The Action of Digestive Enzymes Course Notes

Terms to Know


Peptidase is the term recommended for the protein (enzymes) that causes the hydrolysis of peptide bonds. Peptidases is also called proteases.


An inactive enzyme that becomes active once a small portion of the primary sequence is removed.


The major inactive protease secreated into the stomach.


The active protease that pepsinogen is converted into by either the catalytic effect of 0.1M HCl stomach acid or by pepsin already present in the stomach juice.

Autoactivation or Autocatalysis

The process in which an enzyme catalyzes the activation of more of its own kind.


The reverse of activation. This is any process that makes an active enzyme less active or inactive.


A vomit causing agent

Protein Digestion

Protein digestion really starts with cooking of the food but actually occurs in the stomach. The effect of heat causes an unfolding of the higher order protein structure exposing the primary sequence making it easier for the digestive enzymes to attack and break via hydrolysis, the peptide bonds that hold the amino acid residues together in the primary protein structure.

When food is consumed, protein digestion involves the hydrolysis of peptide bonds, and enzymes that catalyze this reaction are called protease or peptidases. These enzymes are produced by either the cells of the stomach lining or cells of the pancreas in an inactive form called zymogen. The zymogen becomes active once a small portion of the primary sequence is removed. By making zymogens the cells manage to avoid self-digestion.

Food stays in the stomach 2 - 3 hours. During this time pepsinogen the major inactive protease secreated into the stomach is converted to an active protease pepsin by the stomach acid or by pepsin already present in the stomach. Pepsin is the major stomach peptidase.

Pepsinogen + HCl ----> Pepsin

Zymogens in the Intestines

There are three major protease zymogens in the intestines. These zymogens are secreated into the small intestine by the pancreas. These are trypsinogen, chymotrypsinogen, and procarboxypeptidase. Once activated in the small intestine, these zymogens become trypsin, chymotrypsin, and carboxypeptidase.

Trypsinogen + (trypsin or enterokinase) -----> Trypsin

Chymotrypsinogen + trypsin -------> Chymotrypsin + 2 dipeptides

Procarboxypeptidase + trypsin -----> carboxypeptidase

Heavy Metal Poisoning

Heavy metal ions such as Hg2+, Pb2+, Cu2+ and Ag+ are all poisonous enzyme inhibitors because they irreversibly bind to the -SH groups of the cysteine amino acid residues of enzymes. This results in a significant alteration of the tertiary structure of the enzyme and diminishes its catalytic activity. The cyanide anion, CN- also belongs to this class of poisons because it binds to iron atoms that are cofactors of many enzymes.

When a heavy metal ion is ingested, there is no real hazard as far as the digestive enzymes are concerned. These enzymes may be inhibited but they will quickly be expelled from the body and replaced, however once the heavy metal ions leave the stomach and are absorbed through the intestines they will cause havoc to the body's chemistry. The heavy metal binds tightly to proteins and enzymes thus interfering with their functions and diminishing their catalytic activity. Mercury can even alter the chemical structure of proteins. Once the heavy metals leave the stomach they congregate in the liver, kidneys, gastrointestinal tract, and the brain. In the kidneys thay can cause renal failure. In the gastrointestinal tract they disrupt carbohydrate metabolism, blocking the bodies ability to absorb nutrients, cause ulcers and necrosis (cell death). In the brain they can cause alzheimers, tremors, loss of vision and hearing, and coma. Heavy metals act as catalysts which increases the production of free radicals, which can lead to cancer, heart, liver, and kidney diseases.

Egg Whites and Milk Antidotes

Egg whites and milk are used as antidotes for heavy metal poisoning because if taken immediately after the ingestion of the metal poison, their protein content readily combines with the heavy metal ions to form an insoluble solid. The resulting insoluble matter is removed from the stomach by the use of an emetic , thus preventing the digestive enzymes from destroying the denatured protein and once again liberating

Glyceride (Lipid) Digestion

Glyceride digestion occurs in the intestines. Lipids are insoluble in the aqueous media of the gastrointestinal tract. Lipids must be emulsified before enzymes can hydrolyze them into glycerol and fatty acids that can be absorbed by the intestinal mucosa cells. Bile salts produced by the liver from cholesterol and stored in the gallbladder, are the emulsifying agents for lipids. Once secreated through the bile ducts into the small intestine, bile salts form micelles which emulsify dietary lipids like soap emulsifies oil. The enzyme pancreatin also called pancreatic lipase, is a mixture of lipases, and hydrolyze the ester bonds of glycerides into monoglycerides, fatty acids, and some glycerol.

Starch (Carbohydrate) Digestion and Salivary Amylase

Starch is the principle dietary carbohydrate. Starch digestion starts in the mouth where salivary amylase catalyzes the hydrolysis reaction to give a mixture of products including glucose, maltose, and dextrine. Very little carbohydrate digestion occurs in the stomach.

The major site of carbohydrate digestion is the small intestine. In the small intestine, pancreatic amylase hydrolyzes starch and dextrins (low molecular weight polysaccharides) to glucose and maltose. Maltase hydrolyzes maltose to glucose. Glucose is absorbed through the intestinal membrane into the blood.

In the mouth and esophagus:

starch + salivary amylase ------> glucose + maltose + dextrins

In the stomach: Very little starch digestion occurs.

In the small intestine:

starch + pancreatic amylase -------> glucose + maltose + dextrins

dextrins + pancreatic amylase -------> glucose + maltose

maltose + maltase --------> glucose

Starch Digestion and Salivary Amylase Test

starch digestion test

Image from

Left dot - iodine.    Middle dot - starch present.    Right dot - no starch present.

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last updated: March 24, 2008