BIOINORGANIC CHEMISTRY A Short Course Second Edition

CYTOCHROME c OX IDASE 431

generates a transmembrane proton gradient by a different mechanism than
cytochrome bc 1 (see Section 7.6). CcO ’ s substrate, cytochrome c, discussed in
Section 7.7, donates electrons to CcO on the cytoplasmic side of the mitochon-
drial inner membrane (the intermembrane space). Electrons are transferred
from cyto c into CcO, fi rst to a bimetallic copper site, Cu A , then through a
heme iron cofactor, heme a (see Figure 7.38 ), then to CcO ’ s active site, which
contains a heme iron cofactor, heme a 3 (the subscript 3 denotes the site of
dioxygen binding in multiple heme enzymes), and a copper ion, Cu B. The
electrons are used to reduce O 2 to water. In the fully reduced form, all copper
ions would be in the Cu(I) oxidation state and both irons in the Fe(II) state.
In the fully oxidized form, after having transferred four electrons to dioxygen,
the Cu A site contains one Cu(II) ion and one Cu(I) ion (or a mixed - valence
Cu1.5+... Cu 1.5+ moiety; see discussion in Section 7.8.2 ), heme a and a 3 Fe(III)
ions, and a Cu(II) ion at Cu B.
Protons needed for production of two water molecules by cytochrome
c oxidase are taken from the mitochondrial matrix side through two
channels — the so - called D and K channels. These channels — named for the
conserved aspartic acid (D) and lysine (K) amino acids in their respective
channels — pump one proton per electron across the membrane. In contrast to
cytochrome bc 1 — which changes its conformation, depending on redox state
and ligand binding — CcO appears to be static in its tertiary structure. The
bovine enzyme, for example, shows only a minor conformational change when
undergoing reduction and consequent proton pumping — one loop in subunit
I facing the cytoplasm is affected. To understand the mechanistic details of the

Figure 7.38 Heme a structure.

N

N N

CO 2 H

N

Fe

O

H

HO

CO 2 H

heme a

formyl group

hydroxyl farnesylethyl substituent