section 14.1
Mitochondrial Structure and Properties
255
H
I
O
H
A A
h
/ y **
H
o
CH3
0
II
1
II
^ l ~ H
-0 —0
JD— c — CH— CH
H ^
(X)
O
p H
c - H
O—
C-------c — (CH2)— CH3
H
-CH3
'CH3
FIGURE 14-9
Structure of antimycin A, an antibiotic that inhibits electron transport from
CoQ to cytochrome c.
to oxygen in complex IV. The structure of the heme pros-
thetic group (iron-protoporphyrin IX) in cytochromes b,
c, and ci is the same as that present in hemoglobin and
myoglobin but differs from the heme group (heme A) of
cytochromes a and a
3
(Figure 14-10). The heme groups
in cytochromes c and Ci are covalently linked to the
apoprotein by thioether bonds between sulfhydryl groups
of two cysteine residues and the vinyl groups of the
heme. The heme of cytochromes b and a
3
is bound by
strong hydrophobic interactions between the heme and the
apoprotein.
Cytochromes b, ci, a, and a
3
are integral membrane pro-
teins, whereas cytochrome c is a peripheral protein located
on the C side of the membrane and is easily isolated from
mitochondria. The amino acid sequence of cytochrome
c has been established for a wide variety of species and
CH2
CH3
ÇH
o
Heme
(iron-protoporphyrin IX)
consists of 100-113 amino acid residues (M.W. 13,000).
Cytochrome c has played an important role in our un-
derstanding the evolutionary relationships among species.
Twenty-eight residues are invariant among 67 species se-
quenced, presumably because a hydrophobic environment
around the heme appears to be essential. Amino acids
in other positions vary from one species to another and
reflect the times of divergence of the different species.
For example, only two amino acid residues differ in duck
and chicken, but 48 differences are found between horse
and yeast cytochrome c, consistent with their long, diver-
gent evolutionary history. Cytochrome c’s are identical in
chicken and turkey, and in pig, cow, and sheep.
Complex IV
Complex IV, also called cytochrome c oxidase, is the
terminal component of the respiratory chain. It consists of
four redox centers: cytochrome a, cytochrome a3, and two
Cu ions. Like the iron atoms, the copper ions function as
one-electron carriers:
Cu2+ + e“ ^ Cu+
Cytochrome c transfers electrons from cytochrome c1;
the terminal component of complex III, to the four redox
centers of the cytochrome oxidase complex. The transfer
of four electrons from each of the four redox centers of
the cytochrome oxidase complex to an oxygen molecule
C = 0
O
Heme A
FIGURE 14-10
Structure of heme (present in cytochromes b, c, and cj ) and of heme A
(present in cytochromes a and a3).
