IUBMB Enzyme Nomenclature


Accepted name: carotenoid χ-ring synthase

Reaction: carotenoid β-end group + 2 acceptor = carotenoid χ-end group + 2 reduced acceptor

For diagram of reaction click here.

Glossary: okenone = 1'-methoxy-1',2'-dihydro-χ,ψ-caroten-4'-one
renierapurpurin = χ,χ-carotene
synechoxanthin = χ,χ-caroten-18,18'-dioate

Other name(s): crtU (gene name) (ambiguous); cruE (gene name)

Systematic name: carotenoid β-ring:acceptor oxidoreductase/methyltranferase (χ-ring forming)

Comments: The enzyme, found in purple sulfur bacteria (Chromatiaceae) and some cyanobacteria, is involved in the biosynthesis of carotenoids that contain χ-type end groups, such as okenone, renierapurpurin, and synechoxanthin.

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


1. Graham, J.E. and Bryant, D.A. The Biosynthetic pathway for synechoxanthin, an aromatic carotenoid synthesized by the euryhaline, unicellular cyanobacterium Synechococcus sp. strain PCC 7002. J. Bacteriol. 190 (2008) 7966-7974. [PMID: 18849428]

2. Vogl, K. and Bryant, D.A. Biosynthesis of the biomarker okenone: χ-ring formation. Geobiology 10 (2012) 205-215. [PMID: 22070388]

[EC created 2018]

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