Evolving the [Myoglobin, Cytochrome <i>b</i>₅] Complex from Dynamic toward Simple Docking: Charging the Electron Transfer Reactive Patch

We describe photoinitiated electron transfer (ET) from a suite of Zn-substituted myoglobin (Mb) variants to cytochrome <i>b</i>₅ (<i>b</i>₅). An electrostatic interface redesign strategy has led to the introduction of positive charges into the vicinity of the heme edge through D/E → K charge-reversal mutation combinations at “hot spot” residues (D44, D60, and E85), augmented by the elimination of negative charges from Mb or <i>b</i>₅ by neutralization of heme propionates. These variations create an unprecedentedly large range in the product of the ET partners’ total charges (−5 < −<i>q</i>_Mb<i>q</i>_<i>b</i>₅ < 40). The binding affinity (<i>K</i>_a) increases 1000-fold as −<i>q</i>_Mb<i>q</i>_<i>b</i>₅ increases through this range and exhibits a surprisingly simple, exponential dependence on −<i>q</i>_Mb<i>q</i>_<i>b</i>₅. This is explained in terms of electrostatic interactions between a “charged reactive patch” (crp) on each partner’s surface, defined as a compact region around the heme edge that (i) contains the total protein charge of each variant and (ii) encompasses a major fraction of the “reactive region” (Rr) comprising surface atoms with large matrix elements for electron tunneling to the heme. As −<i>q</i>_Mb<i>q</i>_<i>b</i>₅ increases, the complex undergoes a transition from fast to slow-exchange dynamics on the triplet ET time scale, with a correlated progression in the rate constants for intracomplex (<i>k</i>_et) and bimolecular (<i>k</i>₂) ET. This progression is analyzed by integrating the crp and Rr descriptions of ET into the textbook steady-state treatment of reversible binding between partners that undergo intracomplex ET and found to encompass the full range of behaviors predicted by the model. The generality of this approach is demonstrated by its application to the extensive body of data for the ET complex between the photosynthetic reaction center and cytochrome <i>c</i>₂. Deviations from this model also are discussed