The second messenger cyclic AMP (cAMP) is synthesized from ATP by the activity of the enzyme adenylyl cyclase. Cyclic AMP, in turn, activates protein kinase A (PKA), also called cAMP‑dependent protein kinase, which is responsible for most of the effects of cAMP within the cell. Determine the correct steps in the activation of PKA, and then place them in the correct order, starting after the adenylyl cyclase reaction.

Answer: 1. Cyclic AMP (cAMP) concentration increases;

2. cAMP then binds to the two regulatory subunits;

3. A change in the conformation of enzyme results. The autoinhibitory sequences move away from the active site of the catalytic subunits;

4. The catalytic sites are now free to catalyse the various reactions of PKA.

(Note: The question did not list the steps to be placed in the correct order)

Explanation:

The inactive form of Protein Kinase A (PKA) contains two identical sub-units (C) and two identical regulatory sub-units (R). Two auto-inhibitory sequences of the regulatory subunits occupy the catalytic sites of the catalytic subunits. After the reaction of adenylyl cyclase, this inactive form of PKA is activated in the following steps:

1. Cyclic AMP (cAMP) concentration increases;

2. cAMP then binds to the two regulatory subunits;

3. A change in the conformation of enzyme results. The autoinhibitory sequences move away from the active site of the catalytic subunits;

4. The catalytic sites are now free to catalyse the reactions various of PKA which includes the regulation of glycogen, sugar, and lipid metabolism.