Consumption of hydrogen ions in rapid-equilibrium enzyme kinetics.

Enzyme-catalyzed reductase reactions in particular are characterized by large changes in the binding of hydrogen ions Δ(r)N(H). This is a thermodynamic property of the reaction that is catalyzed. For example, in the ferredoxin-nitrite reductase reaction, there is an increase of eight in the binding of hydrogen ions for every molecule of nitrite reduced to ammonia H(2)O. If these hydrogen ions are consumed in the rate-determining reaction, the limiting velocity is proportional to [H(+)](8). This would make it practically impossible to determine the kinetic parameters. This article shows that when n hydrogen ions are consumed in reactions preceding the rate-determining reaction the limiting velocity is not proportional to [H(+)](n) and may only vary with pH according to the pK's of the enzyme-substrate complex that produces products. Rapid-equilibrium rate equations for ordered A + B → products are derived for two mechanisms in which a single hydrogen ion is consumed prior to the rate-determining reaction. Rate equations are tested by calculating velocities for the minimum number of velocity measurements required to estimate the kinetic parameters and using these velocities to estimate the kinetic parameters.