Page 151 - Color Atlas of Biochemistry

P. 151

142 Metabolism

ATP synthesis form complexes with inorganic sulfide and
the SH groups of cysteine residues (see
In the respiratory chain (see p. 140), electrons p. 286). Ubiquinone (coenzyme Q; see
are transferred from NADH or ubiquinol (QH 2 ) p. 104) is a mobile carrier that takes up elec-
to O 2 . The energy obtained in this process is trons from complexes I and II and from re-
used to establish a proton gradient across the duced ETF and passes them on to complex III.
inner mitochondrial membrane. ATP synthe- Heme groups are also involved in electron
sis is ultimately coupled to the return of pro- transport in a variety of ways. Type b hemes
tons from the intermembrane space into the correspond to that found in hemoglobin (see
matrix. p. 280). Heme c in cytochrome c is covalently
bound to the protein, while the tetrapyrrole
ring of heme a is isoprenylated and carries a
A. Redox systems of the respiratory chain
formyl group. In complex IV, a copper ion (Cu B )
The electrons provided by NADH do not reach and heme a 3 react directly with oxygen.
oxygen directly, but instead are transferred to
it in various steps. They pass through at least
10 intermediate redox systems, most of which B. ATP synthase
are bound as prosthetic groups in complexes The ATP synthase (EC 3.6.1.34, complex V) that
+
I, III, and IV. The large number of coenzymes transports H is a complex molecular ma-
involved in electron transport may initially chine. The enzyme consists of two parts—a
appear surprising. However, as discussed on proton channel (F o , for “oligomycin-sensitive”)
p. 18, in redox reactions, the change in free that is integrated into the membrane; and a
enthalpy ∆G—i. e., the chemical work that is catalytic unit (F 1 ) that protrudes into the ma-
done—depends only on the difference in re- trix. The F o part consists of 12 membrane-
dox potentials ∆E between the donor and the spanning c-peptides and one a-subunit. The
acceptor. Introducing additional redox sys- “head” of the F 1 part is composed of three α
tems does not alter the reaction’s overall en- and three β subunits, between which there
ergy yield. In the case of the respiratory chain, are three active centers. The “stem” between
the difference between the normal potential F o and F 1 consists of one γ and one ε subunit.
+
+
of the donor (NAD /NADH+H ,E 0 =–0.32 V) Two more polypeptides, b and δ,form a kind
and that of the acceptor (O 2 /H 2 O, E 0 = of “stator,” fixing the α and β subunits relative
+0.82 V) corresponds to an energy difference to the F o part.
–1
∆G 0 of more than 200 kJ mol .This large The catalytic cycle can be divided into
amount is divided into smaller, more three phases, through each of which the three
manageable “packages,” the size of which is active sites pass in sequence. First, ADP and P i
determined by the difference in redox poten- are bound (1), then the anhydride bond forms
tials between the respective intermediates.It (2), and finally the product is released (3).
is assumed that this division is responsible for Each time protons pass through the F o chan-
the astonishingly high energy yield (about nel protein into the matrix, all three active
60%) achieved by the respiratory chain. sites change from their current state to the
The illustration shows the important redox next. It has been shown that the energy for
systems involved in mitochondrial electron proton transport is initially converted into a
transport and their approximate redox poten- rotation of the γ subunit, whichin turncycli-
tials. These potentials determine the path fol- cally alters the conformation of the α and β
lowed by the electrons, as the members of a subunits, which are stationary relative to the
redox series have to be arranged in order of F o part, and thereby drives ATP synthesis.
increasing redox potential if transport is to
occur spontaneously (see p. 32).
In complex 1, the electrons are passed from
+
NADH+H first to FMN (see p. 104) and then
on to several iron–sulfur (Fe/S) clusters. These
redox systems are only stable in the interior
of proteins. Depending on the type, Fe/S clus-
ters may contain two to six iron ions, which

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