title: Characterization of the rice carotenoid cleavage dioxygenase 1 reveals a novel route for geranial biosynthesis.
abstract: Carotenoid cleavage products--apocarotenoids--include biologically active compounds, such as hormones, pigments and volatiles. Their biosynthesis is initiated by the oxidative cleavage of C-C double bonds in carotenoid backbones, leading to aldehydes and/or ketones. This step is catalyzed by carotenoid oxygenases, which constitute an ubiquitous enzyme family, including the group of plant carotenoid cleavage dioxygenases 1 (CCD1s), which mediates the formation of volatile C(13) ketones, such as beta-ionone, by cleaving the C9-C10 and C9'-C10' double bonds of cyclic and acyclic carotenoids. Recently, it was reported that plant CCD1s also act on the C5-C6/C5'-C6' double bonds of acyclic carotenes, leading to the volatile C(8) ketone 6-methyl-5-hepten-2-one. Using in vitro and in vivo assays, we show here that rice CCD1 converts lycopene into the three different volatiles, pseudoionone, 6-methyl-5-hepten-2-one, and geranial (C(10)), suggesting that the C7-C8/C7'-C8' double bonds of acyclic carotenoid ends constitute a novel cleavage site for the CCD1 plant subfamily. The results were confirmed by HPLC, LC-MS and GC-MS analyses, and further substantiated by in vitro incubations with the monocyclic carotenoid 3-OH-gamma-carotene and with linear synthetic substrates. Bicyclic carotenoids were cleaved, as reported for other plant CCD1s, at the C9-C10 and C9'-C10' double bonds. Our study reveals a novel source for the widely occurring plant volatile geranial, which is the cleavage of noncyclic ends of carotenoids.
authors.affiliation: Faculty of Biology, Institute of Biology II, Albert-Ludwigs University of Freiburg, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Ilg
authors.forename: Peter
authors.initials: P
authors.lastname: Beyer
authors.forename: Salim
authors.initials: S
authors.lastname: Al-Babili
mesh_terms: D055162:Biocatalysis; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism; D049308:Dioxygenases / Q000378:metabolism*; D000077276:Lycopene; D013058:Mass Spectrometry; D015394:Molecular Structure; D012275:Oryza / Q000201:enzymology* / Q000378:metabolism; D013379:Substrate Specificity; D013729:Terpenes / Q000737:chemistry / Q000378:metabolism*
chemical_list: D013729:Terpenes; D002338:Carotenoids; D049308:Dioxygenases; C431939:carotenoid cleavage dioxygenase 1; C043081:decaprenoic acid; D000077276:Lycopene