Enzymes1 are globular proteins that control biological reactions. Digestive2 enzymes speed up the breakdown (hydrolysis3) of food molecules4 into their ‘building block’ components. These reactions occur outside of the cells lining the gut.
Naming and classification of enzymes
There are 2 systems used for naming enzymes:
- The suffix ‘-ase’ is used with the root name of the substance being acted upon, for example, when sucrose (sugar) is digested, it is acted upon by an enzyme5 called sucrase6.
- The type of chemical reaction7 involved as the enzyme functions, for example, when sucrase acts on sucrose, it breaks it into a molecule8 of glucose9 and a molecule of fructose10. This reaction involves adding a water molecule to break a chemical bond and so the enzyme is a hydrolase11. All digestive enzymes belong to this hydrolase class12.
Enzymes are classified according to the type of chemical reaction catalysed. All digestive enzymes are hydrolases, whereas most of the enzymes involved in energy release for muscular contraction13 are oxidation-reduction enzymes such as oxidases, hydrogenases and dehydrogenases.
Chemical structure of enzymes
Enzymes are large protein14 molecules, all of which have their own specific 3D shape. Embedded within the shape is a region known as the ‘active site15’, which can attract other suitably shaped molecules to bind to the site. The analogy16 that is often used to describe this mechanism is that of a key fitting into a lock. The enzyme serves as the lock and the attracted molecule (called the substrate17) is the key.
Once the chemical reaction within this lock and key arrangement has been completed, the products are released and the enzyme is free to attract another substrate molecule.
The rate of reaction for such a process is thousands of substrate molecules per minute. If a solution of sugar is left in a sealed container, it breaks down into glucose and fructose extremely slowly. In the presence of a small amount of the enzyme sucrase, the rate of breakdown is millions of times faster.
Sometimes, chemical substances other than substrates18 can bind with the active sites of enzymes, blocking their normal function. For example, water-soluble compounds of arsenic19 and mercury20 are extremely poisonous21 because they can permanently bind to some enzyme systems, markedly reducing their efficiency. Depending on the dose22, the end result could be death.
Digestive enzymes
Digestive enzymes all belong to the hydrolase class, and their action is one of splitting up large food molecules into their ‘building block’ components. Another unique property23 is that they are extracellular enzymes that mix with food as it passes through the gut. The majority of other enzymes function within the cytoplasm24 of the cell25.
The chemical digestion26 of food is dependent on a whole range of hydrolase enzymes produced by the cells lining the gut as well as associated organs such as the pancreas27. The end goal is to break large food molecules into very much smaller ‘building block’ units. These can then be readily and rapidly absorbed through the gut wall and into the bloodstream for transport to the liver28 and from there to other parts of the body.
The main enzyme-producing structures of the human digestive system29 are the salivary glands30, stomach31, pancreas, liver and small intestine32.
| Digestive juices and enzymes | Substance digested | Product formed |
|---|---|---|
| Saliva | Starch34 | Maltose |
| Gastric juice | Proteins | Partly digested proteins |
| Pancreatic juice | Proteins | Peptides and amino acids37 |
| Intestinal enzymes | Peptides | Amino acids |
| Bile from the liver | Fats globules | Fat droplets |
The following pathway summarises how starch present in a food like bread is broken down chemically into glucose, which can then be absorbed through the intestinal wall and into the bloodstream for transport to the liver and from there to other parts of the body.
Mouth and duodenum
Starch hydrolysed into maltose through the action of the enzyme amylase.
Jejunum
Maltose hydrolysed into glucose through the action of the enzyme maltase.
Related content
Use these articles below to explore some of the science ideas and concepts fundemental to the understanding of digestion chemisty.
- The human digestive system
- Rate of digestion
- Digestion – breaking the large into the small
- Digestion of food – this animated video details how food is processed as it moves through the digestive system39.
The article Catalysing chemical reactions with enzymes includes an animated video outlining in detail how enzymes work.
Activity ideas
Try one of more of these activities with your students.
- Enzyme action – investigate the effect that fruit purees (pineapple, kiwifruit, peach) have on the setting of party jellies.
- Salivary amylase and starch – explore the action of salivary amylase on starch present in cooked rice with simple tests for starch and its digestion product, maltose, are applied.
- Lactose intolerance – investigate the effect of the digestive enzyme lactase on a sugar found in milk called lactose. The digestive system condition40 known as lactose intolerance41 will also be looked at.
