|M.Sc Student||Itay Gonda|
|Subject||Amino-Acid Transaminases-key Enzymes in the Formation of|
Amino-Acid Aroma Compounds in Melons
|Department||Department of Biology||Supervisors||Professor Emeritus Gepstein Shimon|
|Dr. Nurit Katzir|
|Dr. Efraim Lewinsohn|
|Full Thesis text|
The melon (Cucumis melo L., Cucurbitaceae) is a highly polymorphic species and many diverse cultivars are grown throughout the world. The flavor is one of the most important characteristics of the melon fruit quality, and melon aroma is crucial in determining the unique flavor of each cultivar. The aroma of melons is composed of many volatile compounds originating from amino-acids, fatty-acids, carotenoids and terpenes. Amino-acids are known precursors of aroma compounds in melons and also in other organisms. In micro-organisms the biosynthetic pathway of amino-acid derived aroma compounds is well understood, and usually the initial step involves transamination of the amino-acid, forming an α-keto acid intermediate. However, in it was shown that in tomato fruits L-phenylalanine is first decarboxylated en route to aroma compounds, while in petunia and rose petals L-phenylalanine is converted to phenylacetaldehyde in one enzymatic step. The main goal of this work was to assess by which mechanism amino-acids are converted into aroma compounds in melon fruit.
The level of free amino-acids in developing fruit was measured in four melon cultivars and found to increase in ripe fruits of the two climacteric cultivars. Incubation of melon fruit cubes with exogenous amino-acids enhanced the formation of aroma compounds similar to the side chain of the amino-acid supplied, as detected by GC-MS. Exogenous L-[13C6]phenylalanine was also incorporated into volatile aroma compounds in melon cubes, as evidenced by GC-MS analyses. Incubation of melon fruit cubes with exogenous α-keto acids also led to enhanced formation of aroma compounds similar to the side chain of the exogenous keto-acid supplied. Amino-acid transaminase activities were extracted from the flesh of ripe melon fruits. These activities converted L-isoleucine, L-leucine, L-valine, L-methionine or L-phenylalanine into their respective α-keto acids, utilizing α-ketoglutarate as the amine acceptor. Data-mining of the Melon EST Database yielded two novel genes resembling plant amino acid transaminases (CmArAT1 and CmBCAT1). When expressed in bacteria, CmArAT1 and CmBCAT1 had aromatic amino-acid transaminase and branched-chain amino-acid transaminase activities, respectively. The expression of CmBCAT1 and CmArAT1 was examined using qRT-PCR revealing gene expression increases during fruit ripening both in flesh and rind tissues. These results indicate that in melon fruits, the catabolism of amino-acids into aroma volatiles occurs through transamination first, rather than decarboxylation or direct aldehyde synthesis, as has been demonstrated in other plants.