Determining the Conservation and Metabolic Role of Sirtuin Lipoamidase Activity

Abstract

Lipoic acid is a necessary cofactor for several evolutionarily-conserved metabolic enzymes, including the pyruvate dehydrogenase complex (PDH) and alpha-ketoglutarate dehydrogenase complex (KDH). Its cleavage from these enzymes is an important method of metabolic regulation, but much about its function and regulation remains unknown. In mammals, sirtuin 4 (SIRT4) was identified as the first known mammalian cellular lipoamidase. Interestingly, bacteria also possess sirtuin homologs that, similar to human sirtuins, were found to have antiviral properties. CobB in E. coli and SrtN in B. subtilis closely resemble SIRT4 in sequence homology. This research examines the conservation of SIRT4 lipoamidase activity in bacteria and its role in varying metabolic environments. My results indicate that levels of CobB and SrtN inversely correlate with the lipoylation and activity of PDH and KDH. The lipoamidase action of CobB was further confirmed in vitro. I also show CobB interaction with lipoylated metabolic subunits of enzyme complexes including PDH and KDH. Measurement of PDH and KDH activity in cells grown with varying carbon sources indicate that sirtuin activity inhibits PDH and KDH activity in some metabolic environments. These results demonstrate that sirtuin lipoamidase activity is conserved in bacteria and that it is one method of regulating metabolic enzymes. Since the activity of lipoylated enzyme complexes has been linked to several disease states, further examination of the role of sirtuin lipoamidase activity in metabolic regulation could establish a foundation for the development of new therapies.