See our Enzymes Pinterest board for more resource ideas.
Useful link
Read Digestive Enzymes on Biology Online for more information about the various digestive enzymes and the digestion process.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- digestive system: The group of organs that are involved in the breakdown of food in the body, which includes the stomach and intestines.
- hydrolysis: 'hydro' – involving water, 'lysis' – breaking down. When water is involved in the breakdown of a chemical compound into two smaller parts.
- molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- enzyme: A complex protein that acts as a catalyst (speeds up chemical reactions) in specific biochemical reactions. For example, saliva contains an enzyme called amylase that can break down starch into simple sugars.
- sucrase: An enzyme that catalyses the hydrolysis of sucrose into glucose and fructose.
- chemical reaction: A process in which one or more substances are changed into different substances.
- molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- glucose: A simple sugar belonging to the group of carbohydrates called monosaccharides. It is the main form of carbohydrate used by the body.
- fructose: A sugar found in fruits.
- hydrolase: An enzyme that catalyses the hydrolysis (breakdown using water) of a chemical bond. For example, sucrase found in the small intestine hydrolyses the glycosidic bond in sucrose, releasing glucose and fructose.
- class: A classification grouping that ranks above order and below phylum (kingdom > phylum > class > order > family > genus > species).
- contraction: 1. The reduction of the space matter occupies – by becoming smaller or shorter. 2. When muscles become shorter and pull.
- protein: Any of a large class of complex compounds that are essential for life. Proteins play a central role in biological processes and form the basis of living tissues. They have distinct and varied three-dimensional structures. Enzymes, antibodies and haemoglobin are examples of proteins.
- active site: Any location on the surface of a molecule that reacts with another molecule.
- analogy: A comparison between two things, usually using something that is easy to understand to explain something that is more complicated.
- substrate: 1. In ecology, the surface or sediment where an organism lives and grows. 2. In biochemistry, the substance on which an enzyme works.
- substrate: 1. In ecology, the surface or sediment where an organism lives and grows. 2. In biochemistry, the substance on which an enzyme works.
- arsenic: A naturally occurring element that is highly toxic to most organisms.
- mercury: 1. A transition metal in Group 12 of the periodic table – symbol Hg, atomic number 80. 2. Mercury is also the smallest and innermost planet in our Solar System.
- poisonous: Capable of harming or killing by or as if by poison. A poisonous organism only delivers its toxins when eaten, touched or inhaled.
- dose: The amount of a substance that comes into contact with a living organism or some part of a living organism.
- property: A physical or chemical property is a physical or chemical state of a substance that can be measured. Any changes can be used to describe transformations between states.
- cytoplasm: All of the contents of a cell outside of the nucleus.
- cell: 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly. 2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.
- digestion: The mechanical and chemical breaking down of food into smaller components that can be absorbed into a bloodstream.
- pancreas: A greyish-pink organ, about 15 cm long, that stretches across the back of the abdomen, behind the stomach. It produces hormones such as insulin and glucagon (endocrine function) as well as pancreatic juice that contains digestive enzymes (exocrine function).
- liver: A vital organ situated beside the stomach. It is the body’s largest internal organ and plays a major role in metabolism as well as other important and complex functions.
- digestive system: The group of organs that are involved in the breakdown of food in the body, which includes the stomach and intestines.
- salivary glands: Structures found in and around the mouth and throat that produce saliva. The major salivary glands are the parotid, submandibular and sublingual.
- stomach: An organ of digestion with a sac-like shape located between the oesophagus and the small intestine. It temporarily stores food, mixes and churns it with gastric juice and allows digestion of some of the protein content of food to take place.
- small intestine: That part of the gastrointestinal tract that connects the stomach to the large intestine. It consists of three parts: the duodenum, the jejunum and the ileum.
- amylase: An enzyme present in saliva that can digest starch.
- starch: A complex carbohydrate found chiefly in seeds, fruits, tubers, roots and stem pith of plants. Commonly found in foods, such as potatoes, wheat, rice and corn.
- protease: A type of enzyme that speeds up the breakdown of proteins.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- acid: A hydrogen-containing substance that is capable of donating a hydrogen ion to another substance.
- lactose: A type of sugar found in milk and milk products.
- digestive system: The group of organs that are involved in the breakdown of food in the body, which includes the stomach and intestines.
- condition: An existing state or situation; a mode or state of being.
- lactose intolerance: The inability to digest lactose, resulting in discomfort and bloating.
