Distance-Independent Charge Recombination Kinetics
in Cytochrome <i>c</i>–Cytochrome <i>c</i> Peroxidase Complexes: Compensating Changes in the Electronic Coupling
and Reorganization Energies
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Abstract
Charge recombination rate constants
vary no more than 3-fold for
interprotein ET in the Zn-substituted wild type (WT) cytochrome <i>c</i> peroxidase (CcP):cytochrome <i>c</i> (<i>Cc</i>) complex and in complexes with four mutants of the <i>Cc</i> protein (i.e., F82S, F82W, F82Y, and F82I), despite large
differences in the ET distance. Theoretical analysis indicates that
charge recombination for all complexes involves a combination of tunneling
and hopping via Trp191. For three of the five structures (WT and F82S(W)),
the protein favors hopping more than that in the other two structures
that have longer heme → ZnP distances (F82Y(I)). Experimentally
observed biexponential ET kinetics is explained by the complex locking
in alternative coupling pathways, where the acceptor hole state is
either primarily localized on ZnP (slow phase) or on Trp191 (fast
phase). The large conformational differences between the CcP:<i>Cc</i> interface for the F82Y(I) mutants compared to that the
WT and F82S(W) complexes are predicted to change the reorganization
energies for the CcP:<i>Cc</i> ET reactions because of changes
in solvent exposure and interprotein ET distances. Since the recombination
reaction is likely to occur in the inverted Marcus regime, an increased
reorganization energy compensates the decreased role for hopping recombination
(and the longer transfer distance) in the F82Y(I) mutants. Taken together,
coupling pathway and reorganization energy effects for the five protein
complexes explain the observed insensitivity of recombination kinetics
to donor–acceptor distance and docking pose and also reveals
how hopping through aromatic residues can accelerate long-range ET