Page 141 - Color Atlas of Biochemistry

P. 141

132 Metabolism

Molecular models: are taken up by two channels (D and K, not
membrane proteins shown). The mechanism that links proton
transport to electron transfer is still being
The plates show, in simplified form, the struc- investigated.
tures of cytochrome c oxidase (A;complex IV
of the respiratory chain) and of photosystem I B. Reaction center of Synechococcus
of a cyanobacterium (B). These two molecules elongatus
are among the few integral membrane pro-
teins for which the structure is known in de- Photosystem I (PS I) in the cyanobacterium
tail. Both structures were determined by X- Synechococcus elongatus is the first system of
ray crystallography. this type for which the structure has been
solved in atomic detail. Although the bacterial
photosystem differs slightly from the systems
in higher plants, the structure provides val-
A. Cytochrome c oxidase
uable hints about the course of the light re-
Theenzymecytochrome c oxidase (“COX,” EC actions in photosynthesis (see p. 128). The
1.9.3.1) catalyzes the final step of the respira- functioning of the photosystem is discussed
tory chain. It receives electrons from the small in greater detail on p. 130.
heme protein cytochrome c and transfers The functional form of PS I in S. elongatus
them to molecular oxygen, which is thereby consists of a trimer with a mass of more than
6
reduced to water (see p. 140). At the same 10 Da that is integrated into the membrane.
time, 2–4 protons per water molecule formed Only one of the three subunits is shown here.
are pumped from the matrix into the inter- This consists of 12 different polypeptides
membrane space. (gray-blue), 96 chlorophyll molecules (green),
Mammalian COX (the illustration shows 22 carotenoids (orange), several phylloqui-
theenzymefrom bovineheart)is a dimer nones (yellow), and other components. Most
that has two identical subunits with masses of the chlorophyll molecules are so-called an-
of 204 kDa each. Only one subunit is shown in tenna pigments. These collect light energy
detail here; the other is indicated by gray and conduct it to the reaction center,which
lines. Each subunit consists of 13 different is located in the center of the structure and
polypeptides, which all span the inner mito- therefore not visible. In the reaction center, an
chondrial membrane. Only polypeptides I electron is excited and transferred via various
(light blue) and II (dark blue) and the linked intermediate steps to a ferredoxin molecule
cofactors are involved in electron transport. (see p. 128). The chlorophylls (see formula)
The other chains, which are differently ex- are heme-like pigments with a highly modi-
pressed in the different organs, probably fied tetrapyrrole ring, a central Mg 2+ ion, and
have regulatory functions. The two heme an apolar phytol side chain. Shown here is
groups, heme a (orange) and heme a 1 (red) chlorophyll a, which is also found in the re-
are bound in polypeptide 1. The copper center action center of the S. elongatus photosystem.
Cu A consists of two copper ions (green), The yellow and orange-colored carot-
which are coordinated by amino acid residues enoids—e. g., E-carotene (see formula)—are
in polypeptide II. The second copper (Cu B )is auxiliary pigments that serve to protect the
located in polypeptide I near heme a 3 . chloroplasts from oxidative damage. Danger-
To reduce an O 2 molecule to two molecules ousradicalscan be produced during the light
of H 2 O, a total of four electrons are needed, reaction—particularly singlet oxygen.Caroten-
which are supplied by cytochrome c (pink, top oids prevent compounds of this type from
left) and initially given off to Cu A .From there, arising, or render them inactive. Carotenoids
they are passed on via heme a and heme a 3 to are also responsible for the coloring of leaves
the enzyme’s reaction center, which is located seen during fall. They are left behind when
between heme a 3 and Cu B .The reduction of plants break down chlorophyll in order to
the oxygen takes place in several steps, with- recover the nitrogen it contains.
out any intermediate being released. The four
protons needed to produce water and the H +
ions pumped into the intermembrane space

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