A fully microscopical simulation of the rate-limiting hydrogen abstraction catalyzed by soybean lipoxygenase-1 (SLO-1) has been carried out. This enzyme exhibits the largest, and weakly temperature dependent, experimental H/D kinetic isotope effect (KIE) reported for a biological system. The theoretical model used here includes the complete enzyme with a solvation shell of water molecules, the Fe(III)-OH- cofactor, and the linoleic acid substrate. We have used a hybrid QM(PM3/d-SRP)/MM method to describe the potential energy surface of the whole system, and the ensemble-averaged variational transition-state theory with multidimensional tunneling (EA-VTST/MT) to calculate the rate constant and the primary KIE. The computational results show that the compression of the wild-type active site enzyme results in the huge contribution of tunneling (99%) to the rate of the hydrogen abstraction. Importantly, the active site becomes more flexible in the Ile553Ala mutant reactant complex simulation (for which a markedly temperature dependent KIE has been experimentally determined), thus justifying the proposed key role of the gating promoting mode in the reaction catalyzed by SLO-1. Finally, the results indicate that the calculated KIE for the wild-type enzyme has an important dependence on the barrier width. © 2006 American Chemical Society.