Question

In order to study the activity of enzyme X, cells were treated with either glucagon, dibutyryl cAMP, glucagon plus H-8, or glucagon with D-2 at does to achieve a maximal effect. The activity and concentration of enzyme X were then assayed in dilute cell-free extracts. Dibutyryl cAMP is an analog of cAMP that diffuses across cell membranes more easily than does cAMP itself. H-8 is a selective inhibitor of protein kinase A. D-2 is a selective inhibitor of CREB. The results are shown in the following table.

Agent Added	Enzyme Activity (units)	Enzyme Concentration (µg/mL)
None	10	20
Glucagon	100	40
Dibutyryl cAMP	10	19
Glucagon plus H-8	100	21
Glucagon plus D-2	5	9

It can be inferred from these results that glucagon most likely acts to _____________ and the changes or part of the changes in activity of enzyme X are most likely mediated by _____________ .

Select one:

a. increase the catalytic efficiency of enzyme X not through protein kinase A; elevated cAMP level

b. increase the catalytic efficiency of enzyme X through protein kinase A; stimulate transcription of the gene for enzyme X

c. cannot be determined

d. increase the catalytic efficiency of enzyme X not through protein kinase A; stimulate transcription of the gene for enzyme X

e. increase the catalytic efficiency of enzyme X through protein kinase A; elevated cAMP level

Answer 1

Answers

d. increase the catalytic efficiency of enzyme X not through protein kinase A; stimulate transcription of the gene for enzyme X

Explanation: H-8 is the inhibitor of protein kinase and activity of the enzyme-X is same whether H-8 is present or absent (100 units, table). Therefore catalytic efficiency of the enzyme X is not through protein kinase A. CREB (cAMP response transcription factor) requires for the transcription of gene of enzyme-X. D-25 is the inhibitor of CREB. In the presence of D-25, enzyme expression was lowered (9 microgram per ml). From given table, it can be seen that activity of the enzyme depends on the concentration except for glucagon+H-8 and glucagon. This might be due to maximum activity if the enzyme is 100 at the concentration of 21 microgram per ml (for glucagon + H-8) and hence activity did not increase for glucagon alone even at enzyme concentration of 40 microgram per ml.

Explanation for wrong answers

Why protein kinase is not important has already explained above. Cell will have their own cAMP, which will be bind to the CERB and mediate its effect. But, from the given data, it can not possible to confirm effect of cAMP on the expression, though it is understood that cAMP binds to the CERB and activates transcription. However, given table is more conclusive with the "stimulate transcription of the gene for enzyme-X" as it has been shown in table clearly that inhibitor of CERB (D-25) decreases the concentration of enzyme. This decrease is due to the lower expression of enzyme-X.