A novel pathway for lipoic acid synthesis in Gram-positive bacteria

MARTÍN, N; CHRISTENSEN, Q; CRONAN, J; DE MENDOZA, D; MANSILLA, MC

Montecatini Terme

Congreso; 6th Conference on Functional Genomics of Gram-Positive Microorganisms - 16th International Conference on Bacilli; 2011

Lipoic acid (LA) is a covalently bound cofactor essential for the function of several key enzymes involved in oxidative and single carbon metabolism. The genes for LA synthesis were initially identified in Escherichia coli over ten years ago, and the current model for protein lipoylation is based in the mechanism employed by this bacterium. It involves two pathways: one in which exogenous LA is transferred to apoproteins in a process mediated by LA ligase (LplA), and an endogenous one, that involves LipB, which transfers octanoate to target proteins. These octanoylated domains are converted into lipoylated derivatives by lipoyl synthase (LipA). We have previously shown that B. subtilis is able to synthesize LA, and if exogenously provided, ligate it to apoproteins. We have also demonstrated that LipL and LipM are essential for the attachment of octanoic acid to the apoenzymes. However, the role of each of these proteins was not clear. In this work we performed physiological and biochemical characterization of different LA auxotrophic mutants and used in vitro biochemical assays that allowed us to conclude that B. subtilis protein lipoylation is carried out through a novel mechanism. This pathway involves the sequential action of LipM and LipL and a third protein, the glycine cleavage system H protein, which acts as a novel lipoyl/octanoyl carrier. In B. subtilis four proteins, LipM, GcvH, LipL, and LipA, are essential for the endogenous protein lipoylation pathway, instead of the two-protein model of E. coli. Notably, LipL homologues can be found in pathogenic bacteria closely related to B. subtilis such as Staphylococcus aureus and B. anthracis. The strong phenotype of ΔlipL strains suggests that LipL might be an excellent target for the development of new antimicrobials.