Coenzymes, Cofactors & Prosthetic Groups: Function and Interactions

Some enzymes require helpers to recognize a substrate or complete a reaction. These helpers include cofactors, coenzymes, and prosthetic groups, which are required for some enzymes’ functions.

Enzyme Partners

Sometimes we get by with a little help from our friends. There are some tasks, like going to the store for groceries, that are perfectly easy to do by yourself. Some tasks, like moving a large piece of furniture into your house and up a few flights of stairs – well, friends are good to have in situations like this, particularly strong ones who will lift anything for some free pizza.

We all rely on other people at certain times in our lives, so we shouldn’t be surprised that the same thing can happen in a biological process. For example, enzymes benefit from a few close friends, too. Remember that enzymes are proteins that catalyze chemical reactions. In other lessons, we’ve learned how they functioned. In this lesson, we’ll learn about who functions with them, or rather, who helps them catalyze these reactions. Some enzymes need helpers or partners, and some don’t. There are different types of enzyme helpers, too, with different enzymes requiring different helpers or different kinds of friends.

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Cofactors

The first type of enzyme partner is a group called cofactors, or molecules that increase the rate of reaction or are required for enzyme function. Cofactors are not proteins but rather help proteins, such as enzymes, although they can also help non-enzyme proteins as well. Examples of cofactors include metal ions like iron and zinc.

Examples of cofactors that assist proteins
Cofactor Examples

Coenzymes

A specific type of cofactor, coenzymes, are organic molecules that bind to enzymes and help them function. The key here is that they’re organic. ‘Organic’ does not mean you’ll find them in a special aisle in the grocery store. Rather, organic molecules are simply molecules that contain carbon. Don’t let the name ‘coenzymes’ fool you, either; coenzymes are not really enzymes. As the prefix ‘co-‘ suggests, they work with enzymes. Many coenzymes are derived from vitamins.

These molecules often sit at the active site of an enzyme and aid in recognizing, attracting, or repulsing a substrate or product. Remember that a substrate is the molecule upon which an enzyme catalyzes a reaction. Coenzymes can also shuttle chemical groups from one enzyme to another enzyme. Coenzymes bind loosely to enzymes, while another group of cofactors do not.

Coenzymes bind loosely to the active site of enzymes.
image of coenzyme

Prosthetic Groups

Prosthetic groups are cofactors that bind tightly to proteins or enzymes. As if holding on for dear life, they are not easily removed. They can be organic or metal ions and are often attached to proteins by a covalent bond. The same cofactors can bind multiple different types of enzymes and may bind some enzymes loosely, as a coenzyme, and others tightly, as a prosthetic group. Some cofactors may always tightly bind their enzymes. It’s important to note, though, that these prosthetic groups can also bind to proteins other than enzymes.

Prosthetic groups can be either organic or metal ions.
Prosthetic Groups

An enzyme that requires a helper group really isn’t complete without it. Once attached, it’s called a holoenzyme. A holoenzyme is an enzyme with any metal ions or coenzymes attached to it that is now ready to catalyze a reaction.

Examples of Enzyme Partners

There are examples of cofactors, coenzymes, and prosthetic groups in many biological processes. For example, cellular respiration occurs in all of your cells, which is a process that converts food into energy. The details of this process will be saved for other lessons, but you can imagine it as a long series of events that need to occur, like steps in a relay triathlon, except with way more than three events! Enzymes are the proteins that perform many of the steps in cellular respiration, passing the biological baton from the cold-water swim to the bicycle race.

We won’t go over the individual events here, but let’s take for example the steps that convert pyruvate into acetyl-CoA. Don’t be mesmerized by the chemical names; what’s important here is the example. This one step is carried out by a complex of three different enzymes and uses five different partners. These enzyme partners bind to the enzyme active site. Some help move chemical molecules from one step to another. Four out of the five of these partner molecules are derived from different B vitamins, meaning they are organic compounds and therefore coenzymes. Some of these bind tightly to the enzyme tricomplex, making them also prosthetic groups.

Enzyme partners bind to the enzyme active site.
Enzyme Partners Bonding

You get B vitamins from food such as eggs and milk and certain veggies. You can see why these vitamins are so important. They are used to derive these molecular helpers to carry out just one step of an essential cellular process that gives you energy throughout the day! Just think of that the next time you have an omelet. Your friends are helpers that move your furniture, but our bodies derive different types of helpers from the food you eat that allow specific enzymatic processes to take place. It’s hard to imagine that doing something so simple as eating could have such a complicated effect.

Lesson Summary

In this lesson, we’ve learned about some of the help an enzyme can get to perform its reactions. ‘Cofactors‘ is a term broadly given to molecules that increase the rate of reaction or are required for enzyme function. Metal ions are usually cofactors. Coenzymes are a specific type of helper or partner that are organic molecules required for enzyme function that bind loosely to an enzyme. They are often, though not always, derived from vitamins. Prosthetic groups are enzyme partner molecules that bind tightly to an enzyme.

Learning Outcome

By the end of this lesson, you should be able to define and describe the enzyme partners coenzymes, cofactors and prosthetic groups, as well as provide examples of how they function.

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