Nitrous oxide reductase (N2OR)
is a dimeric copper-dependent bacterial enzyme that catalyzes the
reduction
of N2O to N2 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, CuA, a bis-thiolate-bridged
dinuclear
copper center found to date only in N2OR and
cytochrome
c oxidase, and two copper atoms to a second dinuclear
center,
CuZ, presumed to be the 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 CuA
and CuZ 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.