Digestive enzymes play an important role in the process of digestion. In the human digestive system, different chemical reactions and processes occur from time to time. It is important to understand the factors that control these reactions? Digestive enzymes present in our digestive system are essential for digestion because they help speed up chemical reactions. An enzyme speeds up a chemical reaction by reducing the activation energy. As food moves along with the digestive system, enzymes are added along the way.

Digestive Enzymes: Definition

Digestive enzymes refer to the enzymes which can break big macromolecules into smaller molecules so that these can be easily absorbed in the small intestine.

What are the 4 Main Digestive Enzymes?

Fig: Enzymes Breaking Down Food into its Nutrients

We cannot absorb food directly. Foods need to be digested first into their simpler forms so that different nutrients can be absorbed from them. Different enzymes are involved in this digestion process. Digestive enzymes are of the following types-

Protease

1. It refers to the group of enzymes that can digest or break proteins into smaller polypeptides or amino acids. It is also called peptidase or proteinase. In this process, peptide bonds are broken down by hydrolysis, i.e., by the use of water.

Fig: Digestion of Protein Molecules

2. Roles of proteases-
a. Ingested proteins are digested by these enzymes.
b. Old proteins are broken down
c. Cell signalling
3. Proteases actually increase the rate of the reactions involving the digestion of proteins. Without proteases, the digestion of proteins would have taken hundreds of years.
4. While breaking peptide bonds of proteins, some enzymes break terminal peptide bonds of the proteins and are called exopeptidases, while some others break internal peptide bonds and are called endopeptidases.
5. Examples of exopeptidases are aminopeptidases, carboxypeptidase A. Examples of endopeptidases are trypsin, chymotrypsin, pepsin, papain, elastase.
6. Some main proteases are:
a. Trypsin was discovered by 1876 by Wilhelm Kühne. It is secreted by the pancreas in its inactive form called trypsinogen, which gets activated by non- digestive enzyme enterokinase present in the intestinal juice in the intestine.
b. Chymotrypsin is also a pancreatic enzyme secreted in its inactive form called chymotrypsinogen and gets activated by trypsin to active chymotrypsin in the duodenum. This can now digest proteins into peptides or amino acids.
c. Pepsin is secreted in the stomach by the gastric chief cells present in the lining of the stomach. These are secreted in their inactive form called pepsinogen, which gets activated by the HCl secreted by the lining of the stomach wall. It can digest proteins into its smaller peptides.

Amylase

Fig: Digestion of Starch

1. Amylase refers to the carbohydrate digesting enzymes.
2. It catalyses the hydrolysis of carbohydrates like starch into disaccharides and trisaccharides.
3. In humans, the digestion of carbohydrates begins in the mouth itself.
4. In saliva carbohydrate digesting enzyme called “ptyalin”, an amylase is secreted.
5. Amylases break glycosidic bonds of carbohydrates.
6. Amylase can be classified as
   a. α-Amylase
   b. β-Amylase
   c. γ-Amylase

a. α-Amylase
i. In humans, majorly this α-Amylase is found.
ii. Alpha-linked polysaccharides, like starch and glycogen are digested by α-Amylases to produce dextrins, and maltose.
iii. Examples of α-Amylase are- Salivary amylase and Pancreatic amylase.
iv. Salivary amylase breaks starch into maltose and dextrin.
v. It cleaves α(1-4) glycosidic bonds of carbohydrates.
vi. Salivary amylase, when it enters the stomach, gets inactivated due to the presence of acids in the stomach.
vii. Pancreatic amylase breaks down carbohydrates into its simpler form dextrin, maltose, or maltotriose by breaking α(1-4) glycosidic bonds.

Enzymes	Reactions Involved
Maltase	Maltose—> Glucose
Isomaltase	Isomaltose—> Glucose
Sucrase	Sucrose—> Glucose and Fructose
Lactase	Lactose—> Glucose and Galactose

b. β-Amylase
i. It is found in bacteria, fungi, and plants but is absent in animals.
ii. This enzyme produces beta-maltose by an inversion.
iii. These are heat stable.
iv. The sweet flavour of ripe fruit is due to the breakdown of the starch into maltose by β-Amylase.

c. γ-Amylase
i. It breaks α-1,4 glycosidic bonds and α(1–6) glycosidic linkages of carbohydrates and yields glucose.
ii. It is active around pH 3.

Lipase

1. Lipase digests fats.
2. Found in plants, animals and prokaryotes.
3. Lipases act on the glycerol backbone of the lipids.
4. Human pancreatic lipase is an example of lipase secreted from the pancreas.
5. It converts triglycerides into three fatty acids and glycerol.
6. Lipase acts on the emulsified fat.
7. Emulsification increases the efficacy of the lipase enzyme.
8. Intestinal lipase is secreted by intestinal glands. It is released in small amounts.

Nucleic Acid Digesting Enzymes

1. Nucleases digest nucleic acids, which are digested in the small intestine.
2. Nucleic acid digesting enzymes include nucleases, nucleotidases and nucleosidases.
3. Pancreatic juice contains two nucleases, i.e., deoxyribonucleases and ribonucleases, which can split it into its components, i.e., deoxyribonucleotides and ribonucleotides, respectively.
4. Intestinal juice contains nucleotidases and nucleosidases.
5. Nucleotidases digests nucleotides into nucleosides and inorganic phosphate.
6. Nucleosidases digest nitrogenous bases and pentose sugars.

Enzyme	Site of Action	Substrate Digested	End Products
Amylase	1. Mouth 2. Duodenum	Starch	1. Maltose 2. Glucose
Protease	1. Stomach 2. Duodenum	Protein	1. Amino acids
Lipase	1. Duodenum	Fat	1. Fatty acids 2. Glycerol

Deficiency Diseases

Lactose intolerance– It refers to the condition of the body when it is unable to digest lactose present in the milk due to the absence of the lactase enzyme. This lactase enzyme gets reduced with age. The deficiency of this enzyme may also be due to premature birth or an illness or injury.

Exocrine Pancreatic Insufficiency- Exocrine part of the pancreas secretes digestive enzymes. EPI or Exocrine Pancreatic Insufficiency refers to the condition of the body when the exocrine part of the pancreas is unable to secrete pancreatic enzymes. This results in undigested food that causes gas, bloating, diarrhoea and many other digestion problems.

Summary

From the above discussion, we came to know about different digestive enzymes. These enzymes catalyse the chemical reactions that take place to digest the food. There are four groups of digestive enzymes. These are proteases, lipases, amylases and nucleases. Proteases are involved in the digestion of proteins. Proteins are digested into their simpler forms called amino acids. Lipases digest fats into glycerol and fatty acids. Amylases digest carbohydrates into simple sugars. Nucleases digest nucleic acids into nucleotides and nucleosides.

Frequently Asked Questions (FAQs) on the Digestive Enzymes

Frequently asked questions related to digestive enzymes is listed as follows:

Q.1. What are the 4 main digestive enzymes?
Ans: 1. Proteases
2. Amylases
3. Lipases
4. Nucleases

Q.2. What are the benefits of digestive enzymes?
Ans: Digestive enzymes catalyse the chemical reactions that take place to digest the food. This helps in bringing complex forms of food to simpler forms that can be easily absorbed by the body to keep it healthy and strong.

Q.3. Which digestive enzymes are made in the stomach?
Ans: The digestive enzymes made in the stomach are Pepsin and gastric lipase.

Q.4. Which enzyme is responsible for protein digestion?
Ans: Proteases are the enzymes that are responsible for the digestion of proteins.

Q.5. What enzyme digests fat?
Ans: Lipases are the enzymes responsible for the digestion of fats.

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