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Cytochromes and cytochrome-c oxidase

Cyt-b und Cyt-c, and the cytochromes-a and -a 3 of the cytochrome-c oxidase contain haeme
type iron centres. They are distinct by their substitution pattern at the porphyrin ring and the
axial ligands; see Fig. 12. They transfer electrons, moving between the iron oxidation states +II
and +III. Cytochromes may also attain other than simple electron transfer functions. An
example is cytochrome P 450, an oxygenase in which, during turn-over, iron passes through the
oxidation state +IV (and, perhaps, +V).

R 3 L 2 CH 3 Cyt-a: R 1 = vinyl, R 2 = C 17 H 34 OH, R 3 = formyl
L 1 = L 2 = His
R 2
HO 2 (CH 2 ) 2 Cyt-b: R 1 = R 2 = vinyl, R 3 = methyl
N N
L 1 and L 2 free or His
Fe
N N Cyt-c: R 1 = R 2 = CH(CH 3 )-S-CH 2 -C(O)NH, R 3 = CH 3
H 3 C CH 3
L 1 = His, L 2 = Met
Cyt P 450 : R 1 = R 2 = vinyl, R 3 = methyl
R 1
L 1 L 1 = Cys, L 2 = H 2 O (resting form) or free (active form)
HO 2 (CH 2 ) 2
Mb and Hb: R 1 = R 2 = vinyl, R 3 = methyl

L 1 = His, L 2 = free or O 2

Figure 12 . The active centres of selected haeme-type proteins. For Cyt-P 450, see the chapter on
oxigenases.

Cyt-c

e
(Cys)
(His) N S S (Met) N
Cu A Cu HO
N (His) N
+ S
H O (Cys) e
außen N Cu N NH Cu B
HN
(His) N Fe N (His)
a
ca 5 A
Membran
N N
e N Fe Cyt-a
(His) N 3
N
innen (His) N Cu B (O2) Fe (His)
H + a N N
N 3
(His)
N
H

Figure 13. Organisation of the redox-active centres of cytochrome-c oxidase (left). Oxygen
B
activation and reduction occurs at the dinuclear Cu ⋅⋅⋅Cyt-a 3 pair (see expansion to the right).