Environmentally coupled hydrogen tunneling. Linking catalysis to dynamics

Many biological C-H activation reactions exhibit nonclassical kinetic isotope effects (KIEs). These nonclassical KIEs are too large (kH/kD > 7) and/or exhibit unusual temperature dependence such that the Arrhenius prefactor KIEs (AH/AD) fall outside of the semiclassical range near unity. The focus of this minireview is to discuss such KIEs within the context of the environmentally coupled hydrogen tunneling model. Full tunneling models of hydrogen transfer assume that protein or solvent fluctuations generate a reactive configuration along the classical, heavy-atom coordinate, from which the hydrogen transfers via nuclear tunneling. Environmentally coupled tunneling also invokes an environmental vibration (gating) that modulates the tunneling barrier, leading to a temperature-dependent KIE. These properties directly link enzyme fluctuations to the reaction coordinate for hydrogen transfer, making a quantum view of hydrogen transfer necessarily a dynamic view of catalysis. The environmentally coupled hydrogen tunneling model leads to a range of magnitudes of KIEs, which reflect the tunneling barrier, and a range of AH/AD values, which reflect the extent to which gating modulates hydrogen transfer. Gating is the primary determinant of the temperature dependence of the KIE within this model, providing insight into the importance of this motion in modulating the reaction coordinate. The potential use of variable temperature KIEs as a direct probe of coupling between environmental dynamics and the reaction coordinate is described. The evolution from application of a tunneling correction to a full tunneling model in enzymatic H transfer reactions is discussed in the context of a thermophilic alcohol dehydrogenase and soybean lipoxygenase-1.

Temperature-dependent isotope effects in soybean lipoxygenase-1: correlating hydrogen tunneling with protein dynamics

The hydrogen-atom transfer in soybean lipoxygenase-1 (SLO) exhibits a large kinetic isotope effect on k(cat) (KIE = 81) near room temperature and a very weak temperature dependence (E(act) = 2.1 kcal/mol). These properties are consistent with H small middle dot transfer that occurs entirely by a tunneling event. Mutants of SLO were prepared, and the temperature dependence of the KIE was measured, to test for alterations in the tunneling behavior. All mutants studied exhibit KIEs of similar, large magnitude at 30 degrees C, despite an up to 3 orders of magnitude change in k(cat). E(act) for two of the mutants (Leu(754) --> Ala, Leu(546) --> Ala) is larger than for wild-type (WT), and the KIE becomes slightly more temperature dependent. In contrast, Ile(553) --> Ala exhibits k(cat) and E(act) parameters similar to wild-type soybean lipoxygenase-1 (WT-SLO) for protiated substrate; however, the KIE is markedly temperature dependent. The behavior of the former two mutants could reflect increased reorganization energies (lambda), but the behavior of the latter mutant is inconsistent with this description. We have invoked a full H* tunneling model (Kuznetsov, A. M.; Ulstrup, J. Can. J. Chem. 1999, 77, 1085-1096) to explain the temperature dependence of the KIE, which is indicative of the extent to which distance sampling (gating) modulates hydrogen transfer. WT-SLO exhibits a very small E(act) and a nearly temperature-independent KIE, which was modeled as arising from a compressed hydrogen transfer distance with little modulation of the hydrogen transfer distance. The observations on the Leu(754) --> Ala and Leu(546) --> Ala mutants were modeled as arising from a slightly less compressed active site with greater modulation of the hydrogen transfer distance by environmental dynamics. Finally, the observed behavior of the Ile(553) --> Ala mutant indicates a relaxed active site with extensive involvement of gating to facilitate hydrogen transfer. We conclude that WT-SLO has an active site structure that is well organized to support hydrogen tunneling and that mutations perturb structural elements that support hydrogen tunneling. Modest alterations in active site residues increase lambda and/or increase the hydrogen transfer distance, thereby affecting the probability that tunneling can occur. These studies allow the detection and characterization of a protein-gating mode in catalysis.