Elementary plant Biochemistry and Biotechnology (2+1)

Thiamine

• The coenzyme form of thiamine, thiamine pyrophosphate (TPP). Structure of thiamine pyrophosphate.
The two important reactions in which TPP functions as coenzyme are

1. Oxidative decarboxylation of α-keto acids such as pyruvate and α-ketoglutarate.
2. Transketolase reaction


• TPP provides a reactive carbon on the thiazole ring and forms a carbanion stabilized by positively charged ring nitrogen. The carbanion is then free to add the carbonyl of pyruvate (or α-ketoglutarate).
  
• The addition compound is then decarboxylated eliminating CO2 and generating hydroxyethyl-TPP.
• This reaction occurs in a multienzyme complex known as pyruvate dehydrogenase complex (or α-ketoglutarate dehydrogenase complex).
• The acetaldehyde (decarboxylated product) moiety is then transferred to the lipoamide in the complex.
• The role of TPP as a coenzyme in the transketolase reaction is very similar to that of oxidative decarboxylation.
  
• The carbanion of TPP combines with the carbonyl carbon of xylulose 5P.
• Carbon 1 and 2 of xylulose 5P are retained to form hydroxyethyl derivative of TPP.
• Then it is transferred to the carbonyl carbon of ribose 5P to form sedoheptulose 7P.