98 H. A. Krebs 



The succinic semialdehyde TPP complex then reacts with the 

 disulphide form of a-Hpoic acid in such a manner that (a) the 

 aldehyde group of the TPP complex is oxidized to the corres- 

 ponding carboxyl whilst the disulphide is reduced to the 

 dimercaptan; (b) the nascent carboxyl and one of the nascent 

 mercaptan groups condense to form succinyl lipoic acid. TPP 

 is regenerated in this reaction: 



S-CH R.CO.S-CH 



[R . COOH . TPP] + I ^CH2 -> /CH2 + TPP 



S-C HS-C^ 



H2 Hj 



(R' =COOH.(CH2)4) 



The next stage is a transfer of the succinyl group from lipoic 

 acid to coenzyme A, yielding reduced a-lipoic acid and succinyl 

 coenzyme A: 



/R yR 



R.CO.S— CH HS-CH 



')cH2 + HS.CoA-^R.CO.S.CoA+ ^CH2 



HS-CH2 HS-C 



H2 



The reduced lipoic acid interacts with DPN under the 

 influence of lipoic acid dehydrogenase to yield a reduced 

 pyridine nucleotide: 



/R' yR' 



HS-CH S-CH 



:CH 



2 



yCU^ + DPN -> DPNH2 + 

 HS-C^ S-C" 



H2 H2 



Succinyl coenzyme A reacts with GDP and inorganic phos- 

 phate to regenerate reduced coenzyme A and to form GTP 



