254
chapter 14
Electron Transport and Oxidative Phosphorylation
COO"
1
COO“
I
9
o
cn2
1
1
l \
coo
L-c—CH-
II
-c—CF3
II
coo
0
0
(a)
(b)
(C)
F I G U R E 1 4 -7
Inhibitors of complex II. (a) Oxaloacetate, (b) malonate, (c) thenoyltrifluoroacetone, and (d) carboxin.
The hydrogens are accepted by FAD, which is covalently
bound to the apoprotein via a histidine residue. In many
flavoproteins, the flavin nucleotide is bound to the apopro-
tein not covalently but rather via ionic linkages with the
phosphate group. The reducing equivalents of FADH
2
are
passed on to coenzyme Q (CoQ or Q) via the iron-sulfur
centers. Thus, the overall reaction catalyzed by complex
II is
S u c c in a te + FA D — (F e — S )n— E
F u m a ra te + FADH2— (F e — S )„— E
'------------------- Q
FA D — (F e — S )n— E + Q H 2
During the terminal stages of electron transfer in com-
plex II, cytochrome bssg is involved; however, its specific
function is not understood. Oxaloacetate and malonate
are competitive inhibitors of succinate dehydrogenase and
compete with the substrate for binding at the active site
(Chapters
6
and 13). Carboxin and thenoyltrifluoroace-
tone (Figure 14-7) inhibit electron transfer from FADH
2
to CoQ.
Complex III
Complex III contains cytochromes bs
6 2
and bs66,
(collectively called cytochrome b), cytochrome C], and an
iron-sulfur protein. Complex III catalyzes the transport of
reducing equivalents from coQ to cytochrome c:
QH
2
+ 2Cyt c (Fe3+) -* Q + 2Cyt c (Fe2+) + 2H+
Coenzyme Q, also called
ubiquinone
because of its
ubiquitous occurrence in microorganisms, plants, and an-
imals, is lipid-soluble and not tightly or covalently linked
to a protein, although it carries out its electron transport
function together with specific CoQ-binding peptides. It
plays a central role in the electron transport chain be-
cause it collects reducing equivalents from NADH- and
FADH
2
-linked dehydrogenases and passes them on to
the terminal cytochrome system. CoQ is a substituted
1,4-benzoquinone containing a polyisoprenoid side chain
at Cf, (Figure 14-8). In bacteria, CoQ usually contains six
isoprenoid units (Qe), whereas in most mammalian mito-
chondria it has ten (Qio). The reduction of Q to QH
2
(a
hydroquinone) requires two electrons and two protons and
probably occurs via a one-electron intermediate:
Q ,
»
OH'
i
-—
-> QH2
where the dot associated with QH represents an unpaired
electron (a free radical). Antimycin A (a
Streptomyces
antibiotic) inhibits the transfer of electrons from QH
2
to
cytochrome c (Figure 14-9).
Cytochrome c transfers electrons from complex III to
complex IV, and cytochromes a and
3 3
transfer electrons
O
CH,
HJC—Ox'C'v C^'C
CH2—CH=C—CH;
— H
+2H
5 h"
OH
HjC---0„
/ C :
A
^Ç—CH.
HO—o
^ c -
CH3
I
CH.—CH=C—CHj
— H
10
o
OH
C o e n z y m e Q (u b iq u in o n e)
(C o Q o r Q )
H y d ro q u in o n e (ubiquinol)
(CoQHjOrQHj)
F I G U R E 1 4 -8
Structure and redox reaction of coenzyme Q. In most mammalian tissues, CoQ has 10 isoprenoid units. CoQ collects
reducing equivalents from NADH dehydrogenase and from other flavin-linked dehydrogenases.
