Prohormone Processing: How Your Body Activates Peptides
Peptide Hormones
2,600+
Computational mapping of human prohormone convertase cleavage sites identified over 2,600 previously uncharacterized peptide fragments, one of which reduces food intake in mice and pigs.
Coassolo et al., Nature, 2025
Coassolo et al., Nature, 2025
View as imageEvery major peptide hormone in your body starts as something larger. Insulin begins as proinsulin. GLP-1 begins as proglucagon. Beta-endorphin, ACTH, and alpha-MSH all begin as one single precursor protein called POMC. The process that converts these inactive precursors into active signaling molecules is called prohormone processing, and it depends on a small family of highly specific cutting enzymes. Without these enzymes, your body would produce the raw materials for hundreds of peptide hormones but none of the finished products. This article explains how the system works, what happens when it fails, and why it matters for peptide science today. For a broader overview, see Peptide Hormones: The Signaling Molecules Running Your Body.
Key Takeaways
- Prohormone convertases PC1/3 and PC2 are the two primary enzymes that cleave peptide precursors at specific basic amino acid sites to release active hormones (Muller and Lindberg, 1999)
- PC3/PC1 alone is sufficient to process proglucagon into GLP-1, glicentin, and oxyntomodulin in intestinal L cells, while PC2 processes the same precursor into glucagon in pancreatic alpha cells (Rouille et al., 1997)
- PC1/3 is essential for processing the GIP precursor; PC1/3-deficient mice show severely impaired GIP production (Ugleholdt et al., 2006)
- The human pituitary uses dual processing pathways (cathepsin L and prohormone convertases) to cut POMC into ACTH, alpha-MSH, and beta-endorphin (Hook et al., 2009)
- Computational mapping of PC1/3 cleavage sites identified 2,600+ uncharacterized human peptide fragments and a novel 12-mer anti-obesity peptide called BRP (Coassolo et al., Nature, 2025)
- Human PC1/3 deficiency causes severe obesity, malabsorptive diarrhea, and multiple endocrine failures because proinsulin, POMC, and proglucagon cannot be properly processed
Why Peptide Hormones Need Processing
Cells do not produce ready-to-use peptide hormones. Instead, they synthesize large precursor proteins called preprohormones. The "pre" prefix refers to a signal peptide that directs the protein into the endoplasmic reticulum, where it is clipped off to form a prohormone. The prohormone then travels through the Golgi apparatus and into secretory vesicles, where the real processing begins.
This indirect approach exists for three reasons. First, the precursor protein often contains sequences needed for proper folding and disulfide bond formation that are not part of the active hormone. Proinsulin, for example, contains a connecting peptide (C-peptide) that holds the A and B chains of insulin together while disulfide bridges form between them. After the bridges lock into place, C-peptide is cut out, releasing mature insulin.
Second, precursor proteins can contain multiple active peptides within a single chain. POMC is the extreme example: a single 285-amino acid precursor that yields ACTH, alpha-MSH, beta-MSH, gamma-MSH, beta-endorphin, beta-lipotropin, and several other fragments depending on which enzymes are present.[4]
Third, the same precursor can yield different hormones in different tissues. Proglucagon produces glucagon in pancreatic alpha cells but GLP-1 in intestinal L cells. The difference is entirely due to which processing enzymes each cell type expresses. This is not a minor detail. It means that how your body synthesizes peptide hormones is only half the story. The other half is how it processes them.
The Master Cutting Enzymes: PC1/3 and PC2
The two enzymes responsible for most prohormone processing are prohormone convertase 1/3 (PC1/3, also called PC3 or PCSK1) and prohormone convertase 2 (PC2, also called PCSK2). Both are subtilisin-like serine endoproteases that recognize and cleave proteins at pairs of basic amino acids, typically lysine-arginine (KR) or arginine-arginine (RR) motifs.[1]
Muller and Lindberg (1999) detailed the cell biology of these two enzymes and revealed that they are themselves synthesized as inactive precursors requiring their own activation steps.[1]
PC1/3 undergoes rapid propeptide cleavage in the endoplasmic reticulum and becomes fully active after a carboxyl-terminal processing event that occurs later in the secretory pathway. Its activation is relatively fast and does not require strongly acidic conditions.
PC2 has a more complex activation. It folds slowly, exits the ER without propeptide cleavage, and requires association with a chaperone protein called 7B2 (also known as neuroendocrine secretory protein). Without 7B2 binding, PC2 cannot generate a catalytically active mature form. PC2's propeptide is eventually removed by autocatalysis in the maturing secretory granule, and this step requires the acidic pH found in those compartments.
