SPECIES DEPENDENCE OF ENZYME-SUBSTRATE ENCOUNTER RATES FOR TRIOSE PHOSPHATE ISOMERASES

Triose phosphate isomerase (TIM) is a diffusion-controlled enzyme whos e rate is limited by the diffusional encounter of the negatively charg ed substrate glyceraldehyde 3-phosphate (GAP) with the homodimeric enz yme's active sites. Translational and orientational steering of GAP to ward the active sites by the electrostatic held of chicken muscle TIM has been observed in previous Brownian dynamics (BD) simulations. Here we report simulations of the association of GAP with TIMs from four s pecies with net charges at pH 7 varying from -12e to +12e. Computed se cond-order rate constants are in good agreement with experimental data . The BD simulations and computation of average Boltzmann factors of s ubstrate-protein interaction energies show that the protein electrosta tic potential enhances the rates for all the enzymes. There is much le ss variation in the computed rates than might be expected on the basis of the net charges. Comparison of the electrostatic potentials by mea ns of similarity indices shows that this is due to conservation of the local electrostatic potentials around the active sites which are the primary determinants of electrostatic steering of the substrate. (C) 1 998 Wiley-Liss, Inc.