Enzymatic Digestion In Humans
Digestion by saliva
The human digestive tract can be traced from the mouth to the anus. Chemical changes that occur in a meal as it passes through this complex tubular system can also be traced starting from the mouth. Enzymatic digestion starts in the mouth. The saliva contains an enzyme called amylase, seen under a compound light microscope, which begins but does not complete the hydrolysis of starch to glucose. Although amylase produces some glucose, it yields primarily the double sugar maltose, which must be further digested in the intestine. The saliva of many mammals contains no amylase as studied under a compound light microscope; dogs are an example. Doubtless there was little selection pressure for the evolution of such a starch-digesting enzyme in animals that, at least ancestrally, were almost entirely carnivorous.
Digestion in the stomach
Once in the stomach, food is exposed to the action of gastric juice secreted by numerous gastric glands of the stomach wall. When samples of the gastric juice is examined under a compound light microscope, it was discovered that it contains much hydrochloric acid and several enzymes. The acid makes the contents of the stomach very acidic (pH of about 1.5-2.5). Note that, advertisements for many patent medicines to the contrary, an acid stomach is both normal and necessary for proper function.
The principal enzyme of the gastric juice is pepsin, which digests protein. Pepsin does not hydrolyze protein all the way to its amino acid components, as studied under a microscope. It splits the peptide bonds adjacent to only a few amino acids. The specificity of protein-digesting enzymes is readily understandable. When viewed under a microscope, proteins are composed of a variety of building block compounds, not just of one, the structural configuration around the various peptide bonds varies, depending on which two amino acids the bond joins; some of the bonds may fit in the active site of a particular enzyme, and others may not. Pepsin, for example, seems to have an active site complementary to peptide bonds at the amino end of amino acids whose R groups include a six-carbon ring.
Digestion in the Small Intestine
It is in the next section of the digestive tract, the small intestine, that by far the most digestion takes place. When partially digested food passes from the stomach into the duodenum, its acidity stimulates the release of a large number of different digestive enzymes into the lumen, or cavity, of the intestine. These enzymes are secreted from two principal sources, the pancreas and the intestinal glands. The pancreas is a large glandular organ, lying just below the stomach, which originates in the em¬bryo as an outgrowth of the digestive tract; it retains a connection to the duodenum called the pancreatic duct. When food enters the duo¬denum, the pancreas secretes a mixture of enzymes that flows through the pancreatic duct into the duodenum. Included in this mixture are enzymes that digest all three principal classes of foods-carbohy¬drates, fats, and proteins-as well as some that digest nucleic acids. Functions of these enzymes were studied using a microscope.
One of the pancreatic enzymes is pancreatic amylase, which, as its name implies, acts like salivary amylase, splitting starch into the double sugar maltose. It is far more important than salivary amylase, for it carries out most of the starch digestion, as seen under a microscope.
Lipase, also secreted by the pancreas, is the body’s principal fat-digesting enzyme, but it completely hydrolyzes only a relatively small percentage of l the fat to glycerol and fatty acids. Some of the fat is partly digested (e.g. by removal of only one of the three fatty acids), and some is nit digested at all. But since fats, and the products of the partial digestion of fats, are lipid-soluble, they can be absorbed across cell membranes.
Like pepsin, trypsin and chymotrypsin, two of the protein-digesting enzymes of the pancreas, break only the peptide linkages adjacent to certain specific amino acids. Studies using a microscope have shown that the enzyme trypsin splits the peptide bonds adjacent to two amino acids that have positively charged R groups; chymotrypsin splits those adjacent to several amino acids that have six-carbon rings in their R groups.
To summarize it all, then, the action of pepsin in the stomach and of trypsin and chymotrypsin from the pancreas results in a splitting of proteins into fragments of varying lengths, but does not produce many free amino acids. These three enzymes are known as endopeptidases-enzymes that hydrolyze peptide bonds between amino acids located within the protein, not bonds linking terminal amino acids to the chain. Other enzymes, called exopeptidases, hydrolyze off the terminal amino acids, thereby completing the digestive process. There is a great variety of exopeptidases, each highly specific in its action. Intestinal glands secrete most of them, but at least one is produced in the pancreas.
Just as certain enzymes from the intestinal glands complete the digestion of protein, other intestinal enzymes complete the digestion of carbohydrate begun by salivary and pancreatic amylase. These enzymes split double sugars into simple sugars. For example, maltase splits maltose, sucrase splits sucrose, and lactase splits lactose. Absorption of the products of digestion involves active transport.
Bile
One more secretion should be mentioned in this discussion of human digestion. The liver, a critically important organ, produces a fluid called bile, which aids in fat digestion. In human anatomy, the liver is a very large organ occupying much of the space in the upper part of the abdomen. On its surface is a small storage organ, the gallbladder. Bile, produced throughout the liver, is collected by a series of branching ducts and emptied into the gallbladder. When food enters the duodenum, the muscular wall of the gallbladder is stimulated to contract, and the bile is forced down the bile duct into the duodenum.
Bile is not a digestive enzyme; it is not even a protein. It is a complex solution of bile salts, bile pigments, and cholesterol, as seen under a microscope. The bile salts act as emulsifying agents, causing large fat droplets to be broken up into many tiny droplets suspended in water. This action is much like that of a good detergent. The many small fat droplets expose much more surface area to the digestive action of lipase than a few large droplets would.
The bile pigments and cholesterol play no perceptible role in digestion. The pigments are produced through the destruction of red blood cells in the liver; it is they that give the characteristic brown color to feces. The cholesterol, a relatively insoluble compound, sometimes causes trouble by becoming concentrated into hard gallstones, which may block the bile duct and interfere with the flow of bile.
