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 CuII. One class of MCO proteins exhibits a specificity in this reaction towards FeII. 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 FeII to the type 1 CuII. Two features of this electron transfer catalysis stand out. First, the protein provides a binding site for FeII that actually favors FeIII; this coordination sphere places the bound FeII in a state of “entasis” that can be relieved by loss of an electron. In short, the E∘ of the bound FeII 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.