Converting the bis-FeIV state of the diheme enzyme MauG to Compound I decreases the reorganization energy for electron transfer.

The electron transfer (ET) properties of two types of high-valent hemes were studied within the same protein matrix; the bis-Fe(IV) state of MauG and the Compound I state of Y294H MauG. The latter is formed as a consequence of mutation of the tyrosine which forms the distal axial ligand of the six-coordinate heme that allows it to stabilize Fe(IV) in the absence of an external ligand. The rates of the ET reaction of each high-valent species with the type I copper protein, amicyanin, were determined at different temperatures and analysed by ET theory. The reaction with bis-Fe(IV) wild-type (WT) MauG exhibited a reorganization energy (λ) that was 0.39 eV greater than that for the reaction of Compound I Y295H MauG. It is concluded that the delocalization of charge over the two hemes in the bis-Fe(IV) state is responsible for the larger λ, relative to the Compound I state in which the Fe(V) equivalent is isolated on one heme. Although the increase in λ decreases the rate of ET, the delocalization of charge decreases the ET distance to its natural substrate protein, thus increasing the ET rate. This describes how proteins can balance different ET properties of complex redox cofactors to optimize each system for its particular ET or catalytic reaction.