Human chorionic gonadotropin (hCG) is structurally identical to LH in its alpha subunit and shares the same receptor (LH/hCG receptor). The beta subunit of hCG has an additional C-terminal extension (carboxy-terminal peptide, CTP) with O-linked sialic acid residues. The functional consequence of this CTP with sialic acid is:
- A The CTP increases receptor binding affinity 100× over LH at the LH/hCG receptor, making hCG more potent per molecule
- B The CTP enables hCG to cross the placental barrier to stimulate fetal gonadal steroidogenesis directly
- C Sialic acid on CTP enables hCG to stimulate thyroid-stimulating hormone receptor as a secondary function
- D The sialic acid residues on the CTP extend the plasma half-life of hCG from ~2 hours (like LH) to ~24–36 hours, sustaining early corpus luteum function ✓
Explanation
The beta-hCG subunit's carboxy-terminal peptide (CTP, 28–30 extra amino acids compared to LH beta-subunit) carries multiple O-linked oligosaccharide chains heavily decorated with sialic acid. Sialic acid residues protect hCG from renal filtration (large hydrodynamic radius), and from deglycosylation by circulating glycosidases, dramatically extending plasma half-life to ~24–36 hours versus ~2 hours for LH. This long half-life enables hCG — produced from the trophoblast from implantation onward — to continuously stimulate the corpus luteum without needing pulsatile secretion, maintaining progesterone production throughout the first trimester until placental steroidogenesis takes over.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.