Dynamic docking of cytochrome b5 with myoglobin and alpha-hemoglobin: heme-neutralization "squares" and the binding of electron-transfer-reactive configurations.

Intracomplex electron transfer (ET) occurs most often in intrinsically transient, low affinity complexes. As a result, the means by which adequate specificity and reactivity are obtained to support effective ET is still poorly understood. We report here on two such ET complexes:  cytochrome b5 (cyt b5) in reaction with its physiological partners, myoglobin (Mb) and hemoglobin (Hb). These complexes obey the Dynamic Docking (DD) paradigm:  a large ensemble of weakly bound protein−protein configurations contribute to binding in the rapid-exchange limit, but only a few are ET-active. We report the ionic-strength dependence of the second-order rate constant, k2, for photoinitiated ET from within all four combinations of heme-neutralized Zn deuteroporphyrin-substituted Mb/αHb undergoing ET with cyt b5, the four “corners” of a “heme-neutralization square”. These experiments provide insights into the relative importance of both global and local electrostatic contributions to the binding of reactive configurations...