Quantum catalysis? A comment on tunnelling contributions for catalysed and uncatalysed reactions

Appreciation for the contribution of nuclear quantum effects (NQEs) to chemical reactivity predates transition-state theory (TST). Quantum corrections to rate constants for the reactions catalysed by lactate dehydrogenase (LDH) and formate dehydrogenase (FDH) and the same reactions in water are estimated by Bell's one-dimensional approximate method and give tunnelling contributions to catalysis of 1.6 and 0.95, respectively. Published results for NQEs, including both tunnelling and zero-point energies, estimated by the quantum classical path method for LDH, carbonic anhydrase, glyoxylase I and lipoxygenase, together with the corresponding reactions in water, are reviewed: the respective contributions to catalysis are 0.66, 5, 1 and 1. In the absence of better evidence that an enzymic rate enhancement is due to a significantly larger quantum correction for the enzyme-catalysed reaction than for an appropriate uncatalysed reference reaction, it is suggested that the term 'quantum catalysis' should be used with caution and restraint.