Proton-Coupled Electron Transfer in Soybean Lipoxygenase

Abstract

The proton-coupled electron transfer reaction catalyzed by soybean lipoxygenase-1 is studied with a multistate continuum theory that represents the transferring hydrogen nucleus as a quantum mechanical wave function. The inner-sphere reorganization energy of the iron cofactor is calculated with density functional theory, and the outer-sphere reorganization energy of the protein is calculated with the frequency-resolved cavity model for conformations obtained with docking simulations. Both classical and quantum mechanical treatments of the proton donor-acceptor vibrational motion are presented. The temperature dependence of the calculated rates and kinetic isotope effects is in agreement with the experimental data. The weak temperature dependence of the rates is due to the relatively small free energy barrier arising from a balance between the reorganization energy and the reaction free energy. The unusually high deuterium kinetic isotope effect of 81 is due to the small overlap of the reactant and product proton vibrational wave functions and the dominance of the lowest energy reactant and product vibronic states in the tunneling process. The temperature dependence of the kinetic isotope effect is strongly influenced by the proton donor-acceptor distance with the dominant contribution to the overall rate. This dominant proton donor-acceptor distance is significantly smaller than the equilibrium donor-acceptor distance and is determined by a balance between the larger coupling and the smaller Boltzmann probability as the distance decreases. Thus, the proton donor-acceptor vibrational motion plays a vital role in decreasing the dominant donor-acceptor distance relative to its equilibrium value to facilitate the proton-coupled electron transfer reaction.

Hydrogen Tunneling in an Enzyme Active Site: A Quantum Wavepacket Dynamical Perspective

“…19 Quantum mechanical tunneling is believed to be the fundamental reason behind this phenomenon. 7,12,16 (b) In some cases, a weak temperature dependence of the rate constant is seen in conjunction with large primary KIE. 20 (c) An elevated value for the Swain-Schaad exponent 21 [log(k H /k D )/log(k D /k T )] has also been noted.…”

A chemical understanding for the enhanced hydrogen tunnelling in hydroperoxidation of linoleic acid catalysed by soybean lipoxygenase‐1

“…. O) distance in the SLO/linoleic acid complex was estimated as l CO ¼ 2.88 [4] or 2.95 Å , [10] and suggests an even stronger H-bond between the C(11) position of the substrate and SLO. Such a significant H-bond energy involving a CH bond has been explained by the strong electron withdrawal effect of the nearby Fe(III) ion.…”

“…The supposed nonadiabaticity of this PCET motivated its treatment with radiationless transitions theories based on the golden rule of quantum mechanics. [3][4][5][6][7] In adiabatic reactions, the overlap of vibronic wavefunctions gives rise to nuclear tunnelling and is incorporated as a pre-exponential correction to the TST rate expression, Eq (7), while retaining the classical frequency of the nuclear motion along the reaction coordinate, n N % k B T/h.…”

“…[42][43][44] In the case of SLO catalysis of the hydroperoxidation of linoleic acid, the p D -Fe A distance obtained from docking calculations is 5.69 Å and the Fe A -O bond length is 1.86, [4] which lead to an edge-to-edge electron donor-acceptor distance of Dr % 4 Å . Edge-to-edge distance, rather than centre-to-centre distances, are employed for consistency with earlier work on electron transfer reactions.…”

“…The difficulties of TST stimulated the formulation of alternative theories. [2][3][4][5][6][7][8] Indeed, this KIE is much higher than that observed in other H-atom abstractions; for example, that of the H þ H 2 versus H þ D 2 hydrogen (deuterium) transfer is only 9.5 AE 1.4 at 30 8C. [9] The complexity of enzyme catalysis did not dissuade several groups of pursuing more sophisticated versions of TST in the treatment of these systems, to show that tunnelling and recrossing corrections may account for the extreme behaviours sometimes observed.…”