408 IRON-CONTAINING PROTEINS AND ENZYMES
7.6.6 Cytochrome bc 1 Conclusions,
Overall, the conformation of the cytochrome bc 1 complex does not change
appreciably in the absence or presence of substrate or inhibitors except for
the movement of the ISP protein and its [2Fe – 2S] cluster. Researchers looking
for other large changes in tertiary and quaternary structure in the cytochrome
bc 1 complex based on the absence or presence of substrates or inhibitors or
the oxidation state of iron in the heme and [2Fe – 2S] centers must continue
their search. Although a detailed discussion on the cytochrome bc 1 complex
has preceded these conclusions, much has been left out of these sections as
well. For instance, readers may be interested in the large body of knowledge
available for yeast cytochrome,^95 including one structure of the cytochrome
bc 1 complex with its redox partner cytochrome c attached.^96 An X - ray crystal-
lographic structure of bacterialRhodobacter capsulatus cytochrome bc 1 has
been published by the E. A. Berry group.^97 An up - to - date website at http://
sb20.lbl.gov/cytbc1/ lists all the cytochrome bc 1 structures currently in the
Protein Data Bank. A. R. Crofts maintains a cytochrome bc 1 webpage at
http://www.life.uiuc.edu/crofts/bc-complex_site/ containing detailed informa-
tion, graphic displays, movies, and links to other useful bc 1 sites. A review
focusing solely on the mechanism of ubiquinol oxidation by the cytochrome
bc 1 complex has been published recently.^98 Lastly, the reader should remember
that X - ray crystallographic structures are frozen moments in time and space.
While much of the information gained from these structures cannot be accessed
in any other manner at present, confi rmation of structure, function, and mecha-
nism should be always be sought through other experimental means.
7.7 Cytochromes c,
7.7.1 Introduction,
Cytochromes c (see Figure 7.25B ) are ubiquitous in nature and have attributes
that make them one of the most studied proteins in bioinorganic research. The
protein ’ s relatively small size ( ∼ 12.5 kDa), thermodynamic stability, high solu-
bility, and high helical content, as well as the presence of a spectroscopically
accessible iron - containing heme cofactor, have made it a popular study target
for biochemists and biophysicists. Cytochromes c are found outside the cyto-
plasmic membrane of prokaryotes, in the intermembrane space of eukaryotic
mitochondria, or in the lumen of chloroplasts.^99 In eukaryotes, cytochrome c
mediates single electron transfer between mitochondrial inner - membrane
enzymes cytochrome bc 1 (Section 7.6) and cytochrome c oxidase (Section 7.8),
which are complexes III and IV, respectively, of the respiratory chain. Cyto-
chromes c are characterized by the attachment of the heme c cofactor to the
protein chain through thioether linkages provided by two cysteinyl residues
covalently bonded to vinyl substituents of the heme porphyrin ligand (see
Figures 7.25 and 7.32A ).