These activation differences explain why PC1/3 and PC2 cleave prohormones at different stages and in different compartments. PC1/3 acts earlier and faster; PC2 acts later and more slowly. When both enzymes are present in the same cell, they perform sequential cleavages in a strict temporal order.
After the Cut: Finishing the Job
PC1/3 and PC2 perform the initial endoproteolytic cleavages, but several additional enzymes are required to produce the final active peptide:
Carboxypeptidase E (CPE) removes the basic amino acid residues (arginine, lysine) left at the C-terminus after PC1/3 or PC2 cleavage. Without CPE, the peptide retains extra amino acids that prevent receptor binding.
Peptidylglycine alpha-amidating monooxygenase (PAM) converts a C-terminal glycine into an amide group. Many peptide hormones (including GLP-1, calcitonin, and oxytocin) require this amidation for full biological activity.
N-acetyltransferase (N-AT) adds an acetyl group to certain peptides. The conversion of des-acetyl-alpha-MSH to alpha-MSH, for example, requires N-acetylation and dramatically changes the peptide's potency and receptor selectivity.
The full processing pipeline, from gene transcription to secretion of the mature peptide, happens inside secretory vesicles as they mature. The vesicles acidify as they move from the trans-Golgi network toward the cell membrane, and this pH drop activates the processing enzymes in sequence.
The Proglucagon Story: Same Gene, Opposite Hormones
The most striking demonstration of tissue-specific processing involves proglucagon. This 160-amino acid precursor contains the sequences for glucagon, GLP-1, GLP-2, oxyntomodulin, glicentin, and several connecting peptides. Which of these products a cell releases depends entirely on whether it expresses PC1/3 or PC2.
Rouille et al. (1997) demonstrated this directly.[3] In pancreatic alpha cells, which express PC2, proglucagon is cleaved to release glucagon, a hormone that raises blood sugar. In intestinal L cells, which express PC1/3, the same precursor is cleaved at different sites to produce GLP-1, the incretin hormone that lowers blood sugar and suppresses appetite. The cell expressing both hormones' genetic code produces opposite metabolic effects depending on which scissor it uses.
Rouille's team showed that PC3 (PC1/3) alone is sufficient for the complete intestinal processing pathway. When they expressed PC3 with proglucagon in cells that normally have neither enzyme, the cells produced GLP-1, glicentin, and oxyntomodulin but no glucagon. This result confirmed that a single enzyme determines whether proglucagon becomes a blood sugar raiser or a blood sugar lowerer.
Ugleholdt et al. (2006) extended this to another incretin. They demonstrated that PC1/3 is essential for processing the precursor of glucose-dependent insulinotropic polypeptide (GIP).[2] PC1/3-deficient mice had severely impaired GIP production, while PC2-deficient mice showed normal GIP levels. PC1/3 and GIP co-localized in the same intestinal cells. This means both major incretin hormones, GLP-1 and GIP, depend on the same enzyme for their production. The relevance to modern metabolic drugs is direct: tirzepatide targets both GLP-1 and GIP receptors, and both of those targets are created by PC1/3 processing in the gut.
POMC: One Precursor, Ten Peptides
POMC (pro-opiomelanocortin) is the most complex example of prohormone processing in human biology. A single 285-amino acid precursor yields at least 10 biologically active peptides, including ACTH (the stress hormone), alpha-MSH (melanocortin signaling), beta-endorphin (pain modulation), and several other fragments.
Hook et al. (2009) investigated how the human pituitary accomplishes this processing and found something unexpected: it uses not one but two parallel protease pathways.[4] The canonical pathway involves PC1/3 and PC2, but the pituitary also contains cathepsin L, a cysteine protease that can independently cleave POMC at some of the same sites.
The tissue-specific logic applies here too. In anterior pituitary corticotrophs, which express mainly PC1/3, POMC is cleaved into ACTH and beta-lipotropin. In intermediate lobe melanotrophs and hypothalamic neurons, which co-express PC1/3 and PC2, POMC undergoes further processing to yield alpha-MSH, beta-endorphin, and other smaller fragments. The same genetic instruction set produces fundamentally different hormonal outputs depending on cellular context.
This has direct clinical relevance. POMC-derived peptides regulate appetite (alpha-MSH acts on MC4R to suppress food intake), stress response (ACTH triggers cortisol release), pain perception (beta-endorphin activates opioid receptors), and skin pigmentation (MSH drives melanin production in melanocortin pathways). Disrupting POMC processing cascades through multiple physiological systems simultaneously.
