Isotope Exchanges at Equilibrium

Abstract

In the previous chapter, we saw the utility of isotopes (radioisotopes in particular) in elucidating enzyme mechanisms. They were examples when the net chemical reaction was occurring in one direction. Isotope exchange kinetics is also possible in a system at equilibrium. Isotope exchange experiments at equilibrium are useful for defining kinetic mechanisms. Data from carefully executed isotope exchange studies are powerful supplement to steady-state kinetic analysis (Boyer 1978, Rose 1995). More importantly, they provide excellent evidence in discriminating ordered and random sequential mechanisms. We recall that, in some cases, steady-state kinetic data gives quantitative information, and this is used to answer qualitative questions. For example, the presence of a KiA term with finite value is used to conclude against parallel initial velocity patterns (Chap. 19Analysis of Initial Velocity Patterns). However when these values are extremely small, a clear-cut decision becomes difficult. In such cases isotope exchange data gives unambiguous yes–no answers (see below).