Humans and many other organisms are heterotrophs, meaning we eat other plants or animals to obtain energy and nutrients. Our bodies cannot absorb big chunks of food. So, the food needs to be broken down into smaller bits that can be absorbed, what we commonly call food digestion. In this article, we will learn more about digestive enzymes, their definition, the different types there are, their function in food digestion, and some examples.
Digestive Enzymes' Definition in Science
The digestive system processes food in two ways: mechanical and chemical digestion.
Mechanical digestion involves physically breaking down the food into smaller chunks, including chewing in the mouth and peristaltic contractions or muscle contractions that move food through the rest of the digestive tract. But mechanical digestion is not enough. Food particles must be broken down into basic units for the body to absorb them. Chemical digestion does this with the help of digestive enzymes.
Enzymes are biological catalysts, mainly made of proteins, that speed up the metabolic reactions in our bodies.
Enzymes are essential molecules of the body, and life would not be possible without them.
An enzyme binds to one or more reactant molecules to catalyse a chemical reaction. These reactants are the substrates of the enzyme. The part of the enzyme that the substrate binds to is called the active site. It is called the active site because it is where the catalysis occurs.
Catalysis is the process of accelerating a chemical reaction using a catalyst.
Most enzymes are made of proteins, consisting of amino acids that have joined together in a chain. The amino acid sequence present at the active site and their 3D orientation give the active site its distinct size and shape. Because of this, the active site of an enzyme is uniquely designed to bind to a specific target and catalyse a specific reaction. In other words, each reaction needs its own particular enzyme.
The Function of Digestive Enzymes
Digestive enzymes help in the digestion of food. The food that we eat consists of carbohydrates, lipids, proteins, and nucleic acids. We eat these biomolecules because our body needs them for growth and repair. But our body cannot take them up in their large form since they are too big to be absorbed in the intestine and enter the blood. Our digestive enzymes correct this by turning these large essential biomolecules into smaller building blocks by breaking the chemical bonds between them.
Digestive enzymes are produced at various parts of the digestive system and are eventually added to the semi-fluid food passing through the digestive tract.
Types of Digestive Enzymes
Digestive enzymes can be classified into three main categories based on the types of molecules they act on. These three classes are carbohydrases, proteases, and lipases.
Carbohydrase
Carbohydrases are a family of digestive enzymes that hydrolyse complex carbohydrates into simple sugars.
Hydrolysis is a reaction in which water reacts with an organic molecule to break chemical bonds and generate two or more new, simpler molecules.
We eat various types of carbohydrates in our food, and each requires its own carbohydrase enzyme. These enzymes are produced in different sections of the digestive system, such as the salivary glands, pancreas, and small intestine.
Proteases
Proteases, or proteolytic enzymes, refer to a group of enzymes whose catalytic function is to break down protein molecules.
Proteins are long chains of amino acids joined by peptide bonds.
Proteases act by hydrolysing peptide bonds, although the ability of proteases to break different peptide bonds varies. Proteases are produced in the stomach and pancreas, after which they are released into the digestive tract.
Peptides are short strings of amino acids.
Some proteases catalyse the conversion of proteins to short peptides. These peptides often need to be further hydrolysed down to single amino acids. A subfamily of proteases called peptidases does this.
Lipases
Lipases are enzymes secreted by the pancreas that break down lipids (fats) in the duodenum of the small intestine. Lipases turn lipids into their constituting subunits: fatty acids and glycerol.
Bile is a fluid that the liver produces and is stored in the gallbladder. Bile and pancreatic secretions are released into the small intestine during food digestion.
It plays a vital role in the digestion of lipids as it works as an emulsifier, breaking big fat globules into smaller droplets. Emulsified fats have a more extensive surface area for lipases to work and hence are broken down much faster.
Bile is also an excellent solvent. This feature makes it an ideal medium for interacting lipids with lipid-splitting enzymes.
