Cytochrome C1

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Cytochrome C1
Alt. symbolsUQCR4, MC3DN6
Alt. namesComplex III subunit 4, Ubiquinol-cytochrome-c reductase complex cytochrome c1 subunit
Other data
LocusChr. 8 243

Cytochrome C1 (also known as Complex III subunit 4) is a protein encoded by the CYC1 gene. Cytochrome is a heme-containing subunit of the cytochrome b-c1 complex, which accepts electrons from Rieske protein and transfers electrons to cytochrome c in the mitochondrial respiratory chain.[1] It is formed in the cytosol and targeted to the mitochondrial intermembrane space. Cytochrome c1 belongs to the cytochrome c family of proteins.


Cytochrome C1 plays a role in the electron transfer during oxidative phosphorylation. As an iron-sulfur protein approaches the b-c1 complex, it accepts an electron from the cytochrome b subunit, then undergoes a conformational change to attach to cytochrome c1. There, the electron carried by the iron-sulfur protein is transferred to the heme carried by cytochrome c1. This electron is then transferred to a heme carried by cytochrome c. This creates a reduced species of cytochrome c, which separates from the b-c1 complex and moves to the last enzyme in the electron transport chain, cytochrome c oxidase (Complex IV).[2]


CYC1 is a human gene that is conserved in chimpanzee, Rhesus monkey, dog, cow, mouse, rat, zebrafish, fruit fly, mosquito, C. elegans, S. cerevisiae, K. lactis, E. gossypii, S. pombe, N. crassa, A. thaliana, rice, and frog.[3] There are orthologs of CYC1 in 137 known organisms.[4]

In its structure and function, the cytochrome b-c1 complex bears extensive analogy to the cytochrome b6f complex of chloroplasts and cyanobacteria; cytochrome c1 plays an analogous role to cytochrome f, despite their different structures.[5]

Clinical relevance

Mutations in the CYC1 gene are associated with mitochondrial complex III deficiency nuclear type 6. The disease symptoms include early childhood onset of severe lactic acidosis and ketoacidosis, usually in response to infection. Insulin-responsive hyperglycemia is also present, but psychomotor development appears normal. Mutation of CYC1 was observed to cause instability in the cytochrome b-c1 complex, which decreased its ability to create energy through oxidative phosphorylation.[6] Mitochondrial complex III deficiency nuclear type 6 is autosomal recessive.[7]


  1. "CYC1 - Cytochrome c1, heme protein, mitochondrial precursor - Homo sapiens (Human) - CYC1 gene & protein". Retrieved 2016-07-29.
  2. Kokhan O, Wraight CA, Tajkhorshid E (October 2010). "The binding interface of cytochrome c and cytochrome c₁ in the bc₁ complex: rationalizing the role of key residues". Biophysical Journal. 99 (8): 2647–56. doi:10.1016/j.bpj.2010.08.042. PMC 2955499. PMID 20959106.
  3. "CYC1 cytochrome c1 [Homo sapiens (human)]". National Center for Biotechnology Information. U.S. National Library of Medicine. Retrieved 2016-07-29.
  4. "ortholog_gene_1537[group]". National Center for Biotechnology Information. U.S. National Library of Medicine. Retrieved 2016-07-29.
  5. Prince RC, George GN (June 1995). "Cytochrome f revealed". Trends in Biochemical Sciences. 20 (6): 217–8. doi:10.1016/S0968-0004(00)89018-0. PMID 7631417.
  6. Online Mendelian Inheritance in Man (OMIM) Cytochrome C!; CYC1 -123980
  7. "OMIM Gene Map". Online Mendelian Inheritance in Man. Johns Hopkins University. Retrieved 2016-07-29.

External links

This article incorporates text from the public domain Pfam and InterPro: IPR002326