Medical News Today: A brief introduction to enzymes

Medical News Today: A brief introduction to enzymes
Enzymes help speed up chemical reactions in the human body. They bind to molecules and alter them in specific ways. They are essential for respiration, digesting food, muscle and nerve function, among thousands of other roles.

In this article, we will explain what an enzyme is, how it works, and give some common examples of enzymes in the human body.


The basics
Alpha amylase enzyme
The enzyme amylase (pictured), breaks down starch into sugars.
Enzymes are built of proteins folded into complicated shapes; they are present throughout the body.
The chemical reactions that keep us alive - our metabolism - rely on the work that enzymes carry out.
Enzymes speed up (catalyze) chemical reactions; in some cases, enzymes can make a chemical reaction millions of times faster than it would have been without it.
A substrate binds to the active site of an enzyme and is converted into products. Once the products leave the active site, the enzyme is ready to attach to a new substrate and repeat the process.
What do enzymes do?
The digestive system - enzymes help the body break down larger complex molecules into smaller molecules, such as glucose, so that the body can use them as fuel.
DNA replication - each cell in your body contains DNA. Each time a cell divides, that DNA needs to be copied. Enzymes help in this process by unwinding the DNA coils and copying the information.
Liver enzymes - the liver breaks down toxins in the body. To do this, it uses a range of enzymes.
How enzymes work
Enzyme lock and key model
Enzyme lock and key model
The "lock and key" model was first proposed in 1894. In this model, an enzyme's active site is a specific shape, and only the substrate will fit into it - like a lock and key.
This model has now been updated and is called the induced-fit model.
In this model, the active site changes shape as it interacts with the substrate. Once the substrate is fully locked in and in the exact position, the catalysis can begin.
The perfect conditions
Enzymes can only work in certain conditions. Most enzymes in the human body work best at around 37°C - body temperature. At lower temperatures, they will still work but much more slowly.
Similarly, enzymes can only function in a certain pH range (acidic/alkaline). Their preference depends on where they are found in the body. For instance, enzymes in the intestines work best at 7.5 pH, whereas enzymes in the stomach work best at pH 2 because the stomach is much more acidic.
If the temperature is too high or if the environment is too acidic or alkaline, the enzyme changes shape; this alters the shape of the active site so that substrates cannot bind to it - the enzyme has become denatured.

Cofactors
Some enzymes cannot function unless they have a specific non-protein molecule attached to them. These are called cofactors. For instance, carbonic anhydrase, an enzyme that helps maintain the pH of the body, cannot function unless it is attached to a zinc ion.
Inhibition
To ensure that the body's systems work correctly, sometimes enzymes need to be slowed down. For instance, if an enzyme is making too much of a product, there needs to be a way to reduce or stop production.
Enzymes' activity can be inhibited in a number of ways:
Competitive inhibitors - a molecule blocks the active site so that the substrate has to compete with the inhibitor to attach to the enzyme.
Non-competitive inhibitors - a molecule binds to an enzyme somewhere other than the active site and reduces how effectively it works.
Uncompetitive inhibitors - the inhibitor binds to the enzyme and substrate after they have bound to each other. The products leave the active site less easily, and the reaction is slowed down.
Irreversible inhibitors - an irreversible inhibitor binds to an enzyme and permanently inactivates it.
Examples of specific enzymes
There are thousands of enzymes in the human body, here are just a few examples:
Lipases - a group of enzymes that help digest fats in the gut.
Amylase - helps change starches into sugars. Amylase is found in saliva.
Maltase - also found in saliva; breaks the sugar maltose into glucose. Maltose is found in foods such as potatoes, pasta, and beer.
Trypsin - found in the small intestine, breaks proteins down into amino acids.
Lactase - also found in the small intestine, breaks lactose, the sugar in milk, into glucose and galactose.
Acetylcholinesterase - breaks down the neurotransmitter acetylcholine in nerves and muscles.
Helicase - unravels DNA.
DNA polymerase - synthesize DNA from deoxyribonucleotides.
In a nutshell
Enzymes play a huge part in the day-to-day running of the human body. By binding to and altering compounds, they are vital for the proper functioning of the digestive system, the nervous system, muscles, and much, much more.