Which of the following is carried out when cAMP functions as a second messenger Select one Activates all cytosolic protein kinases b. Activates the cAMP-dependent protein kinase Acts second in importance to AMP d Acts outside the cell to influence cellular processes
The Correct Answer and Explanation is:
Correct Answer: b. Activates the cAMP-dependent protein kinase
Explanation
Cyclic AMP (cAMP) is a critical second messenger in many biological signaling pathways. It is synthesized from ATP by the enzyme adenylyl cyclase, which is activated by various first messengers such as hormones (e.g., adrenaline, glucagon) that bind to G-protein-coupled receptors (GPCRs) on the cell membrane.
Once produced inside the cell, cAMP does not act outside the cell; rather, it functions within the cytoplasm to mediate cellular responses. Its primary role is to activate cAMP-dependent protein kinase, also known as Protein Kinase A (PKA). This activation is crucial for the regulation of various physiological processes.
When cAMP binds to the regulatory subunits of PKA, it causes a conformational change that releases the catalytic subunits. These free catalytic subunits then phosphorylate specific target proteins, leading to changes in enzyme activity, gene expression, and other cellular functions.
For example:
- In liver cells, cAMP signaling can lead to the breakdown of glycogen into glucose.
- In cardiac muscle cells, it can increase heart rate and contractility.
- In neurons, it can affect neurotransmitter release and synaptic plasticity.
Let’s also clarify why the other options are incorrect:
- a. Activates all cytosolic protein kinases – This is false because cAMP specifically activates PKA, not all kinases.
- c. Acts second in importance to AMP – Misleading. While AMP has roles in energy metabolism (e.g., activating AMP-activated protein kinase), cAMP is a distinct signaling molecule with its own specific pathway.
- d. Acts outside the cell to influence cellular processes – Incorrect. cAMP functions inside the cell, unlike some first messengers that operate extracellularly.
Thus, the correct answer is b. Activates the cAMP-dependent protein kinase, as this accurately describes the function of cAMP as a second messenger.
