Transient-phase kinetics of alpha-chymotrypsin and other enzyme systems

Time-dependent kinetic complexities in cholinesterase-catalyzed reactions

Cholinesterases (ChEs) display a hysteretic behavior with certain substrates and inhibitors. Kinetic cooperativity in hysteresis of ChE-catalyzed reactions is characterized by a lag or burst phase in the approach to steady state. With some substrates damped oscillations are shown to superimpose on hysteretic lags. These time dependent peculiarities are observed for both butyrylcholinesterase and acetylcholinesterase from different sources. Hysteresis in ChE-catalyzed reactions can be interpreted in terms of slow transitions between two enzyme conformers E and E'. Substrate can bind to E and/or E', both Michaelian complexes ES and Ε'S can be catalytically competent, or only one of them can make products. The formal reaction pathway depends on both the chemical structure of the substrate and the type of enzyme. In particular, damped oscillations develop when substrate exists in different, slowly interconvertible, conformational, and/or micellar forms, of which only the minor form is capable of binding and reacting with the enzyme. Biphasic pseudo-first-order progressive inhibition of ChEs by certain carbamates and organophosphates also fits with a slow equilibrium between two reactive enzyme forms. Hysteresis can be modulated by medium parameters (pH, chaotropic and kosmotropic salts, organic solvents, temperature, osmotic pressure, and hydrostatic pressure). These studies showed that water structure plays a role in hysteretic behavior of ChEs. Attempts to provide a molecular mechanism for ChE hysteresis from mutagenesis studies or crystallographic studies failed so far. In fact, several lines of evidence suggest that hysteresis is controlled by the conformation of His438, a key residue in the catalytic triad of cholinesterases. Induction time may depend on the probability of His438 to adopt the operative conformation in the catalytic triad. The functional significance of ChE hysteresis is puzzling. However, the accepted view that proteins are in equilibrium between preexisting functional and non-functional conformers, and that binding of a ligand to the functional form shifts equilibrium towards the functional conformation, suggests that slow equilibrium between two conformational states of these enzymes may have a regulatory function in damping out the response to certain ligands and irreversible inhibitors. This is particularly true for immobilized (membrane bound) enzymes where the local substrate and/or inhibitor concentrations depend on influx in crowded organellar systems, e.g. cholinergic synaptic clefts. Therefore, physiological or toxicological relevance of the hysteretic behavior and damped oscillations in ChE-catalyzed reactions and inhibition cannot be ruled out.

Damped oscillatory hysteretic behaviour of butyrylcholinesterase with benzoylcholine as substrate

Steady-state kinetics for the hydrolysis of benzoylcholine (BzCh) and benzoylthiocholine (BzSCh) by wild-type human butyrylcholinesterase (BuChE) and by the peripheral anionic site mutant D70G were compared. kcat/Km for the hydrolysis of BzSCh was 17-fold and 32-fold lower than that for hydrolysis of BzCh by wild-type and D70G, respectively. The rate-limiting step for hydrolysis of BzCh was deacylation, whereas acylation was rate-limiting for hydrolysis of BzSCh. Wild-type enzyme and the D70G mutant were found to reach steady-state velocity slowly with BzCh as the substrate. At pH 6, the approach to steady-state for both enzymes consisted of a mono-exponential acceleration upon which a set of damped oscillations was superimposed. From pH 7 to 8.5, the approach to steady-state consisted of a simple exponential acceleration. The damped oscillations were analyzed by both a numerical approximation and simulation based on a theoretical model. BuChE-catalyzed hydrolysis of the thiocholine analogue of BzCh showed neither lags nor oscillations, under the same conditions. The frequency and amplitude of the damped oscillations decreased as the BzCh concentration increased. The apparent induction time for the exponential portion of the lag was calculated from the envelope of the damped oscillations or from the smooth lag. Wild-type BuChE showed a hyperbolic increase in induction time as the BzCh concentration increased (tau max = 210 s at pH 6.0). However, the induction time for D70G was constant over the whole range of BzCh concentrations (tau max = 60 s at pH 6.0). Thus, the induction time does not conform to a simple hysteretic model in which there is a slow conformational transition of the enzyme from an inactive form E to an active form E'. No pH-dependence of the induction time was found between pH 6.0 and 8.5 in sodium phosphate buffers of various concentrations (from 1 mm to 1 m). However, increasing the pH tended to abolish the oscillations (increase the damping factor). This effect was more pronounced for D70G than for wild-type. Although the lyotropic properties of phosphate change from chaotropic at pH 6.0 to kosmotropic at pH > 8.0, no effect of phosphate concentration on the oscillations was noticed at the different pH values, suggesting that the oscillations are not related to a pH-dependent Hofmeister effect of phosphate ions. Simulation and theoretical analysis of the oscillatory behaviour of the approach to the steady-state for BuChE led us to propose a model for the hysteresis of BuChE with BzCh. In this model, the substrate-free enzyme is present as an equilibrium mixture of two forms, E and E'. Substrate binds to E and E', but only Epsilon'S makes products. It is proposed that oscillations originate from a time-dependent change in the local concentration, solvation and/or conformation of substrate in the bulk solution. 1H-NMR measurements provided evidence for a slow equilibrium between two BzCh conformers. Binding of the conformationally preferred substrate conformer leads to products.

Kinetics of a model for zymogen activation: the case of high activating enzyme concentrations

Transient phase and steady state equations have been derived for the following enzyme activation mechanism: (formula; see text) in which the concentrations of activating enzyme, E, and substrate, A, are greatly in excess of that of zymogen, Ei. EEi, EEa, EaA and EaY are four intermediates; W is a peptide related from Ei during EEa formation and X and Y are the products of Ea reaction on A. From the general equations, approximate solutions under certain simplifying conditions have been derived. Finally, some formal particular cases of the above mechanism are considered.