Question

We have just enough time to get our bearings when we see a huge enzyme (called hexokinase) closing in. We are wrapped into the active site of the enzyme. Your classmate sitting on Carbon #6 is brutally launched as the enzyme removes a phosphoryl group from ATP and attaches it to the carbon. We are released from the enzyme and the molecule on which we ride is now Glucose-6-phosphate.

1. What is this type of active site binding called?
2. What type of reaction just occurred with the ATP molecule?

As soon as we realize the molecular changes that have happened to our ‘trusty steed’, we see another enzyme coming. This enzyme is glucose 6-phosphate isomerase (rearranging enzyme). Within the active site of this enzyme, we all have to hold on tight because the enzyme breaks the 6-C sugar ring and re-forms it as a 5-C more highly symmetric ring (fructose 6-phosphate). Your classmate riding on C-1 yells out, “We are reaching the point of no return, is everyone still in?” Of course, we all give her a thumbs-up and see the enzyme approaching us. This enzyme is the most famous of all glycolytic enzymes. It is called Phosphofruktokinase-1 (but goes by its nickname PFK-1). The enzyme wraps around us and immediately you realize why the student on C-1 was the one to yell out. The enzyme hydrolyzes an ATP and transfers the phosphoryl group onto C-1. We now ride on an even more symmetric molecule (Fructose 1,6-bisphosphate).

3. Why is this step considered the point of no return?
4. How many ATP have been consumed at this point?

   We have just enough time to get our bearings when we see a huge enzyme (called hexokinase) closing in. We are wrapped into the active site of the enzyme. Your classmate sitting on Carbon #6 is brutally launched as the enzyme removes a phosphoryl group from ATP and attaches it to the carbon. We are released from the enzyme and the molecule on which we ride is now Glucose-6-phosphate.

5. What is this type of active site binding called?
6. What type of reaction just occurred with the ATP molecule?

7. As soon as we realize the molecular changes that have happened to our ‘trusty steed’, we see another enzyme coming. This enzyme is glucose 6-phosphate isomerase (rearranging enzyme). Within the active site of this enzyme, we all have to hold on tight because the enzyme breaks the 6-C sugar ring and re-forms it as a 5-C more highly symmetric ring (fructose 6-phosphate). Your classmate riding on C-1 yells out, “We are reaching the point of no return, is everyone still in?” Of course, we all give her a thumbs-up and see the enzyme approaching us. This enzyme is the most famous of all glycolytic enzymes. It is called Phosphofruktokinase-1 (but goes by its nickname PFK-1). The enzyme wraps around us and immediately you realize why the student on C-1 was the one to yell out. The enzyme hydrolyzes an ATP and transfers the phosphoryl group onto C-1. We now ride on an even more symmetric molecule (Fructose 1,6-bisphosphate).

8. Why is this step considered the point of no return?
9. How many ATP have been consumed at this point?

Answer 1

This type of active site binding is called enzyme substrate binding.

Hydroslysis reaction occurred with the ATP molecule. By insertion of one molecule of water (hydro) the phosphoester bond of the terminal phosphate group of ATP is broken (lysis).

The step of fructose 1,6 bisphosphate formation step is considered as a point of not return as this is an irreversible reaction, and the product cannot be converted back to fructose 6 phosphate. The bisphosphate product is destined to be utilized in glycolysis only.

2 ATPs have been consumed at this point (one by hexokinase, another by PFK1)