Peptide vs Steroid Hormones for the MCAT: Receptors, Speed and Examples

Peptide hormones are water soluble, bind receptors on the cell surface, and act quickly through second messengers. Steroid hormones are lipid soluble, cross the membrane, bind intracellular receptors, and change gene transcription, which is slower but lasts longer.

Side by side

Peptide signaling in brief

A peptide hormone binds its receptor, often a G protein coupled receptor, and the receptor activates an enzyme such as adenylyl cyclase to make cAMP. cAMP activates protein kinase A, which phosphorylates target proteins. One hormone molecule triggers many second messengers, so the signal is amplified. Insulin is an exception: its receptor is a receptor tyrosine kinase.

Steroid signaling in brief

A steroid hormone diffuses through the membrane, binds its receptor, and the complex binds hormone response elements in DNA to change transcription. Because new proteins must be made, the effect takes hours.

The in-between groups

• Amino acid derived hormones come from a single amino acid. Thyroid hormone (from tyrosine) behaves like a steroid, with an intracellular receptor and slow action, even though it is not made from cholesterol. Epinephrine, also from tyrosine, acts like a peptide hormone through surface receptors.

Common traps

• Giving a steroid hormone a surface receptor. Its receptor is inside the cell.
• Calling thyroid hormone a peptide hormone. It is amino acid derived, but acts like a steroid.
• Forgetting carrier proteins. Steroids need them in blood because they are hydrophobic.

One-minute recap

• Peptide: surface receptor, second messenger, fast, short.
• Steroid: intracellular receptor, gene transcription, slow, long.
• Thyroid hormone acts like a steroid; epinephrine acts like a peptide.