Peptide Hormone Processing
On this page
Direct answer
No peptide hormone is made in its final form. Each begins on ribosomes of the rough endoplasmic reticulum as a prehormone, whose N-terminal signal peptide is recognised by the signal recognition particle and cleaved off as the chain enters the ER lumen; folding and disulfide formation produce a prohormone that travels through the Golgi into secretory granules. There, prohormone convertases (chiefly PC1/3 and PC2, followed by carboxypeptidase E) cut at paired basic residues, and peptidyl alpha-amidating monooxygenase often caps the C-terminus with an amide. Proinsulin becomes insulin plus C-peptide; POMC becomes ACTH and its siblings; one proglucagon gene yields glucagon in the pancreas but GLP-1 in the gut — the same gene read differently by different convertase endowments.
What you must remember
- Signal peptide logic: hydrophobic core with a cleavage site, guides nascent chains into the ER lumen through the SRP-SR receptor; deleting it abolishes secretion (a classic question on why hormones like insulin are made as pre-forms).
- Convertase pair: PC1/3 in the regulated secretory pathway of endocrine cells and PC2 (with its chaperone 7B2) cleave after Lys-Arg or Arg-Arg motifs; furin handles the constitutive pathway.
- Insulin processing detail: PC1/3 and PC2 excise C-peptide from proinsulin at paired basic sites, and carboxypeptidase E trims the leftover basic residues — insulin and C-peptide are stored equimolarly in granules and co-secreted, which is why C-peptide measures endogenous beta-cell function and separates insulinoma (high) from factitious hypoglycaemia (low, with high exogenous insulin).
- POMC cascade: corticotroph PC1/3 makes ACTH and beta-lipotropin; the intermediate lobe (prominent in rodents) uses PC2 to make alpha-MSH and beta-endorphin — the processing map explains the pigmentary and obesity phenotypes of POMC mutations.
- Proglucagon tissue switch: pancreatic alpha cells (PC2) produce glucagon; intestinal L cells and brain (PC1/3) produce GLP-1, GLP-2 and oxyntomodulin — the molecular basis of incretin therapies and of glucagon-like effects in short bowel syndrome.
- Amidation mark: roughly half of mammalian peptide hormones (TRH, GnRH, vasopressin, gastrin among them) need a C-terminal amide from PAM, a reaction requiring copper and ascorbate — deficiency of either impairs multiple endocrine axes subtly.
- Clinical convertase window: tumours may secrete incompletely processed precursors ("big insulin" or proinsulin-rich secretion in insulinoma, "big ACTH" in ectopic ACTH syndrome), and prohormone measurements can outperform mature-hormone assays in specific settings.
One prohormone, three tissues, three hormone sets
Proglucagon is the teaching case. In the pancreatic alpha cell, PC2 cleaves the molecule to glucagon, glicentin-related pancreatic polypeptide and the major proglucagon fragment — the fasting-mobilising hormone. In the intestinal L cell, the very same gene product meets PC1/3 instead, releasing glicentin, oxyntomodulin, GLP-1 and GLP-2: GLP-1 amplifies glucose-stimulated insulin secretion (the incretin axis that GLP-1 receptor agonists borrow), GLP-2 promotes mucosal healing (used as teduglutide in short bowel syndrome), and oxyntomodulin contributes satiety signalling. The mechanism generalises: tissue identity is a convertase identity. Predict consequences in the exam with this rule — a tumour of enteroendocrine lineage will secrete GLP-1-family products, while a beta-cell tumour secretes insulin with disproportionate proinsulin because its granule processing lags behind synthesis.
Where students slip
The recurring confusion is C-peptide's purpose: it has no established receptor action at physiological levels, yet clinically it is the honest witness of beta-cell secretion because hepatic extraction is minimal (unlike insulin's roughly 50% first-pass clearance) — so low C-peptide with high insulin means injected insulin. Second, students call signal peptide removal "activation"; activation is the convertase step in granules, and several peptides (PTH, angiotensinogen) circulate as hormones needing further cleavage, PTH even being secreted first as pre-pro-PTH with a six-amino-acid pro-segment beyond its signal sequence — an exam number worth knowing. Third, the GnRH analogy: amidation and pyroglutamation are not optional cosmetics; without the N-terminal pyro-Glu and C-terminal amide, GnRH loses receptor affinity entirely.
Frequently asked questions
What happens to a prehormone's signal peptide?
It directs the nascent chain into the ER through the signal recognition particle and is cleaved by signal peptidase, never appearing in the mature hormone.
Why is C-peptide clinically useful?
It is secreted equimolarly with insulin, undergoes negligible hepatic extraction, and so indexes endogenous beta-cell function, distinguishing insulinoma from factitious insulin use.
How does one proglucagon gene make different hormones?
Pancreatic PC2 processing yields glucagon, while intestinal PC1/3 processing yields GLP-1, GLP-2 and oxyntomodulin — tissue-specific convertase expression decides.
Which enzymes trim the products of prohormone convertases?
Carboxypeptidase E removes terminal basic residues, and peptidyl alpha-amidating monooxygenase adds the C-terminal amide many peptides require.
What is the significance of elevated proinsulin in an insulinoma?
Granule processing cannot keep pace with synthesis, so tumours oversecrete proinsulin and intermediates, making proinsulin a useful tumour marker.