Functional Characterization of Enzymes Forming Volatile Esters from Strawberry and Banana

Abstract

Volatile esters are flavor components of the majority of fruits. The last step in their biosynthesis is catalyzed by alcohol acyltransferases (AATs), which link alcohols to acyl moieties. Full-length cDNAs putatively encoding AATs were isolated from fruit of wild strawberry (Fragaria vesca) and banana (Musa sapientum) and compared to the previously isolated SAAT gene from the cultivated strawberry (Fragaria 3 ananassa). The potential role of these enzymes in fruit flavor formation was assessed. To this end, recombinant enzymes were produced in Escherichia coli, and their activities were analyzed for a variety of alcohol and acyl-CoA substrates. When the results of these activity assays were compared to a phylogenetic analysis of the various members of the acyltransferase family, it was clear that substrate preference could not be predicted on the basis of sequence similarity. In addition, the substrate preference of recombinant enzymes was not necessarily reflected in the representation of esters in the corresponding fruit volatile profiles. This suggests that the specific profile of a given fruit species is to a significant extent determined by the supply of precursors. To study the in planta activity of an alcohol acyltransferase and to assess the potential for metabolic engineering of ester production, we generated transgenic petunia (Petunia hybrida) plants overexpressing the SAAT gene. While the expression of SAAT and the activity of the corresponding enzyme were readily detected in transgenic plants, the volatile profile was found to be unaltered. Feeding of isoamyl alcohol to explants of transgenic lines resulted in the emission of the corresponding acetyl ester. This confirmed that the availability of alcohol substrates is an important parameter to consider when engineering volatile ester formation in plants.

Effects of packaging and storage conditions on quality and volatile compounds of raspberry fruits

“…After 48 h of storage at 1 ± 1°C, all treatments showed a decrease in total content from that measured at harvest (8.33 µg/kg). The availability of alcohol compounds is thought to be one of the limiting factors for ester production (Beekwilder et al, 2004) and, as suggested by Ke et al (1994), low O 2 and high CO 2 concentrations are responsible for the accumulation of alcohols, which leads to the production of ethyl esters and the reduction in other esters. In our study, the highest total concentration of alcohols (12.49 µg/kg) was observed in fruits wrapped in the biodegradable film under passive MAP (A treatment), due to the combined effect of change in temperature to 18 ± 1°C (96 h) and high CO 2 concentration in the packages, which, moreover, corresponded to the lowest total ester level found.…”

Biochemistry of apple aroma: A review

“…In ripe apples, alcohols constitute 6 to 16 % of the total volatiles (8), whereas esters represent, depending on the apple variety, from 80 % in Golden Delicious and Golden Reinders (14,45) to 98 % in Starking Delicious (36). Esters can have linear or branched chains, and their concentration depends on the availability of precursors (37,46,47) and on the selectivity and activity of the enzymes involved (48)(49)(50)(51). Butyl acetate, hexyl acetate, 2-methylbutyl acetate and ethyl 2-methyl--butanoate are the most important esters due to their high content and impact on apple aroma of several varieties (6,52).…”

Biosynthesis of plant‐derived flavor compounds

“…Alcohol acyl transferases (AAT) are capable of combining various alcohols and acyl CoAs, resulting in the synthesis of a wide range of esters, thus accounting for the diversity of esters. Numerous AAT genes have been isolated and characterized in fruit and vegetables (Aharoni et al, 2000;Beekwilder et al, 2004;El-Sharkawy et al, 2005). Aliphatic esters contribute to the aroma of nearly all fruits and are emitted by vegetative tissues.…”