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2  structures 3033  species 14  interactions 8451  sequences 32  architectures

Family: Cytochrom_B_N (PF00033)

Summary: Cytochrome b(N-terminal)/b6/petB

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This is the Wikipedia entry entitled "Cytochrome b". More...

Cytochrome b Edit Wikipedia article

1l0l opm.png
Symbol Cytochrom_B_N
Pfam PF00033
InterPro IPR005797
SCOP 3bcc
TCDB 3.D.3
OPM superfamily 3
OPM protein 3h1j
CDD cd00284

Cytochrome b is the main subunit of transmembrane cytochrome bc1 and b6f complexes.[1][2]


In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III (EC - also known as the bc1 complex or ubiquinol-cytochrome c reductase. In plant chloroplasts and cyanobacteria, there is an analogous protein, cytochrome b6, a component of the plastoquinone-plastocyanin reductase (EC, also known as the b6f complex. These complexes are involved in electron transport, pumping of protons to the create a PMF. The proton gradient is finally used for the generation of ATP. Concluding, the complexes play a vital part in cells.[3]


Cytochrome b/b6[4][5] is an integral membrane protein of approximately 400 amino acid residues that probably has 8 transmembrane segments. In plants and cyanobacteria, cytochrome b6 consists of two subunits encoded by the petB and petD genes. Cytochrome b/b6 non-covalently binds two heme groups, known as b562 and b566. Four conserved histidine residues are postulated to be the ligands of the iron atoms of these two heme groups.

Use in phylogenetics

Cytochrome b is commonly used as a region of mitochondrial DNA to determine phylogenetic relationships between organisms due to its sequence variability. It is considered to be most useful in determining relationships within families and genera. Comparative studies involving cytochrome b have resulted in new classification schemes and have been used to assign newly described species to a genus, as well as deepen the understanding of evolutionary relationships.[6]

Clinical significance

Mutations in cytochrome b primarily result in exercise intolerance in human patients; though more rare, severe multi-system pathologies have also been reported.[7]

Single-point mutations in cytochrome b of Plasmodium falciparum and P. berghei are associated with resistance to the anti-malarial drug atovaquone.[8]

Human genes

Human genes encoding cytochrome b proteins include:

  • CYB5A – cytochrome b5 type A (microsomal)
  • CYB5B – cytochrome b5 type B (outer mitochondrial membrane)
  • CYBASC3 – cytochrome b, ascorbate dependent 3
  • MT-CYB – mitochondrially encoded cytochrome b


  1. ^ Howell N (August 1989). "Evolutionary conservation of protein regions in the protonmotive cytochrome b and their possible roles in redox catalysis". J. Mol. Evol. 29 (2): 157–69. doi:10.1007/BF02100114. PMID 2509716. 
  2. ^ Esposti MD, De Vries S, Crimi M, Ghelli A, Patarnello T, Meyer A (July 1993). "Mitochondrial cytochrome b: evolution and structure of the protein". Biochim. Biophys. Acta 1143 (3): 243–71. doi:10.1016/0005-2728(93)90197-N. PMID 8329437. 
  3. ^ Blankenship, Robert (2009). Molecular Mechanisms of Photosynthesis. Blackwell Publishing. pp. 124–132. 
  4. ^ Howell N (1989). "Evolutionary conservation of protein regions in the protonmotive cytochrome b and their possible roles in redox catalysis". J. Mol. Evol. 29 (2): 157–169. doi:10.1007/BF02100114. PMID 2509716. 
  5. ^ Esposti MD, Crimi M, Ghelli A, Patarnello T, Meyer A, De Vries S (1993). "Mitochondrial cytochrome b: evolution and structure of the protein". Biochim. Biophys. Acta 1143 (3): 243–271. doi:10.1016/0005-2728(93)90197-N. PMID 8329437. 
  6. ^ Castresana, J. (2001). "Cytochrome b Phylogeny and the Taxonomy of Great Apes and Mammals". Molecular Biology and Evolution 18 (4): 465–471. doi:10.1093/oxfordjournals.molbev.a003825. PMID 11264397. 
  7. ^ Blakely EL, Mitchell AL, Fisher N, Meunier B, Nijtmans LG, Schaefer AM, Jackson MJ, Turnbull DM, Taylor RW (July 2005). "A mitochondrial cytochrome b mutation causing severe respiratory chain enzyme deficiency in humans and yeast". FEBS J. 272 (14): 3583–92. doi:10.1111/j.1742-4658.2005.04779.x. PMID 16008558. 
  8. ^ Siregar JE, Syafruddin D, Matsuoka H, Kita K, Marzuki S (June 2008). "Mutation underlying resistance of Plasmodium berghei to atovaquone in the quinone binding domain 2 (Qo(2)) of the cytochrome b gene". Parasitology International 57 (2): 229–32. doi:10.1016/j.parint.2007.12.002. PMID 18248769. 

External links

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

Cytochrome b(N-terminal)/b6/petB Provide feedback

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Internal database links

External database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR016174

This entry represents a haem-binding domain with a 4-helical bundle structure that is found in transmembrane di-haem cytochromes. The domain contains four transmembrane helices in an up-and-down bundle, and binds two haem groups in between the helices; three of the four haem-binding residues is conserved between family members. Proteins containing this domain include:

  • N-terminal domain of mitochondrial cytochrome b subunit, in which the domain contains an extra transmembrane linker helix that is absent in plant and cyanobacteria subunits [PUBMED:16024040].
  • Cytochrome b6 subunit of the cytochrome b6f complex, which provides the electronic connection between the photosystems I and II reaction centres of oxygenic photosynthesis, and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis [PUBMED:14526088].
  • Cytochrome gamma subunit of formate dehydrogenase-N (Fdn-N), which acts as a major component of Escherichia coli nitrate respiration [PUBMED:11884747].

Please note, this entry also identifies a number of proteins that are cleaved into two chains - a truncated non-functional cytochrome oxidase 1 and an intron-encoded endonuclease.

Gene Ontology

The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan 2heme_cytochrom (CL0328), which has the following description:

This superfamily includes a variety of different heme binding cytochromes.

The clan contains the following 10 members:

COX15-CtaA Cytochrom_B561 Cytochrom_B_N Cytochrom_B_N_2 Cytochrom_C_asm DUF2427 DUF4405 Ferric_reduct Ni_hydr_CYTB PCRF


We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the NCBI sequence database, and our metagenomics sequence database. More...

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We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.

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You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.

External links

MyHits provides a collection of tools to handle multiple sequence alignments. For example, one can refine a seed alignment (sequence addition or removal, re-alignment or manual edition) and then search databases for remote homologs using HMMER3.

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HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...


This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.

Note: You can also download the data file for the tree.

Curation and family details

This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.

Curation View help on the curation process

Seed source: Prosite
Previous IDs: cytochrome_b_N;
Type: Domain
Author: Sonnhammer ELL
Number in seed: 92
Number in full: 8451
Average length of the domain: 164.10 aa
Average identity of full alignment: 19 %
Average coverage of the sequence by the domain: 83.40 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 23193494 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 26.0 26.0
Trusted cut-off 26.0 26.0
Noise cut-off 25.9 25.9
Model length: 188
Family (HMM) version: 14
Download: download the raw HMM for this family

Species distribution

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There are 14 interactions for this family. More...

CytB6-F_Fe-S PetG Cytochrom_B_C PetN Peptidase_M16_C Cytochrom_C1 UCR_TM UCR_Fe-S_N Cytochrom_B_N UcrQ Apocytochr_F_C UCR_14kD UCR_6-4kD Rieske


For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Cytochrom_B_N domain has been found. There are 2 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.

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