H and other transfers in enzymes and in solution: theory and computations, a unified view. 2. Applications to experiment and computations.
Equations obtained in part I for the free-energy barrier to one-step enzymatic reactions between bound reactants are discussed. The rate is expressed in terms of lambdao (protein reorganization energy), DeltaG(o) (standard free energy of reaction of the H-transfer step), bond breaking/bond forming term, w (work terms), and H-transmission property. Two alternative approximations for the coupling of the bond breaking/bond forming and protein are distinguished experimentally in favorable cases by the DeltaG(o) where the maximum deuterium kinetic isotope effect occurs. Plots of log rate versus DeltaG(o) and properties such as DeltaS* and DeltaS(o) are discussed. The weak or zero T-dependence of the kinetic isotope effect for wild-type enzymes operating under physiological conditions is interpreted in terms of vanishing (or isotopically insensitive) w plus transfer from the lowest H-state. Static and dynamic protein flexibility is discussed. While the many correlations accessible for electron transfers are not available for H-transfers in enzymes, a combination of experiment, computation, and analytical approaches can assist in evaluating the utility of the present equations and in suggesting further experiments and computations. A protein reorganization energy lambdao is obtained in the literature from the extended valence bond formalism where diabatic electronic states are used. A method is suggested for extracting it when instead a bond distance difference coordinate is used. The results may provide a bridge between the two approaches.
Marcus
R A
RA
Noyes Laboratory of Chemical Physics, MC 127-72, California Institute of Technology, Pasadena, California 91125-0072, USA. ram@caltech.edu
D003903:Deuterium / Q000737:chemistry; D004735:Energy Transfer; D004798:Enzymes / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D012996:Solutions
D004798:Enzymes; D012996:Solutions; D006859:Hydrogen; D003903:Deuterium