430 IRON-CONTAINING PROTEINS AND ENZYMES
enzyme (Complex IV) in a sequence of membrane - bound electron transfer
proteins in the mitochondrial respiratory chain. The respiratory chain, also
known collectively as mitochondrial membrane oxidative phosphorylation
enzymes (see Figure 1 of reference 78 ), provides much of the free energy
needed for the life processes of aerobic organisms by coupling energy released
in electron transport and proton translocation to the synthesis of adenosine
triphosphate (ATP). Electron equivalents are provided by cytochrome c
enzymes (see Section 7.7) that bind to CcO on the cytosolic side of the mem-
brane (intermembrane space). During the CcO - catalyzed reaction, protons are
pumped across the mitochondrial membrane barrier from the matrix side
toward the cytosolic side (intermembrane space). An electrochemical poten-
tial is thus generated, causing protons to fl ow back into the mitochondrial
matrix. The inward fl ux of protons is coupled to the synthesis (by ATP syn-
thases) of ATP from ADP and inorganic phosphate ( PO 43 −, P i ). Formally, one
can write the reaction catalyzed by CcO as
4442 cytoc^2 ++++ ⎯→O 2 H CcO⎯⎯ cytoc^3 ++ H O 2 (7.11)
Proton and electron balances are given by the equation
O 22 ++ 48 e−+H ()in → 2 H O+ 4 H+( )out (7.12)
Mammalian and yeast CcO complexes contain 13 subunits with a total
molecular mass of approximately 204 kDa. Three subunits are encoded by
mitochondrial genes (subunits I – III), and 10 subunits are encoded by nuclear
genes (subunits IV – XIII). The discussion here will concentrate on subunits I –
III, but all subunits are described in more detail in reference 137b. Figure 3 of
reference 138 displays an overall view of the cytochrome c oxidase complex
with CcO subunits in different colors. Bacterial CcO complexes consist of two,
or at most three, subunits — I, II, III — and these contain the catalytic sites for
all CcOs. Subunit I, located mainly in the transmembrane region and consisting
of 12 transmembrane helices, contains the ligating amino acid residues for the
heme a, heme a 3 , and Cu B sites. Subunit II exhibits a large extramembrane
domain that resides above the cytosolic surface of subunit I. Subunit II ’ s 10 -
strand β barrel contains the ligating aa residues for the bimetallic Cu A site, in
the cytoplasm just above the membrane surface. Subunit II also provides
surface contact for cytochrome c binding through acidic amino acid residues
glu109, asp119, glu127, asp139, glu157, and asp158. Subunit I also provides
cytochrome c contacts via asp50 and asp221, and subunit VIb provides them
via asp74 and glu78. The other subunits, IV – XIII, are believed to be important
for assembly of the complex and regulation of enzyme activity. One website
dedicated to cytochrome c oxidase is found at http://www - bioc.rice.edu/
∼ graham/CcO.html.
Cytochrome c oxidase (CcO) has been characterized as a “ proton pump ”
(see, for instance, M. Saratse ’ s 1999 Science magazine article^78 ), although CcO