2.9 Digestive enzymes
Now we return to the digestion of food inside the body, after it has been mixed with saliva.
In Enzymes,we mentioned the enzyme salivary amylase, a starch-digesting enzyme in the saliva.
In infants lipase is produced in the mouth, and a second lipase is produced by special cells in the stomach wall. Both of these lipases start the digestion of fats, by removing the first of the three fatty acids from some of the triacylglycerol molecules. This is because the production of pancreatic lipase has not started yet. (The structure of triacylglycerols was described in Fats and oils in Week 1.)
In adults lipase is made in the pancreas. However, once the food is thoroughly mixed with the acid in the stomach, the pH is too low for these enzymes to continue working efficiently. One of the protein-digesting enzymes called pepsin is also produced by cells in the stomach wall. This does work well in the acidic conditions and it begins to digest the coagulated proteins to amino acids.
A short way along the small intestine, a little tube (duct) empties more digestive fluids from the pancreas and gall bladder onto the food. Bile from the gall bladder is alkaline and neutralises acid from the stomach. This makes the conditions better for other enzymes to work. Bile also helps to emulsify the fats into tiny droplets, enabling one of the enzymes from the pancreas (lipase) to work better at breaking down the fats in the food.
Pancreatic lipase breaks one or two of the fatty acid tails off the triacylglycerol. This leaves just one attached, creating a monoacylglycerol. The pancreas also produces more proteases to complete the digestion of proteins into individual amino acids. The remaining digestion of carbohydrates – breaking them down into sugars – is done by amylases produced by the walls of the small intestine itself.
One of these amylases is called lactase, which breaks down the sugar in milk (lactose). Lactase is, of course, essential in babies and young children whose main dietary component is milk. But it is unusual for the adults of one species (us) to drink the milk of another (cows), so you would not expect adults to produce lactase.
In fact, that is the case in about 80–95% of adult African and East Asian people. If they consume milk, the lactose is not digested and remains in the intestine. The populations of gut bacteria that can digest lactose increase, producing unpleasant-smelling gases. This can cause bloating and wind, abdominal pain and diarrhoea. These can be symptoms of lactose intolerance. However, most Northern European people, and their descendants elsewhere in the world, have retained their ability to produce lactase. They can continue to drink cow’s milk without ill effects throughout their lives.
Table 1 Enzymes and their role in digestion
|Site||Enzyme||Role in digestion|
|mouth||salivary amylase||breaks down starches into disaccharides|
|Lingual Lipase||begins to break down fats into fatty acids|
|stomach||pepsin||breaks down proteins into large peptides|
|amylase||continues the breakdown of starch|
|trypsin||continues the breakdown of protein|
|lipase||breaks down fat|
|Small intestine||Maltase, sucrase, lactase||Breaks down remaining disaccharides into monosaccharides|
|Peptidase||Breaks down dipeptides into amino acids|