Identifying signaling molecules Which of the following is <em>not</em> a signaling molecule but rather an intracellular effector enzyme in many pathways?

Introduction:
Signaling molecules (first messengers) are extracellular cues such as hormones and neurotransmitters. In contrast, enzymes like adenylyl (adenylate) cyclase reside inside the cell and generate second messengers, translating receptor activation into intracellular signals.

Given Data / Assumptions:

• Testosterone, insulin, thyroxine, and epinephrine are canonical extracellular signals.
• Adenylate cyclase synthesizes cAMP from ATP when activated by Gαs or other inputs.
• Second messengers act within the cytosol to propagate signals.

Concept / Approach:

Differentiate outside-in signals from inside-cell effectors. The former bind receptors; the latter are activated downstream to create the intracellular messenger landscape that controls kinases and channels.

Step-by-Step Solution:

Verification / Alternative check:

Biochemistry shows receptor activation of Gαs increases adenylyl cyclase activity, raising cAMP and activating PKA—hallmarks of intracellular effectors.

Why Other Options Are Wrong:

Testosterone, insulin, thyroxine, and epinephrine are well-established extracellular messengers.

Common Pitfalls:

Confusing first messengers (outside) with second messengers and the enzymes that make them (inside).

Final Answer:

Adenylate cyclase