Saliva Contains α-Amylase and Lysozyme

The main function of saliva is not the digestion of nutrients but the conversion of food into a homogeneous mass during mastication. The only noteworthy enzymes in saliva are α-amylase and lysozyme. Both are endoglycosidases that cleave internal glycosidic bonds in a polysaccharide substrate. Exoglycosidases, in contrast, cleave glycosidic bonds at the ends.

α–Amylase cleaves α-1,4-glycosidic bonds in starch. Starch occurs in two forms. Amylose is a linear polymer of glucose, linked by α-1,4-glycosidic bonds. Amylopectin, which usually forms the larger part of the starch in plants, is a branched molecule with α-1,6-glycosidic bonds at the branch points.

α-Amylase does not act on disaccharides and trisaccharides, and it does not cleave α-1,6 bonds. Therefore it produces maltose, maltotriose, and α-limit dextrins rather than free glucose (Fig. 19-1). Maltose is a disaccharide, and maltotriose is a trisaccharide of glucose residues in α-1,4-glycosidic linkage. α-Limit dextrins are oligosaccharides containing an α-1,6-glycosidic bond.

Figure 19.1 Pattern of starch digestion by α-amylase. This enzyme acts strictly as an endoglycosidase. It is unable to cleave the bonds in maltose, maltotriose, and the α-limit dextrins. Arrows indicate cleavage sites.

The salivary α-amylase is active at the normal salivary pH of 6.5 to 7.0 but is rapidly denatured in the acidic environment of the stomach. Therefore it makes only a minor contribution to starch digestion. Its main function is to keep the teeth clean by dissolving starchy bits of food that remain lodged between the teeth after a meal. Cancer patients whose salivary glands have been destroyed by radiation therapy develop rapid tooth decay.

The other salivary endoglycosidase, lysozyme, hydrolyzes β-1,4-glycosidic bonds in the bacterial cell wall polysaccharide peptidoglycan (Fig. 19.2). Lysozyme kills some types of bacteria. However, other bacteria are resistant because their peptidoglycan is protected from the enzyme by other cell wall components or, in the case of gram-negative bacteria, by an overlying outer membrane. The members of the normal bacterial flora in the mouth (including those that cause bad breath) are resistant to lysozyme. However, many bacteria from other ecosystems are killed by lysozyme, and animals make use of this effect by licking their wounds. They use their saliva as an antiseptic.

Figure 19.2 Structure of peptidoglycan, the substrate of lysozyme. NAG, N-acetylglucosamine; NAM, N-acetylmuramic acid.

Protein and Fat Digestion Start in the Stomach

With a pH close to 2.0, the stomach is a forbidding place. The proton gradient between gastric juice and the blood—an almost million-fold concentration difference—is the steepest ion gradient anywhere in the body. The gastric acid has three major functions:

1. It kills most microorganisms. Because solid foods remain in the stomach far longer than do fluids, pathogens are more likely to establish an intestinal infection when they are ingested in water or other fluids than in solid food. People with achlorhydria (lack of gastric acid) and those who have had a gastrectomy (surgical removal of the stomach) have an increased risk of intestinal infections.

2. It denatures dietary proteins. This helps with protein digestion because it makes the peptide bonds more accessible for proteases.

3. It is required for the action of pepsin. Pepsin is a protease with an unusually low optimum pH of 2.0. It is considered an endopeptidase, but it also cleaves peptide bonds at the ends of the polypeptide. Pepsin cleaves only some peptide bonds, with a preference for bonds formed by the amino groups of large hydrophobic amino acids. Therefore it produces a mix of oligopeptides with some free amino acids. This mix is known as peptone. Protein digestion has to be completed by other enzymes in the small intestine (Table 19.2).

Table 19.2 Enzymes of Protein Digestion

In addition to protein, 10% to 20% of dietary fat is digested by an acid-tolerant gastric lipase in the stomach. Neither gastric acid nor the gastric enzymes are essential for life, and patients can live reasonably normal lives after total gastrectomy.

The Pancreas Is a Factory for Digestive Enzymes

Pancreatic juice supplies a cocktail of enzymes for the digestion of nearly all major nutrients. α-Amylase is secreted in large amounts. This enzyme is different from the salivary α-amylase, which has a slightly different structure and is encoded by a separate gene. Closely related enzymes that catalyze the same reaction but differ in molecular structure, physical properties, and reaction kinetics are called isoenzymes.

For protein digestion, the pancreas supplies the endopeptidases (and exopeptidases) trypsin, chymotrypsin, and elastase. All three are serine proteases (see Chapter 4), but with different cleavage specificities (see Table 19.2). Their action is complemented by exopeptidases. Carboxypeptidase A cleaves nonpolar amino acids from the carboxyl end of peptides, and carboxypeptidase B cleaves basic amino acids. Other pancreatic enzymes include pancreatic lipase, various phospholipases, and nucleases.