CYTOCHROME bc 1 : A BACTERIAL CYTOCHROME 391
heart bc 1 complexes^79 (PDB: 1QCR) and for chicken heart cytochrome bc 184
(PDB: 1BCC, 3BCC). In the P6 5 22 form (PDB: 1BE3), the average distances
are [2Fe – 2S] – heme c 1 = 15.5 Å and [2Fe – 2S] – heme b L = 35.5 Å , while in the
P6 5 structure (PDB: 1BGY) the average distances are 27.5 and 31.0 Å , respec-
tively. In addition, there is a hydrogen bond between the his161 ligand of the
ISP and a propionate of heme c 1 in the PDB: 1BE3 crystal. The authors pos-
tulate that the 15.5 - Å distance and hydrogen bonding facilitate fast electron
transfer between the [2Fe – 2S] cluster and cytochrome c 1.
A second research group postulating the movement of the [2Fe – 2S] domain
is that of Edward A. Berry.^84 This group ’ s X - ray crystallographic structures of
chicken heart cytochrome bc 1 as the native enzyme (PDB: 1BCC) and the
enzyme in the presence of co - crystallized inhibitors stigmatellin and antimycin
A (PDB: 3BCC) showed decidedly different positions for the Rieske iron –
sulfur protein (ISP). The native enzyme, PDB: 1BCC, exhibits a shorter dis-
tance between the [2Fe – 2S] center and heme c 1 than that for the enzyme with
co - crystallized inhibitors, PDB: 3BCC. Conversely, the distance between the
[2Fe – 2S] center and heme b L is longer for the native enzyme than that for
PDB: 3BCC. The distance data are collected in Table 7.6. Figure 1 of reference
84 illustrates the change graphically.
7.6.3 Cytochrome bc 1 Metal Cofactor Specifi cs,
Every cytochrome bc 1 complex contains three subunits with active redox
centers: (1) cytochrome c 1 ; (2) cytochrome b; and (3) the “ Rieske ” [2Fe – 2S]
iron – sulfur protein (ISP). Most researchers studying the structure and func-
tion of the complex believe that the ISP is a mobile domain and moves about
a pivot point during the catalytic cycle, possibly to facilitate electron transfer.
The electron - carrying moieties of cytochrome bc 1 involve the following: (1)
the Fe(II) – Fe(III) centers, in order through the bc 1 complex from the inter-
membrane space (p side), of (a) heme c 1 in cytochrome c 1 , (b) the high poten-
tial [2Fe – 2S] center of the Rieske iron – sulfur protein (ISP) of the opposite
monomer, and (c) the b L (also called b 565 ) and the b H (also called b 562 ) hemes
of cytochrome b; and (2) ubiquinone/ubiquinol. Cytochrome c 1 , containing
heme c 1 , is an all α - helix protein resembling other members of the Type I
cytochrome c family. In PDB: 1BE3, the helices are named: α 1, residues 23 – 38;
α 1 ′ , 57 – 73; α 1 ′′ , 97 – 106; α 2, 114 – 119; α 3, 123 – 132; α 5, 178 – 195; α 6, 197 – 238.
These correspond to the numbering system used for cytochrome c; however,
theα 4 cytochrome c helix appears to be replaced by a distorted double -
stranded β - sheet. This and other insertions and deletions are unique to cyto-
chrome c 1 and are believed to related to its specifi c functions. One of these
functions is to provide a binding surface for cytochrome c to which it transfers
electrons out of the cytochrome bc 1 complex. Helix α 1 contains at least eight
acidic aa residues that help form a large negative docking area for the highly
basic cytochrome c. Using the numbering system found for residues in the
bovine enzyme (PDB: 1BE3, for instance), heme c 1 ’ s iron ion ligands are his41