Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation

Abstract

Summary Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as “swinging arms” that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well-described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like “arm” of biotin were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise and the BioH esterase for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl-ACP of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyl transferase followed by sulfur insertion at carbons C6 and C8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized and thus there is no transcriptional control of the synthetic genes. In contrast transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system, exerted through BirA a dual function protein that both represses biotin operon transcription and ligates biotin to its cognate protein.

Lipoic acid metabolism and mitochondrial redox regulation

“…Lipoic acid (LA) metabolism has been intensely investigated by Lester Reed (1), John Cronan (4,5), and others through genetic and biochemical studies using prokaryotes and S.cerevisiae (6). Homologous enzymes have been identified in plants (7,8) and mammalian systems (9).…”

“…This step is similar to biotin synthesis in that the reaction mechanism requires sulfur donation from an [4Fe-4S] cluster within the enzyme and reductive cleavage of s-adenosylmethionine (SAM) to generate 5'deoxyadenosyl 5'-radicals (5'-dA • ) that remove a hydrogen atom from C6 and C8 and facilitate insertion of the sulfur atoms (5,18,19). The requirements for this enzyme are similar to that of LipB in that E. coli can utilize the LA salvage pathway to compensate for loss of LipA in lipoate containing media (15).…”

The Streptomyces coelicolor Lipoate-protein Ligase Is a Circularly Permuted Version of the Escherichia coli Enzyme Composed of Discrete Interacting Domains

“…In Escherichia coli, lipoic acid may be directly scavenged from the environment or synthesized de novo (2)(3)(4)(5)(6). Studies in our laboratory and others (8,27) have elucidated two independent pathways in E. coli depending on the source of lipoic acid (Fig.…”

“…It is required for the function of several key enzymes involved in central carbon metabolism (1). The 2-oxoacid dehydrogenases and the glycine cleavage system contain lipoyl domains (LDs) 2 that are highly conserved and contain a specific lysine residue to which lipoic acid is attached (2,3) by an amide linkage between the lipoic acid carboxyl group and the lysine residue ⑀-amino group. The lipoyl moiety plays a unique role in catalysis.…”

An Eight-Residue Deletion in Escherichia coli FabG Causes Temperature-Sensitive Growth and Lipid Synthesis Plus Resistance to the Calmodulin Inhibitor Trifluoperazine

“…The resulting intermediate is dehydrated by a ␤-hydroxyacyl-ACP dehydratase (FabA or FabZ) to an enoyl-ACP and is finally reduced again by an enoyl-ACP reductase (FabI), in an NADH-dependent reaction, to give an acyl-ACP that is two carbons longer and poised to reenter the cycle until the appropriate chain length for complex lipid synthesis is achieved. Some of the early intermediates are siphoned off by FAS-dependent synthetic pathways such as those for biotin (4) and lipoic acid (4), whereas longer fatty acids are predominantly utilized for the synthesis of the membrane components, lipid A (5), and phospholipids (5).…”