The lipoate acyltransferase subunits of the 2‐oxo acid dehydrogenase complexes are post‐translationally modified with one or more covalently‐bound lipoyl cofactors. Two distinct lipoate‐protein ligase activities, LPL‐A and LPL‐B, have been detected in E. coli by their ability to modify purified lipoyl apo‐domains of the bacterial pyruvate dehydrogenase complex. Both enzymes require ATP and Mg2+, use L‐lipoate, 8‐methyllipoate, lipoyl adenylate and octanoyl adenylate as substrates, and both activate lipoyl‐deficient pyruvate dehydrogenase complexes. In contrast, only LPL‐B uses D‐lipoate and octanoate and there are differences in the metal‐ion and phosphate requirements. It is suggested that LPL‐B may be responsible for the octanoylation of lipoyl domains observed previously under lipoate‐deficient conditions.