Haem type	Formal iron oxidation/spin states
Haem b	FeII (S=0); FeIII (S=1/2)

Cytochromes b (cyt b) can be defined as electron­transfer proteins having one or two haem b groups, noncovalently bound to the protein. The fifth haem iron ligand is always provided by a histidine residue. Cyt b possess a wide range of properties and function in a large number of different redox processes [1, 2]. Some haemoproteins, such as P450s and nitric oxide synthase (NOS), have been termed `cytochromes b' as well [2]. Although both P450s and NOS can be considered as b­type cytochromes, their main function is catalysis. The recommended name for the group of enzymes which includes P450s and NOS is `haem-thiolate proteins' [3].

Cytochromes b in PROMISE

PROMISE ID	Description
BACFERRITIN	Bacterioferritin (aka cytochrome b1 and cytochrome b557)
CYTB5	Cytochrome b5 family
CYTB562	Soluble cytochrome b562
CYTBHBL	Cytochrome b/b6 (bHbL)

Cytochromes b in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
BACFERRITIN	PR00601	BACTERIOFERRITIN	PS00549	BL00549
-	-	CYTOCHROME_B_HEME CYTOCHROME_B_QO	PS00192 PS00193	BL00192
CYTOCHROMEB5	PR00363	CYTOCHROME_B5	PS00191	BL00191
-	-	CYTOCHROME_B559	PS00537	BL00537
NIHGNASECYTB	PR00161	NI_HGENASE_CYTB_1 NI_HGENASE_CYTB_2	PS00882 PS00883	BL00882
-	-	SDH_CYT_1 SDH_CYT_2	PS01000 PS01001	BL01000

References

1. von Jagow, G. and Sebald, W. (1980) b­Type cytochromes. Annu. Rev. Biochem. 49, 281-314.
2. Kiel, J.L. (1995) Type­b Cytochromes: Sensors and Switches. CRC Press, Boca Raton.
3. Nomenclature Committee of the International Union of Biochemistry (NC­IUB) (1991) Nomenclature of electron­transfer proteins. Recommendations 1989. Eur. J. Biochem. 200, 599-611.