Transketolase

Transketolase is an enzyme of the pentose phosphate pathway catalyzing the reactions below:

Xylulose-5-Phosphate + Ribose-5-Phosphate <=> Glyceraldehyde-3-Phosphate + Sedoheptulose-7-Phosphate

and

Xylulose-5-Phosphate + Erythrose-4-Phosphate <=> Glyceraldehyde-3-Phosphate + Fructose-6-Phosphate

In the Calvin cycle, transketolase also catalyzes the reversals of these reactions above.

The reactions involve transfer of a two carbon acetaldehyde fragment from xylulose-5-phosphate to an acceptor sugar phosphate. The reactions require thiamine pyrophosphate (TPP) as a cofactor, with the two-carbon fragment bound transiently to carbon 2 of the thiazole ring of TPP.

INTERNET LINKS:

1. Pentose Phosphate Pathway

2. CO2 Fixation