Substrate entasis and electronic coupling elements in electron transfer from FeII in a multicopper ferroxidase

Outersphere electron transfer in multicopper oxidases occurs at the type 1, blue Cu-II. One class of MCO proteins exhibits a specificity in this reaction towards Fe-II. In work carried out in collaboration with the Solomon lab over the past 7 years, we have delineated the structural motifs that support this ferroxidase specificity and have quantified the contributions that each makes to this outersphere electron transfer reaction from Fe-II to the type 1 Cu-II. Two features of this electron transfer catalysis stand out. First, the protein provides a binding site for Fe-II that actually favors Fe-III; this coordination sphere places the bound Fe-II in a state of entasis'' that can be relieved by loss of an electron. In short, the E degrees of the bound Fe-II is lowered relative to that of aqueous ferrous iron making electron transfer thermodynamically favorable. Second, carboxylates within this coordination sphere provide an electronic coupling pathway for the electron transfer via their H-bond network with type 1 Cu histidine ligands thus making electron transfer kinetically efficient. This brief report breaks down these contributions to ferroxidase specificity in terms of the semi-classical Marcus equation describing outersphere electron transfer. (c) 2007 Elsevier B. V. All rights reserved.