Insights into the formation of carlactone from in‐depth analysis of the CCD8‐catalyzed reactions

Abstract

Strigolactones are a new class of phytohormones synthesized from carotenoids via carlactone. The complex structure of carlactone is not easily deducible from its precursor, a cis-configured β-carotene cleavage product, and is thus formed via a poorly understood series of reactions and molecular rearrangements, all catalyzed by only one enzyme, the carotenoid cleavage dioxygenase 8 (CCD8). Moreover, the reactions leading to carlactone are expected to form a second, yet unidentified product. In this study, we used C and O-labeling to shed light on the reactions catalyzed by CCD8. The characterization of the resulting carlactone by LC-MS and NMR, and the identification of the assumed, less accessible second product allowed us to formulate a minimal reaction mechanism for carlactone generation.

The Arabidopsis DWARF27 gene encodes an all-trans-/9-cis-β-carotene isomerase and is induced by auxin, abscisic acid and phosphate deficiency

“…In the next step, the carotenoid cleavage dioxygenase 7 (CCD7) [41] catalyzes the stereospecific cleavage of 9-cis--carotene at the C9'-C10' double bond, leading to 9-cis--apo-10′-carotenal and -ionone [39,42]. 9-cis--Apo-10′-carotenal is the substrate of CCD8 that mediates a combination of reactions including isomerization, repeated deoxygenation and intramolecular rearrangement, which form carlactone (CL) and ω-OH-(4-CH3)heptanal [39,43]. However, CCD8 can also cleave all-trans--apo-10′carotenal, supposedly through classical dioxygenase cleavage reaction, into all-trans-apo-13-carotenone [44].…”

Carotenoid biofortification in crop plants: citius, altius, fortius

“…In the next step, 9-cis-β-apo-10'-carotenal is converted by CCD8 into carlactone [144]. It is assumed that CCD8 catalyzes a combination of different reactions, including isomerization, intramolecular rearrangement and repeated oxygenation, when exposed to the 9-cis-β-apo-10'-carotenal [148].…”

Strigolactone Biosynthesis and Signal Transduction

“…The authors also used 18 O2 and 13 C-labeling, which demonstrated that the three O-atoms of carlactone derive from atmospheric oxygen and that the C11-atom in carlactone (C2'-atom in SLs) corresponds to the C11 atom in 9-cis-β-apo-10′-carotenal. Based on these results, Bruno et al, (2017) developed a proposal for the mechanism of carlactone formation, which includes a series of reactions composed of isomerization, repeated oxygenation and intramolecular rearrangements. Furthermore, it is proposed that the stereo-configuration of the C9-C10 double bond determines whether the enzymes build a dioxetane intermediate, leading to β-apo-13carotenone, or a cyclic endoperoxide intermediate, to form carlactone (Bruno et al, 2017).…”

“…Figure 8. A proposal for the mechanism of CCD8-catalyzed reactions (modified from Jia et al, 2018 andBruno et al, 2017). (A) A proposal for the formation of -apo-13-carotenone by CCD8 from all-trans-β-apo-10'-carotenal, the second substrate converted by CCD8 (though at much lower rate).…”