All known effects of cyclic AMP in eukaryotic cells results from
**Core Concept**
Cyclic AMP (cAMP) is a key second messenger in eukaryotic cells, playing a crucial role in signal transduction pathways. It is generated from ATP by the action of adenylate cyclase, an enzyme that responds to various stimuli, including hormones and neurotransmitters. cAMP then acts as a molecular switch, influencing various cellular processes.
**Why the Correct Answer is Right**
The correct answer is protein kinase A (PKA). cAMP binds to and activates PKA, a protein kinase that phosphorylates and thereby activates or inhibits various downstream targets. This includes enzymes, ion channels, and transcription factors, ultimately leading to changes in gene expression, metabolism, and cellular behavior. PKA is a key effector of cAMP, mediating many of its known effects in eukaryotic cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cGMP, another cyclic nucleotide, has distinct effects in eukaryotic cells, primarily through protein kinase G (PKG). While cGMP and cAMP can interact and influence each other, they have distinct signaling pathways.
**Option B:** This option is incorrect because calmodulin, a calcium-binding protein, is involved in signaling pathways that are activated by changes in intracellular calcium concentrations, not cAMP.
**Option C:** This option is incorrect because protein kinase C (PKC) is a family of protein kinases that are activated by diacylglycerol (DAG) and calcium ions, not cAMP.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that cAMP and PKA are key regulators of cellular metabolism, including glucose and lipid metabolism. Abnormalities in cAMP signaling have been implicated in various diseases, such as diabetes and cancer.
**Correct Answer:** C. protein kinase A (PKA)