PhD ceremony: Ms. A.M. Pudlik, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen
Dissertation: Citrate driven transamination for flavor production in Lactococcus lactis
Promotor(s): prof. A.J.M. Driessen
Faculty: Mathematics and Natural Sciences
The study focuses on the production of flavor compounds that are important in the dairy industry. The flavor compounds are produced from amino acids derived from milk proteins and the goal of the study was to drive the conversion by citrate in Lactic Acid Bacteria (LAB). Transamination, the first step in the breakdown of amino acids leading to production of aroma compounds is often limited by lack of an α-keto donor. A key point of the study of Agata Pudlik was redirection of the first intermediate of the citrate metabolic pathway, the keto donor oxaloacetate, to the transamination reaction rather than to further be metabolized in the citrate pathway. The internal pool of oxaloacetate is controlled by the enzyme oxaloacetate decarboxylase and inactivating this enzyme was proposed to do the job. The redirection of the citrate metabolic pathway into the transamination route in L. lactis turned out to be successful resulting in flavor compounds derived from branched-chain amino acids (isoleucine, leucine, valine), aromatic amino acids (phenylalanine, tryptophan, tyrosine), and methionine. Citrate is fermented by a subspecies of the cheese bacterium Lactococcus lactis, i.e. subsp. lactis biovar diacetlylactis, in which the gene encoding the citrate transporter CitP is located on an endogenous plasmid, while the citrate metabolic enzymes are clustered on the chromosome. In Pudliks study, a model strain of L. lactis subsp. lactis biovar diacetlylactis IL1403 with a pFL3 plasmid containing CitP was used.
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