Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases

Abstract

We have calculated the reorganisation energy for the intramolecular electron transfer between the reduced type 1 copper site and the peroxy intermediate of the trinuclear cluster in the multicopper oxidase CueO. The calculations are performed at the combined quantum mechanics and molecular mechanics (QM/MM) level, based on molecular dynamics simulations with tailored potentials for the two copper sites. We obtain a reorganisation energy of 91-133 kJ/mol, depending on the theoretical treatment. The two Cu sites contribute by 12 and 22 kJ/mol to this energy, whereas the solvent contribution is 34 kJ/mol. The rest comes from the protein, involving small contributions from many residues. We have also estimated the energy difference between the two electron-transfer states and show that the reduction of the peroxy intermediate is exergonic by 43-87 kJ/mol, depending on the theoretical method. Both the solvent and the protein contribute to this energy difference, especially charged residues close to the two Cu sites. We compare these estimates with energies obtained from QM/MM optimisations and QM calculations in vacuum, and discuss differences between the results obtained at various levels of theory.

Catalytic Cycle of Multicopper Oxidases Studied by Combined Quantum- and Molecular-Mechanical Free-Energy Perturbation Methods

“…This problem was solved by scaling down the point charges of the other QM system by a factor of 2 in both terms. 43 The point-charge model of each QM system was obtained by a fit to the electrostatic potential (ESP) calculated for a wavefunction polarised by a point-charge model of the surroundings, but without the point charges when the ESP was calculated. 43,61 The ESP points were sampled with the Merz-Kollman approach 62 as implemented in Turbomole 58 .…”

“…73 The entropy term was ignored, because entropy effects are expected to be quite small in these electron-and proton-transfer reactions. 43 Two variants of QM/MM-PBSA-2QM were tested. They differ in how the QM energy and the charges on the QM atoms are calculated.…”

“…30,32,[34][35][36][37][38] In particular, several groups have estimated and rationalised the redox potentials of Cu T1 sites in proteins 36,[39][40][41][42] and the TNC in MCOs has also been studied. 43 Likewise, many methods have been developed to calculate acidity constants, both of small molecules in solution and of various groups in proteins. 33,34,[44][45][46][47][48][49] The former are typically based on QM calculations, to obtain the intrinsic proton affinity of the molecules of interest, combined with continuum-solvation methods to estimate the effect of transferring the reactants from vacuum to solvent.…”

On the Possibility of Uphill Intramolecular Electron Transfer in Multicopper Oxidases: Electrochemical and Quantum Chemical Study of Bilirubin Oxidase

“…The argument we put forward is that the ET pathway between the Cu-T1 site and Cu-T23 cluster is buried in the interior of the protein and the ET, which does not change the total charge of the protein, should not involve any major reorientation of solvent molecules. A recent methodological study [70] of reorganization energies for a MCO shows that the solvent contribution to λ in MCOs is expected to be ~0.35 eV and that the protein contribution (~0.6 eV) is dominated by residues close to the copper sites.…”

Theoretical studies of the active-site structure, spectroscopic and thermodynamic properties, and reaction mechanism of multicopper oxidases

“…However, estimating λ os normally requires more complicated methods, based on molecular-dynamics We have studied the total λ for the electron transfer between the reduced Cu-T1 site and the TNC in the PI state in the CueO protein [95]. To this end, a tailored MM parameterisation of the Cu-T1 and TNC sites in the MCO (i.e.…”