BIOINORGANIC CHEMISTRY A Short Course Second Edition

CYTOCHROME c OX IDASE 433

nonhelical loop region between helices VII and VIII. In its fully reduced Cu +
state ( S = 0) the heme a 3 – Cu B center would be EPR silent for low - spin Fe 2+.
In the oxidized state (Fe 3+ , S = 5/2, CuB^2 +,S = 1/2) the two metal ions are anti-
ferromagnetically coupled and again EPR silent. Heme a 3 and Cu B constitute
the O 2 binding and reduction to water site, and it was hoped that CcO X - ray
crystallographic structures would show dioxygen - binding mode(s). The X - ray
crystallographic structure of oxidized CcO (PDB: 1OCC) was not at high
enough resolution (2.8 Å ) to indicate bridging ligands between the heme a 3 Fe
and Cu B ions. However, the higher - resolution PDB: 2OCC structure (2.3 Å )
does indicate a bridging peroxo or hydroperoxo ligand between the heme a 3
Fe and Cu B ions. The positioning of this ligand has been questioned by work
on model compounds of the fully oxidized state as will be discussed in Section
7.8.4. (See also Table 7.9 .) As of late 2006, no other X - ray crystallographic
structures have been published showing a peroxo, hydroperoxo, or superoxo
ligand positioned between heme a 3 Fe and Cu B ions in CcO. Recently, an X - ray
crystallographic structure has been reported that indicates a hydroxide ligand
or water molecule bridging between the Fe ion of heme a 3 (Fe – O = 2.13 Å )
and the Cu B ion (Cu – O = 2.96 Å )^139 (PDB: 2GSM).
The zinc ion site, residing outside the CcO on the matrix side, was found to
have tetrahedral coordination to four cysteine ligands (cys60, cys62, cys82,
cys85 from subunit Vb). The polypeptide sequence from cys60 to cys82 exhibits
a zinc fi nger motif (see Section 2.4 ). The magnesium ion site was less well
defi ned in the reference 137a report, however, asp369 and his368 from subunit
I, glu198 in subunit II, and several water molecules are implicated as possible
magnesium ligands. In reference 137b , the magnesium ion is postulated to be
part of a system/channel for removal of water from the catalytic site. In this

Figure 7.39 Schematic diagram for cytochrome c oxidase. Distances taken from
bovine heart X - ray crystallographic structure (PDB: 2OCC). Entry and exit channels
for dioxygen, protons, and water are schematic only. (Adapted with permission from
Figure 1 of reference 138. Copyright 2004 American Chemical Society.)

4 e-

CuA (CuII-CuII or CuII-CuI)

4 cyto c

Fe

his

his

Fe

his

CuB

his hishis-tyr

O 2

membrane

intermembrane space, cytosol, P side, out

matrix, N side, in

e-

e-

subunit II ligands

subunit I ligands

~13 Å

22.3 Å

13.2 Å

4.9 Å

Cu - Cu = 2.20 Å

heme a

heme a 3

8 Å

8 H+

4 H+

2 H 2 O