Which molecule is NOT considered a second messenger?

• A. Cyclic AMP (cAMP)
• B. Cyclic GMP (cGMP)
• C. Calcium ions
• D. Protein Kinase A

Answer: (D)

Second messengers are molecules that transmit signals from a receptor on the cell surface to target molecules inside the cell, thereby facilitating cellular responses to extracellular stimuli. Cyclic AMP (cAMP), cyclic GMP (cGMP), and calcium ions are all examples of second messengers that play critical roles in signaling pathways.

Cyclic AMP acts typically in response to hormones like adrenaline, while cyclic GMP is involved in signaling pathways mediated by nitric oxide. Calcium ions serve a variety of functions in signaling, including muscle contraction and neurotransmitter release, thus they qualify as second messengers as well.

In contrast, Protein Kinase A (PKA) is not a second messenger itself. Instead, it is an enzyme that is activated by cyclic AMP. PKA then propagates the signal by phosphorylating target proteins. Thus, while PKA participates in the signaling cascade initiated by a second messenger (cAMP), it does not serve the function of transmitting the initial signal; therefore, it is classified differently from the molecules listed as second messengers.