## 4 Volatile Aroma Compounds Biosynthetic Pathways and Related Enzymes
### Carotenoid Pathway  
Carotenoid and their apocarotenoid derivatives are isoprenoid molecules important for the primary and secondary metabolisms of plants and other living organisms [171]. The oxidative cleavage ofcarotenoids leads to the production of apocarotenoids and is catalyzed by a family of carotenoid cleavage dioxygenases (CCDs) [172]. Apocarotenoid volatiles are synthesized only at the latest stage of ripening, even though the CCD enzymes are present throughout fruit development [173]. CCDs often exhibit substrate promiscuity, which probably contributes to the diversity of apocarotenoids found in nature [172]. CCDs also differ for their subcellular localization: some, like CCD1, are predicted to be cytosolic; others possess transit peptides for plastid or plastoglobule targeting [174]. In general, the biosynthesis of carotenoid-derived volatile compounds occurs via three steps: an initial dioxygenase cleavage yielding apocarotenoids, followed by enzymatic transformations of these apocarotenoids leading to the formation of polar aroma precursors, and finally acid-catalyzed conversions of these precursors to volatile compounds [22]. However, in some cases a volatile product is the result of the initial dioxygenase cleavage step, as was shown for \(\beta\)-ionone in Arabidopsis (_Arabidopsis thaliana_) [175], tomato (_Solanum lycopersicum_) and petunia (_Petunia hybridea_) [172,176].  
CCDs exhibit specificity for the double bond that they cleave but many are promiscuous in their substrate choice. In plants, CCD1 and CCD7 cleave the 9,10 double bonds of their respective carotenoid substrates, _At_CCD1 cleaves linear and cyclic carotenoids at the 9,10 and 9',10' positions. For example, when \(\beta\)-carotene serves as the substrate, _At_CCD1 produces two C\({}_{13}\) products (both \(\beta\)-ionone) and a central C\({}_{14}\) dialdehyde (Scheme 4). CCD7 can cleave both linear and cyclic carotenoid substrates at the 9,10 double bond [177]. [PERSON] _et al._ determined that CCD7 cleaves \(\beta\)-carotene asymmetrically, generating a C\({}_{13}\) ketone (\(\beta\)-ionone) and a C\({}_{27}\) aldehyde (10'-apo-\(\beta\)-carotenal). CCD8 can then cleave the C\({}_{27}\) aldehyde at its 13,14 double bond [178]. Subsequent data demonstrate, however, that CCD8 can act directly upon carotenoid substrates [179]. In addition, _Bo_LCD from _Bixa orellana_ was reported to cleave at the lycopene 5,6 double bond [179], and _Cs_ZCD from _Crocus sativus_ was found to cleave at the 7,8 double bond of zexananthin [180].  
**Scheme 4.** Carotenoids and their degradation products. 1, 5,6-epoxy-3 hydroxy-\(\beta\)-ionone; 2, geranayl acetone (6,10-dimethyl-5,9-undecadien-2-one); 3, \(\beta\)-ionone; 4, C\({}_{14}\) dialdehyde (4,9-dimethyldodeca-2,4,6,8,10-pentaene-1,12-dial); 5, 3-hydroxy-\(\beta\)-ionone; 6, the cleavage product of 9-_cis_-neoxanthin on the 9,10 double bond; 7, the cleavage product of 9-_cis_-neoxanthin on the 9',10' double bond; 8, 4,9-dimethyldodeca-4,6,8-trienedial; 9, \(\alpha\)-ionone; 10, pseudoonone (6,10-dimethyl-3,5,9-undecatrien-2-one); 11, 3-hydroxy-\(\alpha\)-ionone.