In general, enzymes are what kinds of molecules?

A. Proteins.

B. Minerals.

C. Carbohydrates.

D. Lipids.

E. Nucleic acids.

The correct answer is A: Proteins.

Enzymes are complex molecules that function to speed up the chemical reactions. Many chemical reactions would not be possible if enzymes were not present to activate them. In general, enzymes are proteins and therefore have many features typical of proteins.

Proteins are biological molecules that fold to form complex structures. Enzymes work by lowering the energy needed to activate a reaction. The molecule that the enzyme works on is known as a substrate.

The enzyme helps to convert the substrate into a product or products. Enzymes are needed for all the metabolic processes of the cell, and thus enzymes are essential for life.

Diagram showing enzyme action
Diagram showing enzyme action

The function of enzymes is dependent on their structure and shape which is influenced by the fact that they are proteins. Proteins can only function within a specific range of environmental parameters of temperature and pH.

Proteins can become denatured at temperatures and pH values that are outside of their optimal ranges. Denaturing is a change in the shape and structure of the protein molecule.

Since enzymes are proteins it means they can become denatured and if this happens then the enzyme can no longer function since the shape and structure has changed.

Enzyme structure

Most enzymes that are found are actually protein molecules. They, therefore, have the structure of a protein molecule. Proteins have several levels of structure, with the simplest level being the primary sequence which consists of amino acids that are linked together by peptide bonds that form between atoms.

The next level is the secondary structure in which hydrogen bonds form between various groups of peptides. Tertiary structure is the three-dimensional structure that is formed from the secondary structure polypeptides folding and bonding further by hydrogen and disulfide bonds.

The quarternary structure is an even higher level of folding and bonding that occurs to form a large protein molecule. An example of an enzyme with quarternary structure is the enzyme DNA polymerase.

The active site

Enzymes are often of a larger size than the substrate that they act on. Only a small part of the enzyme is involved in the reaction. In fact, this region of the enzyme that is involved is often not more than a few amino acids in size.

The part of the enzyme involved in the reaction is called the active site. This active site consists of two regions that are known as the catalytic site and a region that is known as the binding site of the enzyme.

The binding site is the region where molecules can bind and chemical bonds can form. The catalytic site is the region that is involved in speeding up the enzyme reaction. Enzymes are very specific and the active site will only bind and act on a specific substrate.

Enzyme function

An enzyme binds with a specific substrate to form what is known as an enzyme-substrate complex. The substrate binds to a region of the enzyme that is called the active site of the enzyme.

When the substrate and enzyme bind with each other the enzyme tends to change its shape slightly to form a complex; this is called an induced fit.

The reaction then occurs and the products are formed from the substrate. Products are released and the enzyme is then made available to catalyze more reactions.

The way the enzyme works is often referred to as a lock-and-key mechanism since the substrate has to bind with the enzyme in a very specific manner. In addition, an enzyme only works with a specific type of substrate.

For example, the enzyme sucrase breaks down the sugar sucrose into the products fructose and glucose; while the enzyme lactase breaks down milk sugar lactose into glucose and galactose.

The enzyme is able to change its shape back after the reaction has occurred. In this way, the enzyme can be recycled and used in more reactions.


Some enzymes need small molecules called coenzymes in order to work properly. Coenzymes can help to carry electrons needed for reactions to occur. For example the coenzyme nicotinamide adenine dinucleotide, which is important in cellular respiration.

Enzyme regulation

Enzyme function is carefully regulated by various feedback loops. It is important that enzymes are only released when they are needed.

Therefore there are molecules that bind to the enzyme to stop their activity. These molecules bind to the regulatory sites of the enzyme and bring about what is called allosteric regulation.

Enzyme action can also be regulated by phosphorylation. This is the process which involves the transfer of phosphate groups. This can either trigger an enzyme to work or stop it from working.

Enzyme denaturation

Denaturing occurs because an enzyme is a protein. Denaturation is the process in which the shape of the protein changes due to unfavorable conditions of temperature or pH.

Since enzyme function depends on the shape, a change in shape of the enzyme causes a problem with the function of the enzyme. The change in shape will often mean that the active site of the enzyme has a different shape from normal.

The consequence of this is that the substrate will not be able to bind with the active site. The result of this is that the enzyme will then not be able to catalyze the reaction.

Enzymes are so sensitive to the environment that this is the reason that different parts of the digestive system have different pH values. For example, the stomach in humans has a low pH of about 2 because the enzymes that work in the stomach work at this pH.

In comparison, the pH of the small intestine is about 7.5 since that is the optimal condition for those enzymes to work. Without these enzymes working we would not be able to digest our food.

Application and significance of enzymes

The more information we have about what enzymes exist and what reactions they catalyze, the more helpful in fields such as medicine.

For instance, many medications have been developed that work by mimicking the substrate of a particular enzyme. Using such mimics we can block the action of an enzyme.


  1. Editors of Encyclopedia Britannica (2018). Enzyme. Retrieved from
  2. Editors of Encyclopedia Britannica (2018). Protein. Retrieved from
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  5. GM Cooper (2000). The cell: A molecular approach, 2nd edition. Sunderland: USA, Sinauer Associates.


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