mechanism: sulfur is first inserted at C6 of octanoyl substrate to form an enzyme bound intermediate. In a subsequent rate determining step, the second sulfur atom is inserted at C8, suggesting an energy profile of the reaction reflecting the relative bond strengths of the primary and secondary C-H bonds to be cleaved at C8 and C6, resp.
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protein N6-(octanoyl)lysine:sulfur-(sulfur carrier) sulfurtransferase
This enzyme is a member of the 'AdoMet radical' (radical SAM) family, all members of which produce the 5'-deoxyadenosin-5'-yl radical and methionine from AdoMet [i.e. S-adenosylmethionine, or S-(5'-deoxyadenosin-5'-yl)methionine], by the addition of an electron from an iron-sulfur centre. The radical is converted into 5'-deoxyadenosine when it abstracts a hydrogen atom from C-6 and C-8, leaving reactive radicals at these positions so that they can add sulfur, with inversion of configuration . This enzyme catalyses the final step in the de-novo biosynthesis of the lipoyl cofactor, with the other enzyme involved being EC 188.8.131.52, lipoyl(octanoyl) transferase. Lipoylation is essential for the function of several key enzymes involved in oxidative metabolism, as it converts apoprotein into the biologically active holoprotein. Examples of such lipoylated proteins include pyruvate dehydrogenase (E2 domain), 2-oxoglutarate dehydrogenase (E2 domain), the branched-chain 2-oxoacid dehydrogenases and the glycine cleavage system (H protein) [2,5]. An alternative lipoylation pathway involves EC 184.108.40.206, lipoate---protein ligase, which can lipoylate apoproteins using exogenous lipoic acid (or its analogues) .
lipoyl-bearing subunit of the glycine cleavage system (H-protein) is a substrate for LipA. 5'-deoxyadenosyl radical acts directly on the octanoyl substrate. 2 equivalents of S-adenosyl-L-methionine are cleaved irreversibly in forming 1 equivalent of [lipoyl]H-protein and are consistent with a model in which two LipA proteins are required to synthesize one lipoyl group
coexpression of lipA with groESL, trxA, or fragments of the isc operon such as iscSUA orhscBAfdx does not improve expression levels of soluble holo-LipA. Coexpression of lipA with iscSUA and hscBAfdx on a multi-cistronic plasmid does improve the expression of soluble LipAH and increases the molar ratios of iron and sulfide per LipAH
Lipoic acid metabolism in Escherichia coli: isolation of null mutants defective in lipoic acid biosynthesis, molecular cloning and characterization of the E. coli lip locus, and identification of the lipoylated protein of the glycine cleavage system