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Lipoic Acid

Lipoic acid was discovered in the year 1953 by reed and Gunsalus who extracted it from the liver. Its utility as a growth factor in certain bacteria and protozoans was established, hence it is considered to be an essential dietary requirement. Liver and yeast are rich sources of lipoic acid.
Lipoic acid functions as a cofactor in the transfer of hydrogen, occurring in oxidative decarboxylation reactions. Lipoic acid occurs in oxidized as well as reduced forms.

lipoic acid

Biochemical functions  Lipoic acid is firmly bound to protein and can be released from it by hydrolytic enzymes, or acid or alkali treatment. It is covalently bound to the amino acid lysine as N-lipolyl – lysin having a striking similarity with biocytin (N-biotinyl-lysine). Lipoic and participates in complex reactions catalyzed by pyruvate dehydrogenase system and α -ketoglutarate dehydrogenase complex.

riboflavin and riboflavin mnophosphate or flavin monnucleotide(FMN)

flavin adenine dinucleotide

oxidized flavin and reduced flavin

lipoic acid in pyruvate dehydrogenase complex

Acting as both oxidizing agent and receptor of the succinyl group. In this reaction the lipoyl group is reduced to dihydrolipoyl complex with the succinyl group attached as thiol ester.

In the next reaction, transsuccinylase transfers succinyl group to coenzyme A to form succinyl coenzyme A. This step completes the oxidative decarboxylation of α ketoglutarate, and the eletrons removed from the substrate are present in the thiol groups of dihydrolipoic acid .Now a third enzyme, dihydrolipoamide hehydrogenase catalyzes the oxidation of transsuccinylase and production of NADH. The enzyme dihydrolipoamide dehydrogenase is a flavoprotein and it seems surprising that this enzyme appears to be a special case. It is observed that the reaction is catalyzed by the cooperative action of the FAD coenzyme which regenerates the oxidized lipoyl component.

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