Member database | PROSITE profiles |
PROSITE profiles type | domain |
Short name | CYTB_CTER |
Description
In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is
a component of respiratory chain complex III (EC 1.10.2.2) - also known as the
bc1 complex or ubiquinol-cytochrome c reductase. This complex is the middle
component of the mitochondrial respiratory chain, coupling the transfer of
electrons from ubihydroquinone to cytochrome c with the generation of a proton
gradient across the mitochondrial membrane. Every bc1 complex contains three
common subunits with active redox centers (cytochrome b, cytochrome c1, and
the "Rieske" [2Fe-2S] protein (ISP)). The mitochondrial
system contains additional subunits not present in the bacterial complexes. In
plant chloroplasts and cyanobacteria, there is a analogous protein of
cytochrome b, cytochrome b6, a component of the plastoquinone-plastocyanin
reductase (EC 1.10.99.1), also known as the b6f complex.
Cytochrome b/b6
[3][1] is an integral membrane protein of approximately 400
amino acid residues that has 8 transmembrane segments and four horizontal
helices on the intermembrane side. The two hemes, bL and
bH, are in the center of a four alphahelical bundle formed by helices 1 to 4
[2].
In plants and cyanobacteria, cytochrome b6 consists of two subunits encoded by
the petB and petD genes. The sequence of petB is colinear with the N-terminal
part of mitochondrial cytochrome b, while petD corresponds to the C-terminal
part. Cytochrome b/b6 non-covalently binds two heme groups, known as b562 and
b566. Four conserved histidine residues are postulated to be the ligands of
the iron atoms of these two heme groups.
Apart from regions around some of the histidine heme ligands, there are a few
conserved regions in the sequence of b/b6. The best conserved of these regions
includes an invariant P-E-W triplet which lies in the loop that separates the
fifth and sixth transmembrane segments. It seems to be important for electron
transfer at the ubiquinone redox site - called Qz or Qo (where o stands for
outside) - located on the outer side of the membrane.
A schematic representation of the structure of cytochrome b/b6 is shown below.
+---Fe-b562----+
| +---Fe-b566--|-+
| | | |
xxxxxxxxxxxHxHxxxxxxxxxxxxHxHxxxxxxxxxxPEWxxxxxxxxxxxxxxxxxx
{------------------Cytochrome-b----------------------------}
{----Cytochrome-b6-petB---------}{--Cytochrome-b6-petD-----}
We developed two profiles for cytochrome b/b6, one that spans the N-terminal
region and also recognizes the petB subunit of plant b6 complex; the other
profile is directed against the C-terminal region and recognizes also the
plant petD subunit.
References
1.Mitochondrial cytochrome b: evolution and structure of the protein. Esposti MD, De Vries S, Crimi M, Ghelli A, Patarnello T, Meyer A. Biochim. Biophys. Acta 1143, 243-71, (1993). View articlePMID: 8329437
2.Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex. Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK. Science 281, 64-71, (1998). View articlePMID: 9651245
3.Evolutionary conservation of protein regions in the protonmotive cytochrome b and their possible roles in redox catalysis. Howell N. J. Mol. Evol. 29, 157-69, (1989). View articlePMID: 2509716