Where does the problem in the biochemical pathway lie

Question

1. Consider the following biochemical pathway:

   A --(1)--> B --(2)--> C --(3)--> D

   (A is the initial substrate and D is the final product. The intermediate products are B and C. A is converted to B through the action of enzyme 1, B is converted to C through the action of enzyme 2, and C is converted to D through the action of enzyme 3.)

   Use the above biochemical pathway to answer the following questions:

   1. There is an accumulation of product C and no detectable amount of product D. What can you say about the pathway?

   2. There is no product D being formed when substrate A is added. Adding product B does not correct the problem, but adding product C does. Where does the problem in the biochemical pathway lie?

   3. Imagine that enzyme 3 is defective. Can you expect to overcome the problem and obtain the final product D by adding an excess of B? Why or why not?