Bacterial l-leucine catabolism as a source of secondary metabolites

Abstract

l-Leucine can be assimilated by bacteria when sugars or other preferential carbon sources in the habitat are depleted. The l-leucine catabolism is widely spread among bacteria and has been thoroughly studied. Its pathway is comprised by multiple reactions and converges with other catabolic routes, generating acetoacetate and acetyl-CoA as its final products. The initial three steps are conserved in most bacteria, constituting the first steps of the branched-chain amino acids catabolic pathway. The main product of these sequential reactions is the 3-methylcrotonyl-CoA metabolite, which undergoes further enzymatic steps towards the production of acetoacetate and acetyl-CoA. These, however, are not always the final products of l-leucine catabolism, as intermediates of the pathway can further synthesize fatty acids or feed other secondary metabolism pathways in order to produce diverse compounds which can exhibit biological activities. This alternative metabolism typically leads to the accumulation of products bearing industrial relevance, including volatile compounds used in the food industry, compounds with antimicrobial activity, production of biofuels and biopolymers. In anaerobic bacteria, the l-leucine catabolism may induce the accumulation of a variety of organic compounds acids, such as isovaleric, isocaproic, and 2-methylbutyric acids. In conclusion, the usage by bacterial species of l-leucine as an alternative carbon and nitrogen source may contribute to their environment adaptability and, more importantly, the diverse products that can be obtained from l-leucine metabolism may be represent a valuable source of compounds of biotechnological interest.