Your digestive system has evolved to break down the complex molecules you consume as food into small, simple units of amino acids, sugars and fats. These basic molecules are then used to build organs and tissues throughout your body. The process of protein digestion begins with denaturation, the first step in simplifying these complex molecules into forms that can be absorbed. Denaturation begins in the stomach with the activation of pepsin enzymes.
Protein Structure
When you consume a meal, the protein contents of the food are still in their natural structures; just like proteins in your body, animal and plant protein molecules have secondary and tertiary structures. Rather than existing as simple chains of amino acids, the protein's folding pattern is determined by its sequence; secondary and tertiary structures make the molecule difficult to digest. So the first step in protein digestion is denaturation, the process by which polypeptide bonds and salt bridges are disrupted, reducing proteins to their linear, primary structure.
Once proteins are denatured, your digestive enzymes have easy access to the protein's individual amino acid bonds, which facilitates the catabolic process, or the breakdown of complex molecules into simpler units. Increasing the surface area of contact between the protein and the digestive enzymes will also increase the rate of digestion.
Zymogens
Digestive enzymes are dangerous machines; appropriately, the body has devised an ingenious method to harness the chemical strength of these proteins, waiting to release their power until the precise moment they're needed. Gastric digestive enzymes are produced in cells lining the stomach in the "inactivated" form; also known as zymogens, these powerful enzymes are chemically restrained from breaking down the protein in your own cells and tissues. The stomach protein enzyme "pepsin" is produced as the zymogen "pepsinogen"; the zymogen's activity is inhibited until food reaches the stomach.
Gastric Secretion
Pepsinogen molecules are activated by secretions of hydrochloric acid (HCl), a potent acid released by parietal cells lining the walls of your stomach. The acid serves several other purposes, such as preventing bacterial growth and gut infection and even combatting food allergies. Acid secretion is stimulated by hormones, including gastrin, histamine, and acetylcholine; somatostatin is the primary inhibitor of acid secretion. As more acid is secreted, more pepsinogen molecules are converted to the active pepsin.
Aspartic Proteinases
According to a 2006 article published in "Aliment Pharmacology and Therapy," gastric secretions contain several types of proteinases; the activity of these enzymes is dependent on the placement of two aspartic acid molecules in the region of its "active site." These proteins contain at least seven enzymatically active areas; the major subtypes of enzyme secreted are "pepsin 3b" and "pepsin 5." The activity of each pepsin type depends on the target protein and the pH of solution, which can reach pH 4.5 with significant digestion still occurring.
References
- "Aliment Pharmacology and Therapy"; Human Pepsins -- Their Multiplicity, Function and Role in Reflux Disease; N.B. Roberts; September 2006
- "Scandinavian Journal of Gastroenterology"; Pepsins and the Mucus Barrier in Peptic Ulcer Disease; A. Allen, et.al; 1988
- "Current Opinion Gastroenterology"; Gastric Exocrine and Endocrine Secretion; M.L. Schubert; November 2009
- "Clinical Ter": Exocrine Gastric Secretion and Gastritis, Pathophysiological and Clinical Relationships; E. Colacci, et.al; January 2011
- "Cell and Molecular Life Sciences": Pepsinogens, Progastricsins, and Prochymosins, Structure, Function, Evolution, and Development; T. Kageyama; February 2002
Member Comments