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Last modified: 2 February 1999

Class IV cytochromes c
Haem Haem type Haem iron coordination Axial iron ligands Formal iron
oxidation/spin
states
1, 2, 3
Haem c image
Haem c
His-Haem-Met image
Hexacoordinate
NepsilonHis;

SdeltaMet

FeII (S=0);
FeIII (S=1/2)
4
His-Haem-His image
Hexacoordinate
NepsilonHis;

NepsilonHis

Cytochromes c (cyt c) can be defined as electron­transfer proteins having one or several haem c groups, bound to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cyt c possess a wide range of properties and function in a large number of different redox processes [1, 2].

Ambler [3] recognised four classes of cyt c. Class IV was originally created to hold the complex proteins that have other prostethic groups as well as haem c, e.g. flavocytochrome c and cytochromes cd. Alternatively, Moore and Pettigrew [2] have suggested that Class IV cyt c are tetrahaem proteins containing both bis­His and His­Met coordinated haems, with a 3­D structure exemplified by that of the photosynthetic reaction centre (PRC) cyt c of purple bacteria, and form a structurally homogeneous family. We adopt this latter classification.

PRCs are membrane­spanning complexes of polypeptide chains and cofactors that catalyse the first steps in the conversion of light energy to chemical energy during photosynthesis. PRCs of photosynthetic purple bacteria consist of at least three protein subunits termed L (light), M (medium) and H (heavy); in Rhodopseudomonas viridis and Thiocapsa pfennigii, the four­haem cyt c is the fourth (and the largest) protein subunit [4]. R. viridis PRC cyt c is a lipoprotein; the lipid­binding group is the N­terminal Cys which is linked to a diglyceride [2], which helps attach the cyt c to the surface of the membraneous PRC; most part of the cyt c is located in the aqueous periplasmic compartment.

The 3­D structure of the PRC of R. viridis has been determined [4]. PRC cyt c consists of an N­terminal segment, two pairs of haem binding segments, and a segment connecting the two pairs. Each haem binding segment consists of an alpha­helix with an average length of 17 residues followed by a turn and the 

CxxCH
sequence typical of the c­type cytochromes. The haems are connected to the Cys residues via thioether linkages in such a way that the haem planes are parallel to the helix axes. The sixth ligands to three of the four haems are the Sdelta atoms of a Met residue within the helices. Haem 4 is bis­His coordinated (His­124, located within haem binding segment 2, serves as a sixth ligand for the iron). The two pairs of haem binding segments, containing haems 1, 2 and 3, 4, respectively, are related by a local twofold symmetry (see Figure 1PRCC). The properties of the haem groups of R. viridis PRC cyt c are summarised in the table [2]:

Haem Ferro­alpha­band (nm) Redox potential (mV) Axial ligands
1
554
-80
 His­91, Met­74
2
556
320
 His­136, Met­110
3
559 (552)*
400
 His­248, Met­233
4
552
20
 His­309, His­124

* The alpha­band of haem 3 is split into major (559 nm) and minor (552 nm) components [5].

Class IV cytochromes c in motif databases

PRINTS ID PRINTS AC PROSITE/BLOCKS ID PROSITE AC BLOCKS AC
-
-
 CYTOCHROME_C PS00190 BL00190

Class IV cytochromes c in SWISS­PROT/TREMBL

C554_CHLAU Cytochrome c­554 precursor; Chloroflexus aurantiacus
CYCR_ERYSP Photosynthetic reaction center cytochrome c subunit (fragment); Erythrobacter sp. (strain OCH 114)
CYCR_RHOGE Photosynthetic reaction center cytochrome c subunit precursor; Rhodocyclus gelatinosus (Rhodopseudomonas gelatinosa)
CYCR_RHOVI Photosynthetic reaction center cytochrome c subunit precursor (c558/c559); Rhodopseudomonas viridis
Q52727 Photosynthetic reaction center cytochrome c subunit (pufC); Roseobacter denitrificans
P95617 Tetrahemic cytochrome c (pufC); Rubrivivax gelatinosus
O32413 Photosynthetic reaction center cytochrome subunit (pufC); Rhodospirillum molischianum
O30845 Reaction center four­heme cytochrome subunit (pufC); Ectothiorhodospira shaposhnikovii

Class IV cytochromes c in alignment databases

Protein Superfamily Pfam LPFC 3­D alignment
03784; cytochrome c554
03789; photosynthetic reaction center cytochrome
-
-

Class IV cytochromes c in 3­D databases

PRC cyt c contains four covalently bound haem c groups (see Figure 1PRCC).

PDB scop BSMRELI
Base		 Header
1prc 1prc 1prc 1prc Photosynthetic reaction centre (complex with ubiquinone­1, menaquinone­7, dihydroneurosporene and lauryl dimethylamine­oxide); Rhodopseudomonas viridis

References

  1. Pettigrew, G.W. and Moore, G.R. (1987) Cytochromes c. Biological Aspects. Springer­Verlag, Berlin - Heidelberg - New York.
  2. Moore, G.R. and Pettigrew, G.W. (1990) Cytochromes c. Evolutionary, Structural, and Physicochemical Aspects. Springer­Verlag, Berlin - Heidelberg - New York.
  3. Ambler, R.P. (1991) Sequence variability in bacterial cytochromes c. Biochim. Biophys. Acta 1058, 42-47.
  4. Deisenhofer, J. and Michel, H. (1992) High­resolution crystal structures of bacterial photosynthetic reaction centers. In Ernster, L., Ed. Molecular Mechanisms in Bioenergetics. Elsevier, Amsterdam, pp. 103-120.
  5. Dracheva, S.M., Drachev, L.A., Konstantinov, A.A., Semenov, A.Yu., Skulachev, V.P., Arutjunjan, A.M., Shuvalov, V.A. and Zaberezhnaya, S.M. (1988) Electrogenic steps in the redox reactions catalyzed by photosynthetic reaction­centre complex from Rhodopseudomonas viridis. Eur. J. Biochem. 171, 253-264.
Bibliography on structural studies of Class IV cytochromes c