INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
congresos y reuniones científicas
Electrostatically-driven second sphere ligand switch between high and low reorganization energy forms of native cytochrome c
DAMIAN ALVAREZ-PAGGI; MARIA ANA CASTRO; VERÓNICA TÓRTORA; RAFAEL RADI; DANIEL H. MURGIDA
Congreso; ISE-BES Bioelectrochemistry 2013; 2013
Natural selection has evolved specialized redox proteins able to perform fast intra- and inter-molecular long range electron transfer (ET) reactions, in the milliseconds time scale and below, in spite of involving nearly null thermodynamic driving forces. Most of these nonadiabatic reactions are well described by the high temperature limit expression of Marcus semiclassical theory. Accordingly, the ET rate is proportional to the square of the electronic coupling matrix element, and to a Frank-Condon exponential term that accounts for the thermal accessibility of such degeneracy. The latter is dominated by the reorganization energy (λ) that accounts for both the energy required to distort the reactives towards the equilibrium configuration of the products and the reorganization of the solvent around the redox center. This parameter is of difficult experimental accessibility, and even in the case of model proteins such as cytochrome c (Cyt), a wide range of values have been reported, from 0.23 to 1.1 eV and beyond. We have been able to rationalize these results employing a combination of protein film voltammetry, time-resolved vibrational spectroelectrochemistry and molecular dynamics simulations to evaluate the reorganization energy of Cyt in biomimetic electrostatic complexes. The results reveal the existence of two native-like conformations of Cyt that present significantly different λ values. Conversion from the high to the low λ forms is triggered by electrostatic interactions, and involves the rupture of a weak H-bond between first (Met80) and second (Tyr67) sphere ligands of the heme iron as a distinctive feature of the conformational switch. Moreover, we identified the two flexible Ω loops as the transducers of the electrostatic signal. This fine-tuning effect is abolished in the Y67F Cyt mutant, which presents a λ value similar to the WT protein in electrostatic complexes. We propose that interactions of Cyt with the natural redox partner proteins activate a similar mechanism to minimize the reorganization energy of inter-protein electron transfer.