E1: pyruvate decarboxylase
                     - thiamine pyrophosphate  - TPP
         TPP removes -COOH from pyruvate leaving
         2 carbon fragment and links the 2C-acyl
         fragment to the TPP-S via a thiol.

 E2: dihydrolipoyl transacetylase
                     - lipoate & coenzyme-A
         2 carbon acyl group is transferred  to one
         lipoate arm, and then to the CoA-SH,
         resulting fully reduced lipoate.  

 E3: dihydrolipoyl dehydrogenase - FAD, NAD⁺
         the reduced lipoates transfers 2 :H's to 
         E-FAD -->  E-FADH₂,  and then
         E-FADH₂ passes :H to NAD⁺ --> NADH
                O                           O 
                ||                            ||
           CH₃-C-COOH   ----->  CH₃-C-SCoA 
                         NAD -----------> NADH     
     
                 Model of PDH action *