IUBMB Enzyme Nomenclature


Accepted name: β-carotene 4-ketolase

Reaction: (1) β-carotene + 2 reduced acceptor + 2 O2 = echinenone + 2 acceptor + 3 H2O
(2) echinenone + 2 reduced acceptor + 2 O2 = canthaxanthin + 2 acceptor + 3 H2O

For diagram of reaction click here.

Glossary: echinenone = β,β-caroten-4-one
canthaxanthin = β,β-carotene-4,4'-dione
zeaxanthin = β,β-carotene-3,3'-diol
astaxanthin = 3,3'-dihydroxy-β,β-carotene-4,4'-dione

Other name(s): BKT (ambiguous); β-C-4 oxygenase; β-carotene ketolase; crtS (gene name); crtW (gene name)

Systematic name: β-carotene,donor:oxygen oxidoreductase (echinenone-forming)

Comments: The enzyme, studied from algae, plants, fungi, and bacteria, adds an oxo group at position 4 of a carotenoid β ring. It is involved in the biosynthesis of carotenoids such as astaxanthin and flexixanthin. The enzyme does not act on β rings that are hydroxylated at position 3, such as in zeaxanthin (cf. EC, zeaxanthin 4-ketolase). The enzyme from the yeast Xanthophyllomyces dendrorhous is bifuntional and also catalyses the activity of EC, β-carotene 3-hydroxylase.

Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, CAS registry number:


1. Lotan, T. and Hirschberg, J. Cloning and expression in Escherichia coli of the gene encoding β-C-4-oxygenase, that converts β-carotene to the ketocarotenoid canthaxanthin in Haematococcus pluvialis. FEBS Lett. 364 (1995) 125-128. [PMID: 7750556]

2. Breitenbach, J., Misawa, N., Kajiwara, S. and Sandmann, G. Expression in Escherichia coli and properties of the carotene ketolase from Haematococcus pluvialis. FEMS Microbiol. Lett. 140 (1996) 241-246. [PMID: 8764486]

3. Steiger, S. and Sandmann, G. Cloning of two carotenoid ketolase genes from Nostoc punctiforme for the heterologous production of canthaxanthin and astaxanthin. Biotechnol. Lett. 26 (2004) 813-817. [PMID: 15269553]

4. Ojima, K., Breitenbach, J., Visser, H., Setoguchi, Y., Tabata, K., Hoshino, T., van den Berg, J. and Sandmann, G. Cloning of the astaxanthin synthase gene from Xanthophyllomyces dendrorhous (Phaffia rhodozyma) and its assignment as a β-carotene 3-hydroxylase/4-ketolase. Mol. Genet. Genomics 275 (2006) 148-158. [PMID: 16416328]

5. Tao, L., Yao, H., Kasai, H., Misawa, N. and Cheng, Q. A carotenoid synthesis gene cluster from Algoriphagus sp. KK10202C with a novel fusion-type lycopene β-cyclase gene. Mol. Genet. Genomics 276 (2006) 79-86. [PMID: 16625353]

6. Kathiresan, S., Chandrashekar, A., Ravishankar, G.A. and Sarada, R. Regulation of astaxanthin and its intermediates through cloning and genetic transformation of β-carotene ketolase in Haematococcus pluvialis. J. Biotechnol. 196-197 (2015) 33-41. [PMID: 25612872]

[EC created 2018]

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