Page 149 - Color Atlas of Biochemistry

P. 149

140 Metabolism

Respiratory chain zyme-bound FADH 2 and the electron-trans-
porting flavoprotein (ETF; see p. 164). Ubiq-
The respiratory chain is one of the pathways uinol passes electrons on to complex III, which
involved in oxidative phosphorylation (see transfers them via two b-type heme groups,
p. 122). It catalyzes the steps by which elec- one Fe/S cluster, and heme c 1 to the small
+
trons are transported from NADH+H or re- heme protein cytochrome c.Cytochrome c
duced ubiquinone (QH 2 )to molecular oxygen. then transports the electrons to complex
Dueto the wide differencebetween theredox IV—cytochrome c oxidase. Cytochrome c oxi-
+
potentials of the donor (NADH+H or QH 2 ) dase contains redox-active components in the
and the acceptor (O 2 ), this reaction is strongly form of two copper centers (Cu A and Cu B )and
exergonic (see p. 18). Most of the energy re- hemes a and a 3 , through which the electrons
leased is used to establish a proton gradient finally reach oxygen (see p. 132). As the result
across the inner mitochondrial membrane of the two-electron reduction of O 2 ,the
(see p. 126), which is then ultimately used to strongly basic O 2– anion is produced (at least
synthesize ATP with the help of ATP synthase. formally), andthisisconverted into waterby
binding of two protons. The electron transfer
is coupled to the formation of a proton gradi-
A. Components of the respiratory chain
ent by complexes I, III, and IV (see p. 126).
The electron transport chain consists of three
protein complexes (complexes I, III, and IV),
which are integrated into the inner mitochon- B. Organization
drial membrane, and two mobile carrier mol- Proton transport via complexes I, III, and IV
ecules—ubiquinone (coenzyme Q) and cyto- takes place vectorially from the matrix into
chrome c. Succinate dehydrogenase, which ac- the intermembrane space. When electrons
tually belongs to the tricarboxylic acid cycle, are being transported through the respiratory
+
is also assigned to the respiratory chain as chain, the H concentrationinthis space in-
complex II. ATP synthase (see p. 142) is some- creases—i. e., the pH value there is reduced by
times referred to as complex V,although it is about one pH unit. For each H 2 Omolecule
+
not involved in electron transport. With the formed, around 10 H ions are pumped into
exception of complex I, detailed structural in- the intermembrane space. If the inner mem-
formation is now available for every complex brane is intact, then generally only ATP syn-
of the respiratory chain. thase (see p. 142) can allow protons to flow
All of the complexes in the respiratory back into the matrix. This is the basis for the
chain are made up of numerous polypeptides coupling of electron transport to ATP synthe-
and contain a series of different protein sis, which is important for regulation pur-
bound redox coenzymes (see pp. 104, 106). poses (see p. 144).
These include flavins (FMN or FAD in com- As mentioned, although complexes I
plexes I and II), iron–sulfur clusters (in I, II, through V are all integrated into the inner
and III), and heme groups (in II, III, and IV). membrane of the mitochondrion, they are not
Of themorethan 80polypeptides in the res- usually in contact with one another, since the
piratory chain, only 13 are coded by the mi- electrons are transferred by ubiquinone and
tochondrial genome (see p. 210). The remain- cytochrome c. With its long apolar side chain,
der are encodedbynuclear genes, andhaveto ubiquinone is freely mobile within the mem-
be imported into the mitochondria after brane. Cytochrome c is water-soluble and is
being synthesized in the cytoplasm (see located on the outside of the inner membrane.
p. 228). NADH oxidation via complex I takes place
Electrons enter the respiratory chain in var- on the inside of the membrane—i. e., in the
ious different ways. In the oxidation of matrix space, where the tricarboxylic acid
+
NADH+H by complex I, electrons pass via cycle and β-oxidation (the most important
FMN and Fe/S clusters to ubiquinone (Q). Elec- sources of NADH) are also located. O 2 reduc-
trons arising during the oxidation of succinate, tion and ATP formation also take place in the
acyl CoA, and other substrates are passed to matrix.
ubiquinone by succinate dehydrogenase or
other mitochondrial dehydrogenases via en-

144 145 146 147 148 149 150 151 152 153 154