Nucleases
Nucleases are another digestive enzyme class that breaks down nucleic acids into nucleotides. Most foods we eat, whether vegetables, fruit, or meat, consist of cells that were once alive. These cells contain nucleic acids in the form of DNA or RNA. During digestion, nucleases break down these molecules into simpler molecules.
Examples of Digestive Enzymes
The table below gives an overview of some digestive enzyme examples.
Table 1: Summary of digestive enzymes types, the substrate they work on, where they are produced and released into, and some examples.
| Type of enzyme | Examples | Substrate | Production site | Released into |
| Carbohydrase | Salivary Amylase | Starch | Salivary glands | Mouth |
| Pancreatic Amylase | Starch | Pancreas | Small intestine (duodenum) |
| Maltase | Maltose sugar | small intestine | Small intestine |
| Protease | Pepsin | Protein | Stomach (Gastric glands) | Stomach |
| Trypsin | Protein | Pancreas | Small intestine (duodenum) |
| Peptidase | Peptides | Small intestine | Small intestine |
| Lipase | Lipase | Fatty acids | Pancreas | Small intestine (duodenum) |
| Nuclease | Nuclease | Nucleic acids (DNA and RNA) | Pancreas | Small intestine (duodenum) |
Most digestive enzymes are produced by the pancreas, an organ located behind the stomach. During digestion, the pancreas makes pancreatic juices that contain a mixture of digestive enzymes. These enzymes are responsible for breaking down carbohydrates, proteins, lipids, and nucleic acids.
The pancreas also aids the cells in absorbing the digested food by producing hormones such as insulin.
In the rest of this section, we will look closely at two digestive enzymes, amylase and pepsin, which are essential enzymes needed for the chemical breakdown of carbohydrates and proteins, respectively.
Amylase
One of the most common carbohydrates we eat is starch, a long polymer made of glucose sugars. Amylase is the carbohydrase that cleaves the glycosidic bonds in starch to produce individual glucose sugar molecules. Glucose is then transported across the intestinal wall and is absorbed into the bloodstream.
Glycosidic bonds are covalent bonds that link carbohydrates to another carbohydrate.
Amylases are produced at multiple places along the digestive tract. They are first added in the mouth and released by the pancreas in the small intestine (duodenum).
Pepsin
Protein digestion takes place in the stomach and duodenum. Pepsin is one of the main proteases involved in protein digestion and is secreted in the stomach. In addition to pepsin, the stomach produces gastric acid or hydrochloric acid (HCl). This gastric acid lowers the pH in the stomach, making the conditions optimum for the pepsin.
HCl also plays a role in killing most microbes ingested with our food.
The Benefits of Digestive Enzymes
Digestive enzymes are essential for splitting down food particles into substances that can be absorbed. The lack of this can lead to poor digestion and malnutrition. Let's discuss a particular disorder that can arise when certain digestive enzymes are dysfunctional.
Lactose Intolerance
Lactose intolerance is a digestive condition characterised by an inability to fully digest lactose, a significant sugar found in dairy products. Lactose intolerant individuals lack the enzyme lactase that breaks down lactose to galactose and glucose, two smaller absorbable sugars. The lactose sugar cannot be absorbed in the small intestine, so it remains in the digestive tract. As a result, individuals with lactose intolerance may experience symptoms of diarrhea, gas, and bloating after consuming dairy products.
Lactose intolerance is harmless, but its symptoms can be unpleasant and bring discomfort and pain to a sufferer.
To avoid these symptoms, people with lactose intolerance can eat lactose-free dairy products in which the lactose has already been split into galactose and glucose by the addition of a lactase enzyme.
Enzymes - Key takeaways
The digestive system processes food in two ways: mechanical and chemical digestion.
During chemical digestion, digestive enzymes break down food particles into simpler molecules that can be absorbed across the intestine.
There are three major types of digestive enzymes: carbohydrases, proteases, and lipases.
Lack of digestive enzymes can lead to poor digestion and malnutrition, as seen in individuals who suffer from lactose intolerance.
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