Abstract
In saffron, the cleavage of zeaxanthin by means of CCD2 generates crocetin dialdehyde, which is then converted by an unknown aldehyde dehydrogenase to crocetin. A proteome from saffron stigma was released recently and, based on the expression pattern and correlation analyses, five aldehyde dehydrogenases (ALDHs) were suggested as possible candidates to generate crocetin from crocetin dialdehydes. We selected four of the suggested ALDHs and analyzed their expression in different tissues, determined their activity over crocetin dialdehyde, and performed structure modeling and docking calculation to find their specificity. All the ALDHs were able to convert crocetin dialdehyde to crocetin, but two of them were stigma tissue-specific. Structure modeling and docking analyses revealed that, in all cases, there was a high coverage of residues in the models. All of them showed a very close conformation, indicated by the low root-mean-square deviation (RMSD) values of backbone atoms, which indicate a high similarity among them. However, low affinity between the enzymes and the crocetin dialdehyde were observed. Phylogenetic analysis and binding affinities calculations, including some ALDHs from Gardenia jasmonoides, Crocus sieberi, and Buddleja species that accumulate crocetin and Bixa orellana synthetizing the apocarotenoid bixin selected on their expression pattern matching with the accumulation of either crocins or bixin, pointed out that family 2 C4 members might be involved in the conversion of crocetin dialdehyde to crocetin with high specificity.
Highlights
Saffron spice is produced from the dried stigmas of Crocus sativus L., a member of the large Iridaceae family, which has been cultivated in the Near East and the Mediterranean Basin since the Late Bronze Age [1]
The non-volatile apocarotenoid crocetin dialdehyde is obtained by the action of the plastidic enzyme CCD2 [20,21] over the carotenoid zeaxanthin, which is further oxidized by an unknown aldehyde dehydrogenase to obtain crocetin
The four aldehyde dehydrogenases (ALDHs) gene sequences coded for members of three ALDH protein families: ALDHs 20158 and 11367 belong to family 2, which includes mitochondrial and cytosolic enzymes with relatively broad substrate specificity [24], ALDH54788 falls into family 3, having members with different subcellular localization including cytosol, chloroplasts and endoplasmic reticulum [35,36,37], while ALDH3898 shows identity with members of family 6, known as methyltmalonyl semialdehyde deshydrogenase, the only ALDH family able to use coenzyme A (CoA) as a cofactor [28]
Summary
Saffron spice is produced from the dried stigmas of Crocus sativus L., a member of the large Iridaceae family, which has been cultivated in the Near East and the Mediterranean Basin since the Late Bronze Age [1]. The long scarlet stigmas of saffron are highly valued for flavoring and coloring foods and are among the most expensive spices in the world. The compounds responsible for the organoleptic properties of the spice are the apocarotenoids crocins, the glucosylated derivatives of the apocarotenoid crocetin, and picrocrocin, a monoglucosylated derivative of 4-hydroxy-2,6,6-trimethyl-1-cyclohexene-1-carboxaldehyde (HTCC), which gives the stigma its red coloration and provides saffron with its bitter taste, respectively [2]. The accumulation of crocins is not restricted to saffron. Wild crocus varieties such as C. ancyrensis, Gardenia jasminoides and some species of lamiales do, in minor quantities [9]
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