One-substrate-one-product enzymic reactions: The relationship between isotope-exchange kinetic, steady-state kinetic and equilibrium parameters

Abstract

Expressions are derived for the parameters that can be obtained from (1) steady-state kinetics, (2) isotope-exchange kinetics at equilibrium, and (3) equilibrium binding experiments for the following two one-substrate-one-product enzymic mechanisms: E + S ⇌ X 1 ⇌ X 2 ⇌ … X n ⇌ E + P ( mechanism I ) E 1 + S ⇌ X 1 ⇌ X 2 ⇌ … X n ⇌ E m + P E m ⇌ E m − 1 ⇌ X m − 2 ⇌ … E 2 ⇌ E 1 } ( mechanism II ) It is shown that the exchange constants for both mechanisms are identical with the reciprocals of the equilibrium parameters ( Darvey, 1973 ) that can be obtained from binding experiments. Therefore, expressions relating isotope exchange constants and steady-state kinetic parameters are analogous to those relating equilibrium parameters and steady-state kinetic parameters. The paper also presents, for mechanisms I and II, relationships between the maximum rate of isotope-exchange at equilibrium and the steady-state kinetic parameters. These relationships provide a new criterion for distinguishing experimentally between these two mechanisms.