Acyl-CoA Carboxylases in Actinomycetes: from polyketide biosynthesis to structure-based inhibitor design

HUGO GRAMAJO

Rosario, Argentina

Conferencia; Congreso Sociedad Argentina de Microbiología General; 2008

The acetyl-CoA carboxylase (ACC) catalyzes the first committed step in the biosynthesis of long-chain fatty acids in all animals, plants and bacteria. The reaction catalyzed by this enzyme involves two half-reactions and three components. In the first reaction the biotin carboxylase (BC) transfers the carboxyl group from carboxyphospate to the biotin that is attached to the biotin carboxyl carrier protein (BCCP). In the second reaction, catalyzed by carboxyltransferase (CT), the carboxyl group is transferred from biotin to acetyl-CoA to form malonyl-CoA, the building block of fatty acid biosynthesis. The function, organization and stoichiometry of ACC are highly diverse among different organisms. In actinomycetes, for example, the system is different than in most bacteria, because they do not have a canonical acetyl-CoA carboxylase , instead they posses enzyme complexes commonly named as acyl-CoA carboxylases (ACCase), due to their relaxed substrate specificity, e.g. they can carboxylate other substrates besides acetyl-CoA like propionyl- and butyryl-CoA to yield methyl- and ethylmalonyl-CoA. We became interested in these enzymes complexe in the context of two different genera of actinomycetes, Streptomyces and Mycobacterium, for completely different reasons. In Streptomyces, because the ACCases provide the building blocks (malonyl-CoA, methylmalonyl-CoA, ethylmalonyl-CoA) for the synthesis of a vast array of secondary metabolites (polyketides) with important biological properties. In Mycobacterium because these enzymes provide the substrate for the synthesis of all the exquisite  variety of complex lipids present in these bacteria, like the mycolic acids and mutly-methyl branched fatty acids, that are key constituents of the cell envelope and are essential for the survival and the pathogenicity of these microorganisms. In this talk I will present the biochemical and structural characterization of two ACCase complexes from Streptomyces coelicolor and Mycobacterium smegmatis and the molecular basis of their substrate specificity. I will also present genetics and physiological experiments that confirm which is the dedicated acetyl-CoA carboxylase in both micro-organisms. Finally, I will address how we used the knowledge gained by studying these enzymes for the synthesis of secondary metabolites (polyketides ) in heterologous host like E. coli and for the structure-based identification of  ACCase inhibitors and its use as putative anti-mycobacterial agents.