Dynamic docking and electron transfer between Zn-myoglobin and cytochrome b₅

We present a broad study of the effect of neutralizing the two negative charges of the Mb propionates on the interaction and electron transfer (ET) between horse Mb and bovine cyt b₅, through use of Zn-substituted Mb (ZnMb, 1) to study the photoinitiated reaction, (³ZnP)Mb + Fe³⁺cyt b₅ → (ZnP)⁺Mb + Fe²⁺cyt b₅. The charge neutralization has been carried out both by replacing the Mb heme with zinc-deuteroporphyrin dimethylester (ZnMb(dme), 2), which replaces the charges by small neutral hydrophobic patches, and also by replacement with the newly prepared zinc-deuteroporphyrin diamide (ZnMb(diamide), 3), which converts the charged groups to neutral, hydrophilic ones. The effect of propionate neutralization on the conformation of the zinc-porphyrin in the Mb heme pocket has been studied by multinuclear NMR with an ¹⁵N labeled zinc porphyrin derivative (ZnMb(¹⁵N-diamide), 4). The rates of photoinitiated ET between the Mb's (1-3) and cyt b₅ have been measured over a range of pH values and ionic strengths. Isothermal titration calorimetry (ITC) and NMR methods have been used to independently investigate the effect of charge neutralization on Mb/b₅ binding. The neutralization of the two heme propionates of ZnMb by formation of the heme diester or, for the first time, the diamide increases the second-order rate constant of the ET reaction between ZnMb and cyt b₅ by as much as several 100-fold, depending on pH and ionic strength, while causing negligible changes in binding affinity. Brownian dynamic (BD) simulations and ET pathway calculations provide insight into the protein docking and ET process. The results support a new "dynamic docking" paradigm for protein-protein reactions in which numerous weakly bound conformations of the docked complex contribute to the binding of cyt b₅ to Mb and Hb, but only a very small subset of these are ET active, and this subset does not include the conformations most favorable for binding; the Mb surface is a large "target" with a small "bullseye" for the cyt b₅ "arrow". This paradigm differs sharply from the more familiar, "simple" docking within a single, or narrow range of conformations, where binding strength and ET reactivity increase in parallel. Likewise, it is distinct from, although complementary to, the well-known picture of conformational control of ET through "gating", or a related picture of "conformational coupling". The new model describes situations in which tight binding does not correlate with efficient ET reactivity, and explains how it is possible to modulate reactivity without changing affinity. Such "decoupling" of reactivity from binding clearly is of physiological relevance for the reduction of met-Mb in muscle and of met-Hb in a red cell, where tight binding of cyt b₅ to the high concentration of ferrous-Mb/Hb would prevent the cytochrome from finding and reducing the oxidized proteins; it likely is of physiological relevance in other situations, as well.