Conversely, in the final step of the reaction, the bound serine oxygen forms a hydrogen bond with a protonated histidine, which allows for easier cleavage from the substrate. This allows a facilitated nucleophilic attack of the hydroxyl oxygen on the substrates carbonyl group. Initially, hydrogen bonding between the enzymes histidine and serine side chains weakens the bond of serine’s O-H.Because water is involved in the final, slowest step of the mechanism, deuterating the water would decrease the rate of the overall reaction from 5- to 30-fold.This means that the amount of ES and E + S is constantly at equilibrium, and thus the change of either with respect to time is 0. As soon as ES is converted to *ES, another mole of ES is produced from an infinite supply of E + S. The ES complex is formed from E and S at a faster rate than any other step in the reaction. This is pre-equilibrium kinetics in action.
The catalytic rate constant can be deduced from the graph by simply determining the slope of the line where the reaction demonstrates 0-order kinetics (the linear part).Solve for \(-r_S\), the negative rate of substrate conversion, obtaining the Michaelis-Menten expression describing the kinetics of the given situtation.
