Ieties serve as carriers of reaction intermediates among the multiple active sites of these multienzyme complexes (141). Our IRC-022493 cancer knowledge of the pathways of lipoic acid synthesis, attachment and function has progressed rapidly in the last 10 years largely due to complementary genetic and biochemical analyses in E. coli. I shall first discuss the enzymes that carry and require the cofactor because they are derived from diverse areas of metabolism. Next, the mechanisms of attachment of lipoic acid and its precursor, octanoic acid, to these proteins will be reviewed. Finally, the synthesis of the cofactor itself will be discussed. This organization was chosen because the unusual biosynthetic pathway of lipoic acid is mechanistically intertwined with attachment of the cofactor.Lipoic acid-dependent enzymesPyruvate dehydrogenase (PDH) The PDH reaction mechanism is probably the most thoroughly characterized lipoic aciddependent enzyme. PDH catalyzes the oxidative decarboxylation of pyruvate to the key metabolic intermediate, acetyl-CoA. This very large enzyme complex consists of multiple copies of each of three subunits encoded by the aceE aceF lpd operon. The first subunit (AceE) is a thiamine diphosphate-dependent decarboxylase (E1p) that catalyzes both the decarboxylation of pyruvate and the reductive acetylation of the lipoyl group that is covalently attached to the second subunit, E2p (AceF). The E2p subunit is a dihydrolipoyl acetyltransferase responsible for the transfer of the acyl group from lipoyl moiety to CoA to form acetyl-CoA. The third subunit, E3 (Lpd), is a dihydrolipoyl dehydrogenase that serves to regenerate the disulfide bond of the lipoyl moiety of E2p (143) and thereby prepares the enzyme for another cycle of catalysis. The E2p subunit to which E1 and E3 are bound strongly (but noncovalently) forms the structural core of the multienzyme complex. The oxidative decarboxylation of pyruvate to form acetyl-CoA is the link between glycolysis and the citric acid cycle and therefore PDH activity is essential to cells that rely upon respiration to provide metabolic energy. In most aerobically respiring organisms the PDH complex alsoEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPagesupplies the acetyl-CoA necessary to sustain essential biosynthetic pathways, especially those of fatty acid and amino acid synthesis (144). Synthesis of the PDH complex varies over a 7- to 10-fold range depending on the growth conditions (145?47). It is induced by exogenous pyruvate or when pyruvate is generated endogenously e.g. by thiamine starvation, and it is partially repressed by excess glucose and during growth on acetate or on citric acid cycle intermediates. Regulation by pyruvate or a derivative of pyruvate proceeds through the PdhR repressor (148, 149). PDH synthesis is repressed during anaerobic fermentative growth where the catalytic activity is also inhibited. Under these conditions the conversion of pyruvate to acetyl-CoA is mediated by the S28463 site derepression and activation of pyruvate formate lyase (146, 150). 2-Oxoglutarate dehydrogenase (2-OGDH) The mechanism of 2-OGDH is essentially the same as that of PDH as is the structure of the complex. Indeed, the 2-OGDH complex has been reported to contain low levels of PDH subunits (151). The 2-OGDH complex contains three subunits, a 2-oxoglutarate decarboxylase component (E1o), a trans-succinylase component (E2o) and a dihydrolipoyl dehydrogenase (E3). The E1o and E2o subunit.Ieties serve as carriers of reaction intermediates among the multiple active sites of these multienzyme complexes (141). Our knowledge of the pathways of lipoic acid synthesis, attachment and function has progressed rapidly in the last 10 years largely due to complementary genetic and biochemical analyses in E. coli. I shall first discuss the enzymes that carry and require the cofactor because they are derived from diverse areas of metabolism. Next, the mechanisms of attachment of lipoic acid and its precursor, octanoic acid, to these proteins will be reviewed. Finally, the synthesis of the cofactor itself will be discussed. This organization was chosen because the unusual biosynthetic pathway of lipoic acid is mechanistically intertwined with attachment of the cofactor.Lipoic acid-dependent enzymesPyruvate dehydrogenase (PDH) The PDH reaction mechanism is probably the most thoroughly characterized lipoic aciddependent enzyme. PDH catalyzes the oxidative decarboxylation of pyruvate to the key metabolic intermediate, acetyl-CoA. This very large enzyme complex consists of multiple copies of each of three subunits encoded by the aceE aceF lpd operon. The first subunit (AceE) is a thiamine diphosphate-dependent decarboxylase (E1p) that catalyzes both the decarboxylation of pyruvate and the reductive acetylation of the lipoyl group that is covalently attached to the second subunit, E2p (AceF). The E2p subunit is a dihydrolipoyl acetyltransferase responsible for the transfer of the acyl group from lipoyl moiety to CoA to form acetyl-CoA. The third subunit, E3 (Lpd), is a dihydrolipoyl dehydrogenase that serves to regenerate the disulfide bond of the lipoyl moiety of E2p (143) and thereby prepares the enzyme for another cycle of catalysis. The E2p subunit to which E1 and E3 are bound strongly (but noncovalently) forms the structural core of the multienzyme complex. The oxidative decarboxylation of pyruvate to form acetyl-CoA is the link between glycolysis and the citric acid cycle and therefore PDH activity is essential to cells that rely upon respiration to provide metabolic energy. In most aerobically respiring organisms the PDH complex alsoEcoSal Plus. Author manuscript; available in PMC 2015 January 06.CronanPagesupplies the acetyl-CoA necessary to sustain essential biosynthetic pathways, especially those of fatty acid and amino acid synthesis (144). Synthesis of the PDH complex varies over a 7- to 10-fold range depending on the growth conditions (145?47). It is induced by exogenous pyruvate or when pyruvate is generated endogenously e.g. by thiamine starvation, and it is partially repressed by excess glucose and during growth on acetate or on citric acid cycle intermediates. Regulation by pyruvate or a derivative of pyruvate proceeds through the PdhR repressor (148, 149). PDH synthesis is repressed during anaerobic fermentative growth where the catalytic activity is also inhibited. Under these conditions the conversion of pyruvate to acetyl-CoA is mediated by the derepression and activation of pyruvate formate lyase (146, 150). 2-Oxoglutarate dehydrogenase (2-OGDH) The mechanism of 2-OGDH is essentially the same as that of PDH as is the structure of the complex. Indeed, the 2-OGDH complex has been reported to contain low levels of PDH subunits (151). The 2-OGDH complex contains three subunits, a 2-oxoglutarate decarboxylase component (E1o), a trans-succinylase component (E2o) and a dihydrolipoyl dehydrogenase (E3). The E1o and E2o subunit.
