Mechanisms of Hormone Action
High-Yield Summary
- 3 chemical classes of hormones: peptide (water-soluble, surface receptors), steroid (lipid-soluble, intracellular receptors), amino acid-derived (mixed behavior).
- Peptide hormones act via a second-messenger cascade: hormone (1st messenger) → G protein → adenylate cyclase → cAMP (2nd messenger) → PKA → phosphorylates targets (e.g. CREB). Rapid onset, short duration, high amplification.
- Steroid hormones need carrier proteins in blood (inactive while bound), cross the membrane, bind intracellular receptors, dimerize, and act directly as transcription factors. Slow onset, long duration.
- Amino acid-derived hormones split: catecholamines (epinephrine, norepinephrine) act like peptide hormones; thyroid hormones (T₃, T₄) act like steroid hormones.
- Hormones also classify by target: direct hormones act on the end target (e.g. insulin on muscle); tropic hormones act on another endocrine gland (e.g. GnRH → LH → gonads).
Key Terms
- First messenger
- The hormone itself, binding a surface receptor — never enters the cell.
- Second messenger
- Intracellular signal (e.g. cAMP) triggered by the first messenger, amplifying the signal inside the cell.
- Signaling cascade
- Chain reaction set off by one hormone molecule, amplifying into a large, fast cellular response.
- Dimerization
- Two steroid hormone-receptor complexes pairing up to stabilize DNA binding and drive transcription.
- Tropic hormone
- A hormone that acts on another endocrine gland rather than the final target tissue (e.g. GnRH, TSH, ACTH).
Peptide vs. Steroid Hormones
| Peptide Hormones | Steroid Hormones |
|---|---|
| Water-soluble | Lipid-soluble |
| Travel freely in blood, no carrier needed | Require carrier/transport proteins; inactive while bound |
| Bind surface receptors | Bind intracellular receptors (cytoplasm or nucleus) |
| 1st messenger → 2nd messenger cascade | Hormone-receptor complex acts directly as a transcription factor |
| Rapid onset, short duration | Slow onset, long duration |
Peptide Hormone Second-Messenger Cascade (cAMP)
- 1Hormone binds a surface receptor (1st messenger) — never enters the cell.
- 2Receptor activates a G protein on the inside of the membrane.
- 3G protein activates adenylate cyclase, converting ATP → cyclic AMP (2nd messenger).
- 4cAMP activates protein kinase A (PKA), which phosphorylates target proteins.
- 5PKA may phosphorylate transcription factors (e.g. CREB), altering gene expression in the nucleus.
Common MCAT Trap
- Amino acid-derived hormones don't all behave the same way: catecholamines use surface receptors (peptide-like), but thyroid hormones (T₃/T₄) use intracellular receptors (steroid-like).
- A steroid hormone bound to its blood carrier protein is inactive — it must dissociate first to act on its target cell.
- Direct vs. tropic isn't about chemical class — insulin (peptide) is direct, while GnRH (peptide) is tropic. It's about whether the target is the end tissue or another endocrine gland.
Quick Recall
What is the second messenger in the classic peptide hormone cascade?
Why do steroid hormones have a slower onset than peptide hormones?
Give an example of a 3-step tropic hormone cascade.
Which amino acid-derived hormones act like steroid hormones?