BioPHYSICAL chemistry

I–IV (Figure 6.10). Complex I, also called NADH:ubiquinone oxidoreductase,
is an 850-kDa enzyme composed of over 40 protein subunits, including a
FMN-containing flavoprotein and several iron–sulfur cofactors. Complex I
catalyzes the conversion of NADH to NAD+, which is coupled to electron
transfer to ubiquinone and the pumping of protons from the matrix to
the intermembrane space:

NADH +5H+matrix+Q →NAD++QH 2 +4H+intermembrane (6.23)

Complex II, or succinate dehydrogenase, is
a 140-kDa enzyme that contains a num-
ber of cofactors. This enzyme couples the
electron transfer of succinate to fumarate
with the conversion of FAD to FADH 2
(Figure 6.11). In this reaction, electrons pass
from succinate through FAD and the iron–
sulfur cofactors to ubiquinone. Complexes I
and II, together with the proteins acyl-CoA
dehydrogenase, ETF:ubiquinone oxidoreductase, and glycerol 3-phosphate
dehydrogenase, produce a pool of reduced ubiquinone, QH 2 , that is re-
oxidized by complex III.
Complex III, also called the cytochrome bc 1 complex, is a 250-kDa protein
with 11 protein subunits and a number of hemes and iron–sulfur centers.
Complex III couples the transfer of electrons from the ubiquinones to cyto-
chrome cwith the accompanying transfer of protons from the matrix across
the membrane to the intermembrane space. The net oxidation/reduction
reaction, often termed the Q cycle, couples the transfer of electrons from
the ubiquinones to the transfer of protons across the cell membrane:

QH 2 +2cyt c 1 (oxidized) +2H+matrix
→Q +2cytc 1 (reduced) +4H+intermembrane (6.24)

Complex IV, also named cytochrome oxidase, completes the respiratory
chain. The size of complex IV varies in different organisms, from three or
four small protein subunits in bacteria to 13 in eukaryotic cells. Heme and
copper cofactors perform the overall four-electron reduction of oxygen
by a mechanism that is sequential without the release of intermediates:

4Cyt c(reduced) +8Hmatrix+ +O 2
→4cyt c(oxidized) +4H+intermembrane+2H 2 O (6.25)

In considering the net flow of electrons through the respiratory chain,
electrons from NADH reduce molecular oxygen according to:

(6.26)

NADH++ →H++O NAD +H O

1

2 22

CHAPTER 6 REDOX REACTIONS AND BIOENERGETICS 127

C

C

C

C

COO

Succinate Fumarate

COO H

COO H COO

H



H

H H

EFAD  E FADH 2

Figure 6.11In

respiration, succinate

is converted into

fumarate with the

involvement of FAD.