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Peptide & protein hormones: made on demand, stored ready to go

Most hormones are protein chains. Follow one from gene to bloodstream: written as an oversized draft, trimmed twice, packed into granules, and dumped out in seconds when the signal comes.

Written long, then trimmed

A peptide hormone is a protein, so the cell builds it the way it builds any protein: a gene is read and amino acids are linked into a chain. But the chain that comes off the ribosome is not the finished hormone. It is a longer draft called a preprohormone — the active hormone plus extra pieces front and back.

  1. The ribosome makes the [[preprohormone|preprohormone]] — the longest form. Its leading "pre" piece is a signal sequence that steers the chain into the endoplasmic reticulum.
  2. The signal sequence is clipped off, leaving the [[prohormone|prohormone]] — still not active, but closer.
  3. Inside secretory granules, enzymes make the final cuts, releasing the active hormone (and sometimes useful leftover fragments).

Insulin: the classic worked example

Insulin shows the trimming beautifully. Its prohormone, proinsulin, is a single folded chain. Enzymes snip out a middle segment — the C-peptide — leaving two chains joined by bridges: that is mature insulin. The C-peptide is released in equal amounts alongside insulin, which is why doctors can measure it to ask "how much insulin is this body making on its own?"

PREPRO-INSULIN   [signal]---[B chain]---[ C-peptide ]---[A chain]
      |  clip signal in the ER
      v
PRO-INSULIN              [B chain]---[ C-peptide ]---[A chain]   (one folded chain)
      |  enzymes excise C-peptide inside the granule
      v
INSULIN                  [B chain]==bridges==[A chain]   +   free C-peptide

Secreted together, 1 : 1  ->  measuring C-peptide estimates the body's own insulin output.
From draft to active hormone: insulin and its C-peptide leave the cell in equal numbers.

Stored ahead, released in a flash

Because they are water-loving, peptide and protein hormones can be packed into membrane-wrapped secretory granules and stockpiled inside the cell. This is the great advantage of the family: the hormone is pre-made and waiting. When the right trigger arrives, granules fuse with the cell membrane and spill their contents into the blood within seconds — no time wasted on synthesis.