 upon addition of L-lactate, reecting a transition from mainly tR to rR.…”