Two [4Fe-4S] clusters (203). One cluster is coordinated by the radical SAM CXXXCXXC motif that functions in 5-dA generation. The second cluster is coordinated by the CXXXXCXXXXXC motif, which, thus far, is unique to lipoic acid synthases and suggested to be the source of the sulfur atoms. This has been addressed (as in BioB (63)) by preparation of LipA from E. coli cells grown on an isotopically labeled sulfur source (34S) (250). As expected the lipoic acid formed using this enzyme in vitro was isotopically labeled with 34S. Moreover, when the reactions were performed with equimolar amounts of 32S-labeled LipA and 34S-labeled LipA the lipoic acid molecules formed contained either two 32S atoms or two 3S-atoms (250). Thus both sulfur atoms emanate from the same polypeptide thereby eliminating the possibility that the monothiolated species are released and the second sulfur atom is inserted following rebinding to LipA. If release and rebinding occurred, half of the lipoic acid formed would have one atom each of 32S and 34S. Such mechanistic experiments are facilitated if an octanoylated peptide substrate can be substituted for the octanoylated lipoyl Monocrotaline custom synthesis domain as is the case for an ML390MedChemExpress ML390 octanoyl-tripeptide in the Sulfolobus solfataricus LipA assay (242). This system has recently been used to show that S. solfataricus LipA catalyzed lipoate biosynthesis in a stepwise manner with sulfur first being inserted at C-6 of the octanoyl chain (251). However, this intermediate remained tightly bound to LipA. This is consistent with the finding that both sulfur atoms are derived from the same LipA polypeptide and implies that the sulfur atom of the intermediate may still exist as part of the Fe/S cluster. Incorporation of the second sulfur is much slower perhaps due to rearrangement of the Fe/S cluster. A current model of the LipA reaction is given in Fig. 12. It should be noted that the finding that lipoic acid synthesis proceeds through an octanoyldomain intermediate explains a previously puzzling observation first made by Ail and Guest (182, 223) and subsequently by others (252). These workers found that upon overproduction of a lipoyl domain in E. coli three species of domain were obtained, the expected apo and lipoylated domains plus a third species subsequently shown to be octanoylated domain. Based on conventional biochemistry in which LipA would produce free lipoic acid, the octanoylated domain was thought to be an anomalous product resulting from the lack of sufficient free lipoic acid to modify the overexpressed domain plus a lack of specificity of the attachment enzyme (182, 223). From the present pathway it now seems clear that LipA was limiting (as was proposed), but the octanoylated domain was an accumulated intermediate rather than the aberrant byproduct of overproduction. Finally as expected from the BioB data LipA is inhibited by the 5-deoxyadenosine producd in the reaction. The first indication was that addition of lipoic acid and biotin to a mtm (pfs) strain gave better growth than biotin alone (21). 5-Deoxyadenosine inhibition ofAuthor Manuscript Author Manuscript Author Manuscript Author ManuscriptEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPageLipA and BioB that is potentiated by methionine and reversed by addition of Mtn has recently been demonstrated in vitro (76). LipA and LipB form stable complexes with the 2-oxoacid dehydrogenases of E. coli. The experiment discussed above in which the LipB-acyl enzyme i.Two [4Fe-4S] clusters (203). One cluster is coordinated by the radical SAM CXXXCXXC motif that functions in 5-dA generation. The second cluster is coordinated by the CXXXXCXXXXXC motif, which, thus far, is unique to lipoic acid synthases and suggested to be the source of the sulfur atoms. This has been addressed (as in BioB (63)) by preparation of LipA from E. coli cells grown on an isotopically labeled sulfur source (34S) (250). As expected the lipoic acid formed using this enzyme in vitro was isotopically labeled with 34S. Moreover, when the reactions were performed with equimolar amounts of 32S-labeled LipA and 34S-labeled LipA the lipoic acid molecules formed contained either two 32S atoms or two 3S-atoms (250). Thus both sulfur atoms emanate from the same polypeptide thereby eliminating the possibility that the monothiolated species are released and the second sulfur atom is inserted following rebinding to LipA. If release and rebinding occurred, half of the lipoic acid formed would have one atom each of 32S and 34S. Such mechanistic experiments are facilitated if an octanoylated peptide substrate can be substituted for the octanoylated lipoyl domain as is the case for an octanoyl-tripeptide in the Sulfolobus solfataricus LipA assay (242). This system has recently been used to show that S. solfataricus LipA catalyzed lipoate biosynthesis in a stepwise manner with sulfur first being inserted at C-6 of the octanoyl chain (251). However, this intermediate remained tightly bound to LipA. This is consistent with the finding that both sulfur atoms are derived from the same LipA polypeptide and implies that the sulfur atom of the intermediate may still exist as part of the Fe/S cluster. Incorporation of the second sulfur is much slower perhaps due to rearrangement of the Fe/S cluster. A current model of the LipA reaction is given in Fig. 12. It should be noted that the finding that lipoic acid synthesis proceeds through an octanoyldomain intermediate explains a previously puzzling observation first made by Ail and Guest (182, 223) and subsequently by others (252). These workers found that upon overproduction of a lipoyl domain in E. coli three species of domain were obtained, the expected apo and lipoylated domains plus a third species subsequently shown to be octanoylated domain. Based on conventional biochemistry in which LipA would produce free lipoic acid, the octanoylated domain was thought to be an anomalous product resulting from the lack of sufficient free lipoic acid to modify the overexpressed domain plus a lack of specificity of the attachment enzyme (182, 223). From the present pathway it now seems clear that LipA was limiting (as was proposed), but the octanoylated domain was an accumulated intermediate rather than the aberrant byproduct of overproduction. Finally as expected from the BioB data LipA is inhibited by the 5-deoxyadenosine producd in the reaction. The first indication was that addition of lipoic acid and biotin to a mtm (pfs) strain gave better growth than biotin alone (21). 5-Deoxyadenosine inhibition ofAuthor Manuscript Author Manuscript Author Manuscript Author ManuscriptEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPageLipA and BioB that is potentiated by methionine and reversed by addition of Mtn has recently been demonstrated in vitro (76). LipA and LipB form stable complexes with the 2-oxoacid dehydrogenases of E. coli. The experiment discussed above in which the LipB-acyl enzyme i.
