You have discovered a new gene that encodes an unusual regulatory subunit of PKA

Question

You have discovered a new gene that encodes an unusual regulatory subunit of PKA, which you name Ro. As with the R2C2 version of PKA, Ro forms a dimer and interacts with two molecules of the same catalytic subunit (C) found in PKA to form a Ro2C2 heterotetramer. However, unlike the R in conventional PKA, each Ro subunit binds 8 molecules of cAMP. Which of the following properties would you predict Ro2C2 PKA would display compared to the conventional R2C2 form of PKA ?

A. PKA with Ro subunits would become active at a lower concentration of cAMP.

B. Full activity of PKA with Ro subunits would occur at a higher concentration of cAMP.

C. The dynamic range of activity of PKA with Ro subunits would occur over a narrower range of cAMP concentration.

D. The dynamic range of activity of PKA with Ro subunits would over a wider range of cAMP concentration.

Explanation / Answer

B. Full activity of PKA with Ro subunits would occur at a higher concentration of cAMP. The regular R2C2 complex will bind to two molecules of cAMP, and it is given in the question that Ro molecules will bind 8 molecules. Hence the activity of PKA with Ro subunit will take place with higher concentration of cAMP.

D. The dynamic range of activity of PKA with Ro subunits would over a wider range of cAMP concentration. The concentration of cAMP that will be required generally will depend on the substrate. According to one study, the concentration of the protein kinase in muscle is relatively high (0.23 muM), and under these conditions the apparent activation constant of the enzyme for cyclic AMP is raised so that an increase in cyclic AMP levels in the tissue would cause an accompanying increase in protein kinase activity over a wide range of nucleotide concentration.

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