Transketolase, an enzyme of the Calvin Cycle of photosynthesis, catalyzes the transfer of a 2-carbon fragment from a 5-carbon keto sugar (D-xyulose-5-P) to a 5-carbon aldo sugar (D-ribose-5-P) to form a 7-carbon keto sugar (sedoheptulose-7-P) and a 3-carbon aldo sugar (glyceraldehyde-3-P). The overall reaction is given below.

Thiamin pyrophosphate (TPP) is a cofactor in this reaction.

1. Indicate the chemical steps involved in the transketolase reaction showing the role of TPP.

2. Based on the substrates and products of transketolase, one can make some tentative conclusions concerning the substrate binding site on this enzyme. which of the following compounds would be most likely substitute for either Xu-5-P or S-7-P in the reaction? Explain your reasoning.

3. Which of the sugar phosphate substrates for transketolase will form cyclic hemiacetals or hemiketal? Draw the structures.
4. The bacterial enzyme phosphoketolase is in some respects similar to transketolase. It reacts with D-fructose-6-P (compound II in question 2) and inorganic phosphate to yield acetyl phosphate and erythrose-4-P as shown below.

The first step in this reaction is analogous to transketolase, however, the first intermediate rearranges to yield the following intermediate.