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

Which of the following additional pieces of experimental information would be most useful in elucidating the mechanism of action of glucagon on enzyme X? Choices are the followings:

Select one:

a. Rate of covalent incorporation of isotope ³²P_i into enzyme X

b. Translational efficiency of the mRNA for enzyme X

c. Binding constant of D-2 for purified enzyme X

d. Rate of covalent incorporating of isotope ³⁵S methionine into enzyme X

e. Binding constant of H-8 for purified enzyme X

f. Rate of transcription of the gene for enzyme X

Can someone please answer?

Answer 1

Answer is option e. H-8

H-8 is a protein kinase A inhibitor. Protein kinase A is an important role to play in the signaling pathway of glucagon. But here in presence of an inhibitor of protein kinase A, although the enzyme X concentration drops down as control ( in absence of everything) but the activity remains same as in presence of only glucagon.

Thus the inhibitor has something role to play by binding to the protein kinase A and maintaining its activity high. So the binding constant of H-8 and PKA might explains more about the role played by glucagon on the enzyme X.