Electrostatic Redesign of the [Myoglobin, Cytochrome b5] Interface To Create a Well-Defined Docked Complex with Rapid Interprotein Electron Transfer

Cyt b(5) is the electron-carrier repair protein that reduces met-Mb and met-Hb to their O-2-carrying ferroheme forms. Studies of electron transfer (ET) between Mb and cyt b(5) revealed that they react on a Dynamic Docking (DD) energy landscape on which binding and reactivity are uncoupled: binding is weak and involves an ensemble of nearly isoenergetic configurations, only a few of which are reactive; those few contribute negligibly to binding. We set the task of redesigning the surface of Mb so that its reaction with cyt b5 instead would occur on a conventional simple docking (SD) energy landscape, on which a complex exhibits a well-defined (set of) reactive binding configuration (s), with binding and reactivity thus no longer being decoupled. We prepared a myoglobin (Mb) triple mutant (D44K/D60K/E85K; Mb(+6)) substituted with Zn-deuteroporphyrin and monitored cytochrome b5 (cyt b5) binding and electron transfer (ET) quenching of the 'ZnMb(+6) triplet state. In contrast, to Mb(WT), the three charge reversals around the front-face heme edge of Mb(+6) have directed cyt b(5) to a surface area of Mb adjacent to its heme, created a well-defined, most-stable structure that supports good ET pathways, and apparently coupled binding and ET: both K-a and k(et) are increased by the same factor of similar to 2 x 10(2), creating a complex that exhibits a large ET rate constant, k(et) = 10(6) (1)s(-1), and is in slow exchange (k(off) << k(et)). In short, these mutations indeed appear to have created the sought-for conversion from DD to simple docking (SD) energy landscapes.