Hydride tunnelling mechanism

Figure 13 Hydride tunnelling mechanism coupled to a rotation about the 3-fold axis in the [HFe(CO)4]" anion...

Protium/deuterium/tritium kinetic isotope effects are often used to support hydride transfer mechanisms over single electron transfer mechanisms. However, sequential electron/proton/electron transfer mechanisms can easily show isotope effects as well. Even though the rate limiting step in the overall two electron reduction of flavin or NADH may be the isotope independent endergonic electron tunneling to form a radical intermediate state, once formed, this radical state can return the electron to recreate the... 

The simplest conclusion is then that tunneling occurs both in enzymic hydride-transfer reactions and in related non-enzymic (model) reactions. It remains to be seen whether enzymic rate acceleration has evolved simply to make the existing tunneling reaction much more efficient or instead to create new tunneling mechanisms distinct from those observed in model systems. 

The role of protein motions in dihydrofolate reductase (DHFR) has been examined using molecular dynamics simulations as well as experimental studies of mutant enzymes. DHFR catalyzes transfer of hydride from NADPH to dihydrofolate via a tunneling mechanism. Molecular dynamics simulations of complexes of the enzyme with both substrates show both correlated and anticorrelated motions (see Figure 23). " Mutations... 

The report of Basran et al. (entry 5 of Table 2) contains two studies involving hydride transfer with nicotinamide cofactors. In morphinone-reductase catalyzed reduction by NADH of the flavin cofactor FMN (schematic mechanism in Fig. 5), the primary isotope effects are modest (around 4 for H/D), but exhibit a small value of Ajj/Aq (0.13) and an exalted isotopic difference in energies of activation (8.2kJ/mol) that alone would have generated an isotope effect around 30. The enthalpies of activation are in the range of 35-45 kJ/mol. This is behavior typical of Bell tunneling as discussed above. It can also be reproduced by more complex models, as will be discussed in later parts of this review. 

The study of Karsten et al. (entry 13 in Table 2) is of special interest because the reaction under catalysis (see Figure 5 for the schematic mechanism) may involve hydride transfer simultaneous with the fission of a C-C bond in the decarboxylation component of the reaction. If the two events are concerted (evidence in related enzymes does not provide a clear guideline on this point) then tunneling might become more difficult because of the increased effective mass. 

Apparent NMR equivalence of nuclei can also arise by a quantum mechanical intramolecular tunneling process. In principle, this process may be differentiated from intermolecular exchange processes because although the exchanging nuclei are rendered equivalent insofar as the NMR experiment is concerned, spin-spin splitting by other magnetic nuclei is not washed out. This type of intramolecular exchange is manifested in several boron hydride derivatives. It was first proposed by Ogg and Ray (98) to explain the NMR spectra of aluminum borohydride, whose structure is... 

The enzyme-product complexes of the yeast enzyme dissociate rapidly so that the chemical steps are rate-determining.31 This permits the measurement of kinetic isotope effects on the chemical steps of this reaction from the steady state kinetics. It is found that the oxidation of deuterated alcohols RCD2OH and the reduction of benzaldehydes by deuterated NADH (i.e., NADD) are significantly slower than the reactions with the normal isotope (kn/kD = 3 to 5).21,31 This shows that hydride (or deuteride) transfer occurs in the rate-determining step of the reaction. The rate constants of the hydride transfer steps for the horse liver enzyme have been measured from pre-steady state kinetics and found to give the same isotope effects.32,33 Kinetic and kinetic isotope effect data are reviewed in reference 34 and the effects of quantum mechanical tunneling in reference 35. 

Quantum dynamics effects for hydride transfer in enzyme catalysis have been analyzed by Alhambra et. al., 2000. This process is simulated using canonically variational transition-states for overbarrier dynamics and optimized multidimensional paths for tunneling. A system is divided into a primary zone (substrate-enzyme-coenzyme), which is embedded in a secondary zone (substrate-enzyme-coenzyme-solvent). The potential energy surface of the first zone is treated by quantum mechanical electronic structure methods, and protein, coenzyme, and solvent atoms by molecular mechanical force fields. The theory allows the calculation of Schaad-Swain exponents for primary (aprim) and secondary (asec) KIE... 

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