{PDOC00171}
{PS51002; CYTB_NTER}
{PS51003; CYTB_CTER}
{BEGIN}
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* Cytochrome b/b6 profiles *
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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) (see <PDOC00177>)).  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  [1,2]  is  an integral membrane protein of approximately 400
amino  acid  residues  that  has  8 transmembrane segments and four horizontal
helices  on  the intermembrane side (see <PDB:1BE3; C>). The two hemes, bL and
bH,  are  in the center of a four alphahelical bundle formed by helices 1 to 4
[3].

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.

-Sequences known to belong to this class detected by the first profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Sequences known to belong to this class detected by the second profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Note: These profiles replace two patterns which sensitivity were inadequate.

-Last update: June 2004 / Patterns removed, profile added and text revised.

[ 1] Howell N.
     "Evolutionary conservation of protein regions in the protonmotive
     cytochrome b and their possible roles in redox catalysis."
     J. Mol. Evol. 29:157-169(1989).
     PubMed=2509716
[ 2] Esposti M.D., De Vries S., Crimi M., Ghelli A., Patarnello T., Meyer A.
     "Mitochondrial cytochrome b: evolution and structure of the protein."
     Biochim. Biophys. Acta 1143:243-271(1993).
     PubMed=8329437
[ 3] Iwata S., Lee J.W., Okada K., Lee J.K., Iwata M., Rasmussen B.,
     Link T.A., Ramaswamy S., Jap B.K.
     "Complete structure of the 11-subunit bovine mitochondrial cytochrome
     bc1 complex."
     Science 281:64-71(1998).
     PubMed=9651245

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