Opening a New Path to Lipoic Acid

Lipoic acid (Fig. 1) is a cofactor that is critical to the catalytic function of several key enzymes in intermediary metabolism that are found in all domains of life. There are two pathways to lipoyl-modified proteins. The de novo pathway from octanoyl- acyl carrier protein (ACP) involves the consecutive action of LipB and LipA (Fig. 2A), and the LplA-dependent salvage pathway activates and transfers environmental lipoic acid to the lipoyl protein domains (Fig. 2B). In this issue, Hermes and Cronan (6) identify gain-of-function mutations in LplA that allow cells to bypass the requirement for LipB in de novo lipoic acid formation (Fig. 2C). Normally, lipB null strains require either a lipoate or an octanoate supplement for growth, but these lplA mutants have increased affinities for octanoic acid, permitting them to utilize the low concentrations of intracel- lular octanoate, thus substituting for the LipB reaction. The characterization of these suppressor strains provides new in- formation on the biochemistry of lipoate metabolism and re- veals the existence of intracellular pools of medium-chain fatty acids that previously were not thought to exist. Functions of lipoic acid. Lipoic acid is essential for the functions of several enzymes in oxidative metabolism (2). A good example is pyruvate dehydrogenase (PDH), where co- valently bound lipoate shuttles reaction intermediates between the active sites of a multisubunit complex (11). Lipoic acid is bound to the dihydrolipoyl transacetylase (E2) subunit in its oxidized state as a five-membered, disulfide-linked ring (Fig. 1). The E1 subunit of PDH decarboxylates pyruvate, forming 2-hydroxyethyl-2-thiamine pyrophosphate, and the lipoic acid disulfide is reduced when E2 accepts an acetaldehyde moiety from the thiamine adduct. Acetyl-coenzyme A is then released via a thioester exchange reaction with coenzyme A. The disul- fide ring of lipoate is reformed by the FAD-dependent dihy- drolipoyl dehydrogenase component (E3). Other important bacterial lipoyl enzymes include the glycine cleavage system, -ketoglutarate dehydrogenase, and the branched-chain - ketoacid dehydrogenase that is involved in supplying branched-chain precursors for fatty acid synthesis. In Esche- richia coli, strains defective in lipoic acid synthesis are unable to grow aerobically on glucose minimal medium unless it is supplemented with acetate and succinate to bypass the require- ment for PDH and -ketoglutarate dehydrogenase (5, 13). Bypassing LipB. The de novo biosynthetic pathway in E. coli requires lipA and lipB (Fig. 2A). Genetic studies identified the product of the lipA gene as responsible for inserting the two sulfur atoms into the octanoate precursor (13). LipA is an S-adenosylmethionine-dependent enzyme that inserts sulfur at