Question

Please do not forget to write down all the names of your group members.

1. Please propose the mechanism for α−β C-C bond cleavage of G6P. Draw the mechanism clearly, give the structures of the product, and then explain why steps 2 and 3 of glycolysis are necessary.

2. If glucose is labeled at C1 carbon (the C1 carbon is tagged with radioactive 14C), where would this labeled C atom be (which carbon atom number, draw the structure and label this carbon atom) when glucose has been metabolized to:

A. DHAP

B. GAP (you should be able to figure which carbon atom is the original C1 if you draw the mechanism of step 5)

C. After the reaction with G6PD enzyme (first step in pentose phosphate pathway)

D. After the oxidative decarboxylation step in PPP

E. How many % of the total pyruvate produced via glycolysis will carry this label?

3. A. What would be the fate of glucose in cells that need ATP, but not NADPH and R5P?

B. How about cells that need a lot of NADPH, but not R5P?

C. Finally, what would be the fate of glucose if the cells need a lot of R5P, but not NADPH?

Here are some hints to help you answer Q3.

1) Remember that the reactions in PPP (with the exception of the 3 in oxidative phase) are FULLY reversible.

2) Remember that glycolysis and PPP are linked by the common metabolites: F6P and GAP.

3) Although the stoichiometry of the reactions shown in class is different: glycolysis starts from 1 glucose molecule and PPP needs 3 G6P. In reality, our cells will always have more than 1 glucose molecule. 180 g of glucose (4.5 cans of soft drink) is 6.022 x 1023 molecules of glucose. Bottom line: to answer Q3, please do not worry about stoichiometry and whether there is enough glucose in the cells or not. There is truly plenty of glucose to go to any pathways as needed.

UPDATE: Question 2 refers to question 1

Answer 1

This exercise is about of the glycolysis. Now the first enzyme that intervenes in this metabolic pathway is hexokinase which catalyzes the reaction between glucophate and ATP to form Glucose-6-phosphate. This reaction is important because having a phosphate group the glyocose is now "ionized" and with this it can no longer leave the cell.The next step is the isomerization of glyocose 6 phosphate to fructose 6 phosphate, this is mediated by the enzyme phosphoglucose isomerase converts an aldose into a ketose so that the next enzyme can work in. Fructose 6 phosphate is once more phosphorylated to fructose-1,6-bisphosphate by an enzyme called phosphofructokinase, which is very important because it is a point of regulation for the entire metabolic pathway.

For Exercise 3:
Remember that the pentose phosphate pathway generates NADPH and synthesizes sugars of five carbons.
for a) A cell in need of ATP will use glucose to generate energy, because in this case it does not require reducing power for its other biosynthetic routes

For b) the cell needs to satisfy its demand for NADPH which will make use of the route, the excess of Ribuolosa 5 phosphate can intervonvert it to other metabolites through the trancetolases to not have excess R5P

For c) in C) the cell can use glucose by glycolysis, to be able to synthesize the necessary ribulose can use the transketolases to take glyceralherido 3 phosphate and other sugars (Sedoheptulosa 7 phosphate) to synthesize Ribose and xylulose.