Lipoic acid is often found near the active sites of enzymes, usually bound to the peptide by a long, flexible amide linkage with a lysine residue.

(a) Is lipoic acid a mild oxidizing agent or a mild reducing agent? Draw it in both its oxidized and reduced forms.

(b) Show how lipoic acid might react with two Cys residues to form a disulfide bridge.

(c) Give a balanced equation for the hypothetical oxidation or reduction, as you predicted in part (a), of an aldehyde by lipoic acid.

Lipoic acid is a mild oxidizing agent. Thus, it will oxidise other species and itself gets reduced. Reduction means addition of hydrogen whereas oxidation means, addition of oxygen. Hence, reduced form of lipoic acid will contain SH group.

Oxidized form Reduced form

Lipoic acid reacts with two cysteine residues and forms disulfide bridge. It acts as a oxidising agent and itself gets reduced to the dihydrolipoic acid. Disulfide bridges stabilises protein structure and its formation involves a reaction between sulfhydryl side chains of two cysteine residues.

Disulfide bridge formation

When an aldehyde reacts with lipoic acid, then lipoic acid will acts as an oxidizing agent and will oxidize aldehyde to carboxylic acid and itself gets reduced to dihydrolipoic acid.

Aldehyde Carboxylic acid