Gastric Acid (HCl) Activates Pepsinogen to Pepsin for Protein Digestion

Hydrochloric acid (HCl) secreted by parietal cells creates the acidic milieu (pH 1.5–3.5) essential for protein digestion.

Activation Cascade

1. Chief cells secrete pepsinogen (inactive zymogen, 42 kDa).
2. HCl cleaves off 44‑amino‑acid activation peptide → conformational change → active pepsin (34 kDa).
3. Pepsin auto‑catalyzes further pepsinogen activation (positive feedback).

Pepsin Action

• Endopeptidase: Hydrolyzes internal peptide bonds, preferentially at aromatic amino acids (Phe, Tyr, Trp, Leu).
• Optimal pH 1.5–2.5; inactivated >pH 6.
• Produces large peptides → further digested by pancreatic proteases (trypsin, chymotrypsin) in duodenum.

Additional HCl Roles

• Denatures dietary proteins (exposes cleavage sites).
• Kills ingested microbes.
• Converts dietary ferric iron (Fe³⁺) to absorbable ferrous (Fe²⁺).
• Stimulates secretin/cholecystokinin release.

Clinical Relevance

• PPIs/H2RAs raise gastric pH → impaired protein digestion, potential bacterial overgrowth, reduced iron/B12 absorption.
• Achlorhydria (autoimmune atrophic gastritis) → pepsinogen I low, pepsinogen I/II ratio ↓.