What Happens When Processing Fails
Human PC1/3 deficiency proves how central prohormone processing is to health. People with loss-of-function mutations in both copies of the PCSK1 gene (encoding PC1/3) develop a severe syndrome: neonatal malabsorptive diarrhea requiring controlled nutrition, followed by extreme hyperphagia and obesity as they grow. Additional features include diabetes insipidus, growth hormone deficiency, hypogonadism, adrenal insufficiency, and hypothyroidism.
The obesity mechanism is well understood. Without functional PC1/3:
- Proinsulin cannot be fully processed to insulin, causing elevated proinsulin levels and postprandial hyperglycemia
- POMC cannot be processed to alpha-MSH, removing a key appetite-suppressing signal through the melanocortin pathway
- Proglucagon cannot be processed to GLP-1 and GLP-2, disrupting incretin signaling and intestinal function
- ProGIP cannot be processed to active GIP[2]
The condition is rare (estimated at fewer than 1 in 500,000 births), but heterozygous carriers of PCSK1 mutations, who have one functional copy, show increased obesity risk in population studies. This suggests that even partial reduction in prohormone processing capacity has metabolic consequences.
2,600 Unknown Peptides: The Computational Frontier
A 2025 study published in Nature by Coassolo et al. revealed how much of prohormone processing remains uncharacterized.[5] Using computational drug discovery methods, the team systematically mapped human proteolytic peptide fragments cleaved by prohormone convertases and identified over 2,600 previously uncharacterized peptide sequences.
From this map, they identified a 12-amino acid peptide called BRP (BRINP2-related peptide). When administered to mice and pigs, BRP reduced food intake and produced anti-obesity effects without causing nausea or aversion, side effects that limit current GLP-1 receptor agonists. BRP acts through central FOS activation and operates independently of leptin, GLP-1 receptor, and melanocortin 4 receptor signaling, meaning it represents an entirely new pathway for appetite regulation.
This study illustrates a broader point: the prohormone convertase system likely produces hundreds of bioactive peptides that have never been functionally characterized. The enzymes have been known since the 1990s, but the full catalog of their products is just beginning to be mapped.
The Difference Between Processing and Degradation
Prohormone processing is controlled, precise, and activating. It happens inside secretory vesicles using specific enzymes that cut at defined amino acid sequences. The result is a functional peptide hormone.
Peptide degradation is the opposite: uncontrolled enzymatic breakdown that inactivates hormones after they have been secreted. DPP-4, for example, cleaves and inactivates GLP-1 within minutes of its release. This distinction matters for why peptide hormones cannot be taken as pills: oral administration exposes peptides to digestive proteases that degrade rather than process them.
Understanding this difference also explains why peptide vs steroid hormones have such different pharmacological profiles. Steroid hormones are synthesized through enzymatic modification of cholesterol and do not require proteolytic activation. Peptide hormones are cut from precursors, a process that can be tissue-specific, pH-dependent, and regulated by chaperone proteins.
Open Questions
How many unknown bioactive peptides exist? The Coassolo et al. study mapped 2,600+ fragments from known prohormone convertase substrates. But many PC1/3 and PC2 substrates remain unidentified, and the full map of human prohormone convertase products could be much larger.
Can processing be therapeutically manipulated? If tissue-specific expression of PC1/3 vs. PC2 determines which hormones are produced, could targeted enzyme modulation shift proglucagon processing toward more GLP-1 production? This remains theoretical.
Do processing defects contribute to common metabolic disease? Beyond rare PCSK1 mutations, common genetic variants near prohormone convertase genes are associated with obesity and diabetes risk in genome-wide association studies. Whether these variants cause subtle processing impairments is still under investigation.
What role does the cathepsin L pathway play? Hook et al. showed that human pituitary uses cathepsin L alongside PCs for POMC processing.[4] Whether this dual pathway is redundant, compensatory, or produces distinct peptide ratios remains unclear.
The Bottom Line
Prohormone processing is the system that converts inactive peptide precursors into active hormones through precise enzymatic cleavage. Two enzymes, PC1/3 and PC2, perform most of this work, with additional enzymes handling trimming and modification. The same precursor protein can yield different hormones in different tissues depending on which enzymes are expressed, as demonstrated by proglucagon producing glucagon in the pancreas and GLP-1 in the gut. Human PC1/3 deficiency causes severe metabolic disease, confirming the system's physiological importance. Computational mapping has revealed over 2,600 uncharacterized peptide fragments from prohormone convertase activity, suggesting that the known catalog of peptide hormones is incomplete.