Pentose phosphate catabolism

Transketolase catalyzes a transfer of a two-carbon fragment from xylulose-5-phosphate to ribose-5-phosphate to give a triose phosphate, glyceraldehyde-3-phosphate, and a seven-carbon sugar, sedoheptulose-7-phosphate.

In the final reaction of pentose phosphate catabolism, transketolase acts on another molecule of xylulose-5-phosphate, transferring a glycolaldehyde fragment to erythrose-4-phosphate and generating a three-carbon product and a six-carbon product—glyceraldehyde-3-phosphate and fructose-6-phosphate, respectively.