{PDOC00169}
{PS51007; CYTC}
{PS51008; MULTIHEME_CYTC}
{PS51009; CYTCII}
{PS51010; CYTF}
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* C-type cytochrome superfamily profiles *
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In  proteins  belonging to the c-type cytochrome family [1], the heme group is
covalently  attached  by  thioether  bonds  to two conserved cysteine residues
located  in  the  cytochrome  c  center.  Cytochromes  c typically function in
electron  transfer, but c-type cytochrome centers are also found in the active
sites  of  many  enzymes,  and  in  eukaryotic  cells, cytochrome c has also a
role in apoptosis [2].

The known structures of c-type cytochromes have six different classes of fold.
Of  these,  four are unique to c-type cytochromes [3]. The different folds are
detailed in the example list below.

The  consensus  sequence for the cytochrome c center is Cys-x-x-Cys-His, where
the  histidine  residue  is one of the two axial ligands of the heme iron [4].
This arrangement is shared by all proteins known to belong to the cytochrome c
family, which presently includes:

Monoheme proteins:

 - Cytochrome  c,  an  electron  carrier protein located in the mitochondrial
   matrix.  Cytochrome  c  is a globular protein with an all alpha-helice fold
   (see <PDB:1HRC>).
 - Cytochrome c1. This is the heme-containing component of the cytochrome b-c1
   complex,   which  accepts  electrons  from  Rieske  protein  and  transfers
   electrons  to cytochrome c in the mitochondrial respiratory chain.
 - Bacterial  class  II  cytochromes  c  (c'  and  c556).  Cytochrome  c' is a
   high-spin  protein  and  is  the  most  widely  occurring  bacterial c-type
   cytochrome.  Cytochrome  c556  is  a  low-spin  cytochrome  .  Both  have a
   C-terminal  c-type  cytochrome center. Class II cytochromes are composed of
   four alpha helices (see <PDB:1CPR>).
 - Cytochromes c2, c5 and c6.
 - Bacterial cytochromes c550 to c553 and c555.
 - Chloroplast  and cyanobacteria cytochrome f. It translocates protons across
   the  thylakoid  membrane  and  transfers  electrons  from photosystem II to
   photosystem  I.  Structurally,  cytochrome  f is unique in the cytochrome c
   family as it is an all beta-strand fold (see <PDB:1CTM>).
 - Bacteria cytochrome c oxidase, mono-heme subunit, FixO.

Multiheme   proteins   (prokaryotes).   They  are  frequently  associated  with
electron-transport processes within the nitrogen and sulphur cycles:

 - Cytochrome  c  nitrite  reductase, each  monomere contains five heme groups
   clustered in a pseudo-two fold structure (see <PDB:1QDB>).
 - Cytochrome   c3.  It  participates  in  sulfate  respiration  coupled  with
   phosphorylation  by transferring electrons from the enzyme dehydrogenase to
   ferredoxin. It binds 4 heme groups per subunit.
 - Cytochrome c4. It binds 2 heme groups per subunit.
 - Cytochromes  cc3/Hmc  (High-molecular-weight  cytochrome  c), binds 16 heme
   groups per subunit.
 - Purple bacteria photosynthetic reaction center. It binds four heme groups.
 - HAO   (hydroxylamine   oxidoreductase).   It  catalyzes  the  oxidation  of
   hydroxylamine  to  nitrite.  The  electrons  released  in  the reaction are
   partitioned  to  ammonium  monooxygenase  and  to the respiratory chain. It
   binds eight heme groups per subunit.
 - Cytochrome  c554.  It  is  the immediate acceptor of electrons from HAO. It
   binds four heme groups per subunit.
 - Flavocytochrome  fumarate.  It  catalyzes unidirectional fumarate reduction
   using  artificial  electron  donors  such as methyl viologen. It binds four
   heme groups per subunit.

To  recognize  c-type cytochrome family proteins we have developed 4 profiles.
The  first one recognizes all mono-heme cytochrome c proteins (except class II
and  f-type  cytochromes).  The  second one recognizes cytochrome c that binds
more  than  one heme group. The third one recognizes class II cytochrome c and
the fourth one is directed against cytochrome f family.

-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.

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

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

-Note: These profiles replace a pattern which specificity was inadequate.

-Last update: August 2004 / Pattern removed, profiles added and text revised.

[ 1] Mathews F.S.
     "The structure, function and evolution of cytochromes."
     Prog. Biophys. Mol. Biol. 45:1-56(1985).
     PubMed=3881803
[ 2] Martinou J.-C., Desagher S., Antonsson B.
     "Cytochrome c release from mitochondria: all or nothing."
     Nat. Cell Biol. 2:E41-E43(2000).
     PubMed=10707095; DOI=10.1038/35004069
[ 3] Allen J.W., Daltrop O., Stevens J.M., Ferguson S.J.
     "C-type cytochromes: diverse structures and biogenesis systems pose
     evolutionary problems."
     Philos. Trans. R. Soc. Lond., B, Biol. Sci. 358:255-266(2003).
     PubMed=12594933; DOI=10.1098/rstb.2002.1192
[ 4] Barker P.D., Ferguson S.J.
     "Still a puzzle: why is haem covalently attached in c-type
     cytochromes?"
     Structure 7:R281-R290(1999).
     PubMed=10647174

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