Cytochrome b5 is a membrane-bound hemoprotein which acts as an electron
carrier for several membrane-bound oxygenases . There are two homologous
forms of b5, one found in microsomes and one found in the outer membrane of
mitochondria. Two conserved histidine residues serve as axial ligands for the
heme group. The structure of a number of oxidoreductases consists of the
juxtaposition of a heme-binding domain homologous to that of b5 and either a
flavodehydrogenase or a molybdopterin domain. These enzymes are:
Lactate dehydrogenase (EC 184.108.40.206) , an enzyme that consists of a
flavodehydrogenase domain and a heme-binding domain called cytochrome b2.
Nitrate reductase (EC 1.7.1.-), a key enzyme involved in the first step of
nitrate assimilation in plants, fungi and bacteria [3,4]. Consists of a
molybdopterin domain (see <PDOC00484>), a heme-binding domain called
cytochrome b557, as well as a cytochrome reductase domain.
Sulfite oxidase (EC 220.127.116.11) , which catalyzes the terminal reaction in
the oxidative degradation of sulfur-containing amino acids. Also consists
of a molybdopterin domain and a heme-binding domain.
Yeast acyl-CoA desaturase 1 (EC 18.104.22.168) (gene OLE1). This enzyme
contains a C-termainal heme-binding domain.
This family of proteins also includes:
TU-36B, a Drosophila muscle protein of unknown function .
Fission yeast hypothetical protein SpAC1F12.10c.
Yeast hypothetical protein YMR073c.
Yeast hypothetical protein YMR272c.
We used a segment which includes the first of the two histidine heme ligands,
as a signature pattern for the heme-binding domain of cytochrome b5 family.
PROSITE is copyright. It is produced by the SIB Swiss Institute
Bioinformatics. There are no restrictions on its use by non-profit
institutions as long as its content is in no way modified. Usage by and
for commercial entities requires a license agreement. For information
about the licensing scheme send an email to
or see: prosite_license.html.