For the pyruvate dehydrogenase complex, what is the specific chemical function of the lipoic acid cofactor?

The pyruvate dehydrogenase complex (PDH) is the enzyme complex responsible for converting pyruvate into acetyl-CoA which will then enter the TCA cycle. It is composed of 3 enzymes namely the pyruvate decarboxylase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). 

In the E1, pyruvate is decarboxylated, carbon dioxide is released and replaced by thiamine pyrophosphate (TPP). In the E2, TPP is released and replaced by lipoic acid. The addition of lipoic acid allows the addition of coenzyme A (CoA) to the molecule therefore ultimately producing acetyl CoA.

B. a graduated substance