386 IRON-CONTAINING PROTEINS AND ENZYMES
heme b n and heme x, hemes that are contained within the cytochrome b
portion of the complex. Hemes b p , b n , and x reside completely within the
membrane and within the cytochrome b 6 protein subunit. An uptake of protons
from the negative (n or stromal) side of the membrane results in a proton
electrochemical gradient across the membrane. Figure 2 of the reference 73
also illustrates the process well.
The cytochrome b(6)f electron transfer and proton translocation functions
are similar to those performed by the respiratory cytochrome bc 1 complex
described in Section 7.6. In fact, cytochrome f is functionally, but not structur-
ally, analogous to the high - potential cytochrome c 1 in bc 1 ; the two b hemes, b p ,
and b n , are analogous to the b H and b L hemes of bc 1 ; and the Rieske ISP centers
are similar in the two cytochromes. The cytochrome b(6)f PQH 2 → PQ reac-
tion has its counterpart in the dihydroubiquinone→ ubiquinone reaction of
bc 1. In both cases the lipophilic quinone species from the bulk bilayer phase
move between reduction and proton uptake sites on the negative (n) side of
the membrane to sites for oxidation and proton release on the positive (p)
side. This cycling is known as the Q (quinone) - cycle mechanism. Overall the
core of the b(6)f complex is similar to the analogous respiratory cytochrome
bc 1 complex, but the domain arrangement outside the core as well as the
arrangement of prosthetic groups (hemes and ISP) differ considerably.
7.5.2 Cytochrome b(6)f Metal Cofactor Specifi cs,
Starting with the extrinsic portion, outside the membrane, of the cytochrome
b(6)f complex, one fi rst encounters heme f, which is a cytochrome c - type heme.
(See Figure 7.25B .) It carries one his26 ligand to its iron center. In keeping
with other cytochrome c types, it is attached to the cytochrome f protein via
two thioether linkages — cys22 and cys25 in the PDB: 1VF5 X - ray crystallo-
graphic structure. This typical cytochrome c - type connector also provides an
axial histidine ligand in a common amino acid pattern: CXXXH — that is,
C 22 XXC 25 H 26. Cytochrome f differs from c - type cytochromes in that it has
an abnormal second axial ligand, the amide - N backbone nitrogen of the
N - terminal tyrosine, tyr1. Next in the electron transfer chain, one fi nds the
Rieske - type [2Fe – 2S] cluster, approximately 29 Å distant from heme f toward
the membrane lumen (p) side, outside but very close to the membrane surface.
The 29 - Å distance is believed to be too long for effi cient electron transfer
between cytochrome f and the [2Fe – 2S] cluster. However, the entire iron –
sulfur protein (ISP) is mobile, as will be discussed in more detail for cyto-
chrome bc 1 below, so that electron transfer could take place under other
conformations. The [2Fe – 2S] cluster has bonding contacts with his110, his129,
cys108, and cys126 (see Table 7.5 ). Hemes b p , b n , and x reside in the cytochrome
b 6 protein within the membrane. Heme b p , nearer the lumen (p) side, is held
in place by his86 and his187, of the cytochrome b 6 B and D helices. The iron
ion in heme b n is attached to the protein via the imidazole nitrogens of two
histidine ligands, his100 and his202 from cytochrome b 6 ’ s B and D helices. The