Identification of metabolic pathways involved in the biosynthesis of flavor compound 3-methylbutanal from leucine catabolism by Carnobacterium maltaromaticum LMA 28

Carnobacterium maltaromaticum strains are widely found in food including fish, meat and some dairy products. Producing a malty/chocolate like aroma due to 3-methylbutanal from the catabolism of leucine is a general characteristic of this species. In this study, we investigated metabolic routes responsible for the biosynthesis of this flavor compound from the catabolism of leucine in C. maltaromaticum LMA 28, a strain isolated from mold ripened soft cheese. Depending on the lactic acid bacterium, leucine can be converted into 3-methylbutanal following two possible metabolic pathways: either directly by α-ketoacid decarboxylase (KADC) pathway or indirectly by α-ketoacid dehydrogenase (KADH) pathway. Both KADC (41.0 ± 3.0 nmol/mg protein/min) and KADH (1.43 ± 0.62 nmol/mg protein/min) activities were detected and determined in vitro in C. maltaromaticum LMA 28. C. maltaromaticum LMA 28 slightly reduced the production of 3-methylbutanal from leucine in the presence of a specific inhibitor of KADH enzyme complex, i.e. sodium meta-arsenite, suggesting that both pathways were involved in vivo in leucine catabolism. Moreover the presence of genes encoding aminotransferase, glutamate dehydrogenase, α-ketoacid decarboxylase, α-ketoacid dehydrogenase and aldehyde dehydrogenase was confirmed. C. maltaromaticum is then the first lactic acid bacterium in which presence of both metabolic routes responsible for the biosynthesis of 3-methylbutanal from leucine catabolism was confirmed in vitro and in vivo as well.