The role of binding subsite A, located at the terminal of the six binding subsites of hen egg-white lysozyme, in substrate binding and catalytic reactions was investigated by kinetic studies using a chemical modification method. Computer simulation showed that, although subsite A participates in the binding of the substrate, a decrease in the affinity of subsite A to the sugar residue does not cause a lowering of the rate of substrate consumption but changes the mode of the reaction by changing the distribution of the products formed. The binding free energies of subsites for Asp-101-modified lysozymes were estimated by data-fitting from the experimental time-courses. The contribution of Asp-101 in hen egg-white lysozyme to the substrate binding at subsite A was estimated to correspond to a binding free energy of about -3 kJ/mol, 30% of the total binding free energy of subsite A. Modification of Asp-101 affected not only the binding free energy of subsite A but also that of subsite C.