The role of quantum mechanical nuclear motions in enzymatic reactions is examined by realistic simulations that take into account the fluctuations of an entire enzyme-substrate complex. This is done by using the quantized classical path (QCP) approach which is based on Feynman’s path integral formulation. The calculations evaluate the quantum mechanical activation free energy and deuterium isotope effect for the proton transfer step in the catalytic reaction of carbonic anhydrase. The calculated and observed isotope effects are in very good agreement, thus demonstrating the potential of our approach in extracting mechanistic information. Furthermore, the value of the calculated quantum mechanical rate constant is in a good agreement with the corresponding observed value. This is significant since the evaluation of the ratio between the quantum mechanical rate constants of the reaction in the protein and in aqueous solution does not involve any adjustable parameter. The reliability of our calculations is based on the use of the empirical valence bond (EVB) method. This method does not try to represent the potential surfaces of the reacting atoms by a first principle approach (this is easily done by fitting the EVB surface to experimental and theoretical results) but rather evaluates the effect of moving these atoms from solution to the enzyme active site. The possible catalytic advantage of quantum mechanical nuclear motions is examined by comparing these effects in the enzyme and in a reference solution reaction. It is found that while quantum mechanical corrections to activation free energies of enzymatic reactions can be quite large they are not drastically different than the corresponding corrections in solution. Apparently the largest catalytic effects are due to reduction in the reorganization energy and Delta G(0) by the electrostatic effects of the preorganized environment of the protein active site. Nevertheless, small but non-negligible catalytic contributions can be associated with quantum mechanical effects.