• Prosthetic group features
• Redox properties of the Class III cytochromes c
• Class III cytochromes c in motif databases
• Class III cytochromes c in alignment databases
• Class III cytochromes c in 3­D databases
• References

Haem type	Haem iron coordination	Axial iron ligands	Formal iron oxidation/spin states
Haem c	Hexacoordinate	NHis; NHis	FeII (S=0); FeIII (S=1/2)

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].

Ambler [2] recognised four classes of cyt c. Class III comprises the low redox potential multiple haem cytochromes: cyt c₇ (trihaem), c₃ (tetrahaem), and high­molecular­weight cyt c (HMC; hexadecahaem), with only 30-40 residues per haem group. The haem c groups, all bis­histidinyl coordinated, are structurally and functionally nonequivalent and present different redox potentials in the range 0 to -400 mV [3]. The redox properties of some multihaem cyt c are summarised in the table:

Protein	Species	Haem midpoint potential (mV)				Ref.
		Em1	Em2	Em3	Em4
Cyt c3	Thermodesulfobacterium commune	-140	-280	-280	-280	[4]
Cyt c3	Desulfovibrio gigas	-290	-314	-315	-350	[5]
Cyt c3	Desulfovibrio vulgaris Miyazaki	-235	-323	-333	-358	[6]
Cyt c3	Desulfovibrio vulgaris Hildenborough	-290	-335	-345	-375	[7]
Cyt c3	Desulfovibrio desulfuricans Norway	-150	-300	-330	-355	[8]
Cyt c3, acidic	Desulfovibrio africanus	-210	-240	-260	-270	[9]
Cyt c3, basic	Desulfovibrio africanus	-90	-260	-280	-290	[9]
Cyt c3	Desulfuromonas acetoxidans	-200	-210	-370	-380	[10]
Cyt c7	Desulfuromonas acetoxidans	-102	-177	-177	-	[11]

3­D structures of a number of cyt c₃ have been determined. The proteins consist of 4-5 ­helices and 2 ß­strands wrapped around a very compact core of four non­parallel haems which present a relatively high degree of exposure to the solvent (see Figure 2CY3). The overall protein architecture, haem plane orientations and iron­iron distances are highly conserved [3].

Class III cytochromes c in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
CYTOCHROMEC3	PR00609	CYTOCHROME_C	PS00190	BL00190

Class III cytochromes c in alignment databases

Protein Superfamily	Protein Homology Domain	Pfam	LPFC 3­D alignment
00014; cytochrome c3 00015; cytochrome cc3 03797; Shewanella fumarate reductase flavocytochrome	00180; cytochrome c3	-	cyt3

PDB	scop	BSM	RELI Base	Header	¹
1a2i	-	1a2i	-	Cytochrome c3 (reduced); Desulfovibrio vulgaris, strain Hildenborough	-
1cth	1cth	1cth	1cth	Cytochrome c3; Desulfovibrio vulgaris, strain Hildenborough	MMS94081
1czj	1czj	1czj	1czj	Cytochrome c3 (complex with sulphate); Desulfovibrio desulfuricans, strain Norway NCIB 8310 (syn. Desulfomicrobium baculatum)	MS7RRC5
1new	1new	1new	1new	Cytochrome c7 (cytochrome c551.5) (fully oxidised); Desulfuromonas acetoxidans	MS7GA7
1wad	1wad	1wad	1wad	Cytochrome c3 (complex with Ca2+); Desulfovibrio gigas	MS7LBC43
2cdv	2cdv	2cdv	2cdv	Cytochrome c3; Desulfovibrio vulgaris, strain Miyazaki	-
2cy3	2cy3	2cy3	2cy3	Cytochrome c3; Desulfovibrio desulfuricans, strain Norway	-
2cym	2cym	2cym	2cym	Cytochrome c3; Desulfovibrio vulgaris, strain Hildenborough	MMS92043
2cyr	2cyr	2cyr	2cyr	Cytochrome c3; Desulfovibrio desulfuricans ATCC 27774	MS6MMC19
2new	2new	2new	2new	Cytochrome c7 (cytochrome c551.5) (fully oxidised); Desulfuromonas acetoxidans	MS7GA7

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Pettigrew, G.W. and Moore, G.R. (1987) Cytochromes c. Biological Aspects. Springer­Verlag, Berlin - Heidelberg - New York.
2. Ambler, R.P. (1991) Sequence variability in bacterial cytochromes c. Biochim. Biophys. Acta 1058, 42-47.
3. Coutinho, I.B. and Xavier, A.V. (1994) Tetraheme cytochromes. Methods Enzymol. 243, 119-140.
4. Hatchikian, E.C., Papavassiliou, P., Bianco, P. and Haladjian, J. (1984) Characterization of cytochrome c₃ from the thermophilic sulfate reducer Thermodesulfobacterium commune. J. Bacteriol. 159, 1040-1046.
5. Santos, H., Moura, J.J.G., Moura, I., LeGall, J. and Xavier, A.V. (1984) NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c₃. Eur. J. Biochem. 141, 283-296.
6. Benosman, H., Asso, M., Bertrand, P., Yagi, T. and Gayda, J.P. (1989) EPR study of the redox interactions in cytochrome c₃ from Desulfovibrio vulgaris Miyazaki. Eur. J. Biochem. 182, 51-55.
7. Morimoto, Y., Tani, T., Okumura, H., Higuchi, Y. and Yasuoka, N. (1991) Effects of amino acid substitution on three­dimensional structure: an X­ray analysis of cytochrome c₃ from Desulfovibrio vulgaris Hildenborough at 2 Å resolution. J. Biochem. (Tokyo) 110, 532-540.
8. Guigliarelli, B., Bertrand, P., More, C., Haser, R. and Gayda, J.P. (1990) Single­crystal electron paramagnetic resonance study of cytochrome c₃ from Desulfovibrio desulfuricans Norway Strain. Assignment of the heme midpoint redox potentials. J. Mol. Biol. 216, 161-166.
9. Pieulle, L., Haladjian, J., Bonicel, J. and Hatchikian, E.C. (1996) Biochemical studies of the c­type cytochromes of the sulfate reducer Desulfovibrio africanus. Characterization of two tetraheme cytochromes c₃ with different specificity. Biochim. Biophys. Acta 1273, 51-61.
10. Bruschi, M., Woudstra, M., Guigliarelli, B., Asso, M., Lojou, E., Petillot, Y. and Abergel, C. (1997) Biochemical and spectroscopic characterization of two new cytochromes isolated from Desulfuromonas acetoxidans. Biochemistry 36, 10601-10608.
11. Fiechtner, M.D. and Kassner, R.J. (1979) The redox properties and heme environment of cytochrome c­551.5 from Desulfuromonas acetoxidans. Biochim. Biophys. Acta 579, 269-278.

Bibliography on structural studies of Class III cytochromes c