Cu(A) and Cu(Z) are variants of the electron transfer center in nitrous oxide reductase

Nitrous oxide reductase (N(2)OR) is a dimeric copper-dependent bacterial enzyme that catalyzes the reduction of N(2)O to N(2) as part of the denitrification pathway. In the absence of an x-ray crystal structure, the current model of the nature of the copper sites within the enzyme is based on four copper atoms per monomer and assigns two copper atoms to an electron transfer center, Cu(A), a bis-thiolate-bridged dinuclear copper center found to date only in N(2)OR and cytochrome c oxidase, and two copper atoms to a second dinuclear center, Cu(Z), presumed to be the site of catalysis. Based on detailed analysis of the low temperature magnetic CD spectra of N(2)OR, this paper revises the current model and proposes that both Cu(A) and Cu(Z) are variants of an electron transfer center and hence that all of the observed optical features are due to this electron transfer center. It is proposed further that the presence of these different forms provides a mechanism for the delivery of two electrons to an active site comprising copper ions lacking thiolate coordination.