The Linear Dependence of Log(kcat/Km) for Reduction of NAD+ by PhCH2OH on the Distance between Reactants when Catalyzed by Horse Liver Alcohol Dehydrogenase and 203 Single Point Mutants

Like most dehydrogenases, with horse liver alcohol dehydrogenase (LADH), a bulky amino acid residue (Val 203) is positioned at the face of NAD+ distal to substrate alcohol in order to restrict the separation of reactants and to control the stereochemistry. Molecular dynamics simulations of native (Val203) and single-point mutants (Leu203, Ala203, and Gly203) of LADH·PhCH2OH·NAD+ provide the close contact distances (CCD) between PhCH2OH and NAD+ reactants. It is found that log(kcat/Km) is linearly dependent upon CCD. This linear dependence of log(kcat/Km) upon CCD is expected if hydride transfer is the rate-determining step. Since log(kcat/Km) has been found to be a linear function of tunneling, ln(kH/kT)/ln(kD/ kT), it follows that tunneling in LADH has a linear dependence upon CCD.