Acetylcholinesterase Inhibitors: Examples & Mechanism

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning about inhibitors of the enzyme acetylcholinesterase. We'll learn how these drugs work, different examples and what they are used for.

Acetylcholine

Try to sit perfectly still. It's difficult, and even if you are seasoned in the meditation department, if you're alive you're never really going to be perfectly still. Even when we're not actively moving our appendages our lungs are still expanding, our heart is still pumping, and our digestive system is still contracting.

These movements are mediated by chemical communication between the brain and muscle. One chemical involved in muscle movement is called acetylcholine. Acetylcholine is released by neurons and tells nearby muscle cells to contract. It is also involved in communication between neurons in the brain.

Acetylcholinesterase

It's important not only that muscles contract, but that they also relax. Imagine having your legs fully contracted forever! You would never be able to walk. Your quadriceps on the top of your thighs need to contract to move your legs forward, then relax so your other leg can propel you forward.

More importantly, the muscle that allows your lungs to expand and contract is the diaphragm, which needs to contract when you inhale, but also relax when you exhale. If you can't relax, you won't be able to exhale carbon dioxide and you will shortly suffocate and die.

In order for muscles to relax, the acetylcholine needs to be removed. It's analogous to how roads must have both red lights and green lights. There needs to be control, otherwise all the cars would crash into each other. Similarly, the acetylcholine signal must stop and start again for the body to stay functional.

An enzyme called acetylcholinesterase breaks down acetylcholine, removing it and allowing the muscle to relax. Without acetylcholinesterase, your muscles would stay contracted forever!

Acetylcholinesterase Inhibitors

Acetylcholinesterase inhibitors prevent acetylcholinesterase from doing its job. Some do this for only a little bit, called reversible inhibitors and can be used as medicine in some situations. Others inhibit acetylcholinesterase permanently and can be very dangerous to humans, called irreversible inhibitors. Today, we're going to look at several examples of each.

Acetylcholinesterase inhibitors prevent acetylcholinesterase from breaking down acetylcholine
AchE inhibitor

Donepezil

Imagine waking up and not knowing who your most loved family members are. As you get out of bed, you're unsure of how to get to the bathroom. This doesn't feel like your house. Life is confusing, and often depressing. This bleak scenario is the life of patients with advanced Alzheimer's disease.

Alzheimer's disease is a type of neurodegenerative illness, where brain matter breaks down. One problem is the loss of neurons that use acetylcholine. To increase acetylcholine communication, doctors can prescribe donepezil, a reversible acetylcholinesterase inhibitor.

For acetylcholinesterase to do its job, it must be able to attach to acetylcholine at an opening called the active site. Donepezil binds near the active site of acetylcholinesterase called the peripheral anionic site. Think of this area as the gateway for acetylcholine to get into acetylcholinesterase. If the gateway is blocked, acetylcholine can't get in and can't be destroyed.

So, when patients take donepezil the amount of acetylcholine in their body increases, allowing for more acetylcholine signaling to offset the death of acetylcholinergic neurons in Alzheimer's. Donepezil has been shown to improve cognition in Alzhiemer's patients, however it does not cure the disease.

Sarin Gas

Not all acetylcholinesterase inhibitors are used for good though. Sarin gas was developed in 1938 by Germany and has been used extensively in chemical warfare. Sarin gas is extremely toxic and once released into the air, finds its way to water sources, food, and even sticks to clothing and surfaces, poisoning victims long after the initial exposure. Sarin is so frightening that Germany decided not to use it during World War II. Today, the United Nations has strict rules against chemical warfare like this, however it still persists, recently being used in attacks in Syria in 2017.

A sarin gas victim being rushed to medical treatment in Syria
sarin gas victim

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