Biochemical and Catalytic Properties of Three Recombinant Alcohol Acyltransferases of Melon. Sulfur-Containing Ester Formation, Regulatory Role of CoA-SH in Activity, and Sequence Elements Conferring Substrate Preference

Abstract

Alcohol acyltransferases (AAT) play a key role in the biosynthesis of ester aroma volatiles in fruit. Three ripening-specific recombinant AATs of cantaloupe Charentais melon fruit (Cm-AAT1, Cm-AAT3, and Cm-AAT4) are capable of synthesizing thioether esters with Cm-AAT1 being by far the most active. All proteins, as well as AAT(s) extracted from melon fruit, are active as tetramers of around 200 kDa. Kinetic analysis demonstrated that CoA-SH, a product of the reaction, is an activator at low concentrations and an inhibitor at higher concentrations. This was confirmed by the addition of phosphotransacetylase at various concentrations, capable of modulating the level of CoA-SH in the reaction medium. Site-directed mutagenesis of some amino acids that were specific to the Cm- AAT sequences into amino acids that were consensus to other characterized AATs greatly affected the selectivity of the original protein and the number of esters produced.