Biosynthesis and Attachment
The precursor to lipoic acid, octanoic acid, is made via fatty acid biosynthesis in the form of octanoyl-acyl carrier protein. In eukaryotes, a second fatty acid biosynthetic pathway in mitochondria is used for this purpose. The octanoate is transferred from a thioester of acyl carrier protein to an amide of the lipoyl domain by an octanoyltransferase. The sulfur centers are inserted into the 6th and 8th carbons of octanoate via a radical SAM mechanism, by lipoyl synthase. The sulfurs are from the lipoyl synthase polypeptide. As a result, lipoic acid is synthesized on the lipoyl domain and no free lipoic acid is produced. Lipoic acid can be removed whenever proteins are degraded and by action of the enzyme lipoamidase. Free lipoate can be attached to the lipoyl domain by the enzyme lipoate protein ligase. The ligase activity of this enzyme requires ATP. Lipoate protein ligases proceed via an enzyme bound lipoyl adenylate intermediate.
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