| Digestive enzymes and zymogens
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| Most digestive enzymes in the gut are secreted as inactive precursors
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| With the exception of amylase and lingual (tongue-associated) lipases, digestive enzymes secreted into the gut lumen are present as inactive precursors termed zymogens. The secretion of all gut enzymes is similar in the salivary glands, gastric mucosa and pancreas. These organs contain specialized cells for the synthesis, packaging, and transport of enzymes to the cell surface, and thence to the intestinal lumen. These secretions are termed exocrine, that is, 'secreting to the outside' (as opposed to the endocrine secretion of hormones).
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| Figure 9.2 Digestion of dietary polymers. |
| Enzymes involved in protein digestion (proteases) and the lipase, phospholipase A2, are synthesized as inactive zymogens and are only activated on their release to the gut lumen. In general, these enzymes, once in their active form, can activate their own precursors. Activation of their precursors can occur by either change in pH (e.g. pepsinogen in the stomach
is converted at pH below 4.0 to the active enzyme, pepsin), or by the action of specific enteropeptidases bound to the mucosal membrane of the duodenum (see Fig. 9.1).
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| All digestive enzymes are hydrolases
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All digestive enzymes hydrolyze their substrates. The products of such hydrolytic procedures are oligomers, dimers, and monomers of parent macromolecules. Thus carbohydrates are hydrolyzed to a mixture of disaccharides and monosaccharides. Proteins are broken down to a mixture of di- and tripeptides and amino acids . Lipids, however, are treated differently - they are broken down to a mixture of fatty acids (FA), glycerol, and mono-, and diacylglycerols (Fig. 9.2).
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