Biosynthesis of sinapoyl esters

Sinapoyl esters are phenolic compounds found in members of the Brassicaceae, which includes the model plant Arabidopsis thaliana. The two major sinapoyl esters are sinapoyl malate (3.92) and sinapoyl choline (3.93), which accumulate in leaves and seeds, respectively. Sinapoylmalate plays a role in the protection against UV-radiation (Landry et al., 1995), whereas sinapoyl choline may be used as a storage form of choline in seeds (Shirley and Chappie, 2003). The precursor of these two esters is sinapate (3.35). 

Sinapate is synthesized via the oxidation of sinapaldehyde (3.79) by an aldehyde dehydrogenase, as described in Section 13 of this chapter. Sinapaldehyde, in turn, is derived from the amino acid phenylalanine (3.27) via the general phenylpropanoid pathway (see Section 7), followed by a number of the hydroxylation and methylation reactions described in Section 10. 

The elucidation of sinapoyl ester metabolism was aided by the availability of mutants. The sngl sinapoyl glucose accumulator 1) mutant of Arabidopsis had been identified based on a mutant screen for alterations in the composition of fluorescent compounds in the leaves. The screen was performed by thin layer chromatography and revealed that the leaves of the sngl mutant contained less sinapoylmalate and instead accumulated the precursor sinapoyl glucose (Lorenzen et al. 1996). 

Either the aglycone resulting from the action of a P-glucosidase or the glucoside, or possibly both, undergo cis-trans isomerization under influence of UV-light or possibly mediated by a dimethylallyl transferase. The last step of the biosynthetic pathway is an intramolecular esterification reaction, which can occur spontaneously, to yield coumarin (3.96). The enzymes that involved in these reactions have not been purified. 

---