130 INSTRUMENTAL METHODS
what the authors called the purple CyoA protein.^31 This last protein exhibited
different characteristics for the bimetallic center than for the fi rst two and was
called the Cu*A center (see Figure 3.23 ). First, these researchers noted similari-
ties in the low - temperature magnetic circular dichroism (MCD) spectrum of
COX and the room temperature MCD spectrum of N 2 OR that had its mixed -
valence, Cu(1.5) – Cu(1.5) bimetallic Cu A center detected by EPR previously.
Next, the researchers noted that COX ’ s Cu A center is found in subunit II and
that there was a clear primary sequence similarity between the COX and
N 2 OR copper - binding domains — namely, copper ion binding ligands found in
the sequences cys – X 3 – cys – X 3 – his – X 2 – his, where X stands for any amino acid
residue. To eliminate interference from the other metal centers found in COX,
EPR spectroscopy was carried out on a site - directed mutant form of the
soluble domain ofParacoccus denitrifi cans COX subunit II expressed in E.
Figure 3.23 Experimental EPR spectra of N 2 OR V (spectrum A), PdII (spectrum B),
and purple CyoA (spectrum D). Also shown are simulated spectra for PdII (spectrum
C), and purple CyoA (spectrum E). (Reprinted with permission from Figure 2 of
reference 31. Copyright 1996, American Chemical Society.)
A.B.C.D.
E.0.28 0.302.3 2.2 2.1g-value
2 1.90.32
Field (mT)0.34 0.36