What converts pepsinogen to pepsin?

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Activation of pepsinogen happens in the stomach when gastric acid, specifically hydrochloric acid, creates a very acidic environment. This low pH causes pepsinogen, a secreted inactive enzyme (a zymogen), to undergo a change that cleaves part of its structure and forms pepsin, the active protease. Pepsin can then start digesting proteins and can also help activate more pepsinogen in a positive feedback loop. Bile works in fat digestion in the small intestine, not in activating pepsinogen. Pancreatic enzymes act in the small intestine to digest various nutrients and do not convert pepsinogen in the stomach. Heat isn’t a physiological trigger for this activation. So, acid in the gastric lumen is the correct trigger for converting pepsinogen to pepsin.