CU-A AND CU-Z ARE VARIANTS OF THE ELECTRON-TRANSFER CENTER IN NITROUS-OXIDE REDUCTASE

Nitrous oxide reductase (N2OR) is a dimeric copper-dependent bacterial enzyme that catalyzes the reduction of N2O to N-2 as part of the deni trification 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 c opper center found to date only in N2OR and cytochrome c oxidase, and two copper atoms to a second dinuclear center, Cu-Z, presumed to be th e site of catalysis. Based on detailed analysis of the low temperature magnetic CD spectra of N2OR, 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 presen ce of these different forms provides a mechanism for the delivery of t wo electrons to an active site comprising copper ions lacking thiolate coordination.