Question

Which among the following belongs to the class of second messengers?

• A. Hormones, neurotransmitters
• B. Pharmacological agonists
• C. cAMP, cGMP, DNA binding
• D. Ions, protein kinases

Hint:

• First messengers: Extracellular signals (e.g., hormones, neurotransmitters).
• Second messengers: Intracellular signaling molecules that relay and amplify the signal from a receptor.
• Key second messengers: cAMP, cGMP, Ca\(^{2+}\), IP\(_3\), DAG.
• Protein kinases are enzymes involved in signal transduction, often activated by second messengers, but are not second messengers themselves.

Answer 1

The Correct Option is C

Second messengers are small, non-protein, intracellular signaling molecules that are rapidly synthesized or released in response to an extracellular signal (the "first messenger," e.g., a hormone or neurotransmitter binding to a cell surface receptor). They relay the signal from the receptor to intracellular target proteins, amplifying the signal and triggering cellular responses. Common examples of second messengers include:

• Cyclic AMP (cAMP)
• Cyclic GMP (cGMP)
• Calcium ions (Ca\(^{2+}\))
• Inositol trisphosphate (IP\(_3\))
• Diacylglycerol (DAG)
• Nitric Oxide (NO) (though it can also act as a first messenger in some contexts)

Let's analyze the options: (a) Hormones, neurotransmitters: These are typically first messengers (extracellular signals). (b) Pharmacological agonists: These are drugs that bind to receptors and activate them, mimicking the effect of natural first messengers. Not second messengers themselves. (c) cAMP, cGMP, DNA binding:

• cAMP (cyclic Adenosine Monophosphate) is a classic second messenger.
• cGMP (cyclic Guanosine Monophosphate) is also a well-known second messenger.
• "DNA binding" is a process or property of certain proteins (e.g., transcription factors), not a second messenger molecule.

Despite "DNA binding" being incorrectly included, cAMP and cGMP are definitely second messengers. (d) Ions, protein kinases:

• Certain ions (like Ca\(^{2+}\)) act as second messengers. However, "ions" in general is too broad.
• Protein kinases are enzymes that phosphorylate other proteins. They are often components of signaling pathways activated by second messengers, but they are not second messengers themselves.

Comparing the options, option (c) contains two definitive examples of second messengers (cAMP, cGMP), even though it also includes an incorrect item ("DNA binding"). Given the choices, this is the best fit. \[ \boxed{\text{cAMP, cGMP, DNA binding (recognizing cAMP \& cGMP are key second messengers)}} \]