Where does the body get its own peptides?
The short answer
From genes that code for a longer precursor. Enzymes then cut the precursor into one or several active peptides, often in storage vesicles just before release.
The short read
- A gene makes a long precursor, not the finished peptide.
- Enzymes cut at pairs of basic amino acids such as lysine and arginine.
- One precursor can yield several different messengers.
The cell reads a gene and builds a long chain called a prepropeptide. A short leader sequence steers it into the secretory pathway and is removed. Prohormone convertases then snip the remainder at specific sites. Proinsulin becomes insulin plus a leftover C-peptide, and the precursor POMC is cut into several different hormones depending on the tissue.
The full explanation
Your body does not assemble small peptides from scratch each time. It makes a long chain and then edits it.
The assembly line
- A gene is copied and read, and the cell builds a prepropeptide.
- The first stretch, the signal sequence, directs the chain into the compartments that prepare molecules for export. Then it is clipped off, leaving a propeptide or prohormone.
- Enzymes called prohormone convertases cut the chain, usually after a pair of basic amino acids such as lysine and arginine.
- Smaller tidy-up enzymes trim the ends. Some peptides get a final chemical decoration, for example an amide group at the C-terminus, which they need to work.
- The finished peptides are packed in vesicles and wait for a release signal.
Proinsulin: the textbook example
Cells in the pancreas make proinsulin. It is cut into insulin and a leftover fragment called C-peptide. They are released together, which is why measuring C-peptide in blood is a way to estimate how much insulin the body is making on its own.
One precursor, several messages
A precursor called POMC is cut differently depending on the tissue. In one place it yields ACTH, a stress-axis hormone. In another it produces melanocyte-stimulating hormones and beta-endorphin, a pain-related peptide. One gene, several products, chosen by which enzymes are present.
Why this design
Cutting in the vesicle lets the cell store a large quantity in an inactive form and release it quickly. It also lets a tissue tune its output without changing the gene. The price is that each cut has to be correct.
Education only. Not medical advice, no dosing, no product recommendations. Read this first.
