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Holoenzyme: Definition & Overview

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  • 0:00 Enzyme Review
  • 1:13 What Is a Holoenzyme?
  • 2:04 Function of a Holoenzyme
  • 3:59 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson provides a quick review of the definition and function of an enzyme. It then defines a holoenzyme, explains its functions, and gives a few examples.

Enzyme Review

Your best friend has bought you a present, but he lives far, far away. What's the fastest way for him to get the present to you? He could walk, but that would take a long time. Riding a bike, driving, or even taking a plane would be much faster.

Without enzymes, chemical reactions in a cell occur at a similar speed to your friend walking to bring you that present. Enzymes are proteins that - like the bike, car, or plane that can help your friend get to you faster - speed up (or catalyze) cellular chemical reactions. They bind substrates and turn them into products.

In our analogy, the substrate would be the present from your friend, while the final product would be the delivered gift. So, the enzymes (bikes, cars, or planes) catalyze the change in substrate (wrapped present) to product (usable gift).

There is a special place on enzymes, called the active site, where substrates bind and are changed into products. But, the active site is not the only place where molecules can bind to enzymes. Molecules can also bind to other areas on enzymes to regulate (or control) enzyme activity. Non-substrates that bind enzymes and help catalyze reactions are called cofactors.

What Is a Holoenzyme?

A plane can take off with just a pilot, but he or she needs the assistance of a co-pilot for various tasks, including pre-flight checks to ensure everything is a go. Likewise, many enzymes need the help of cofactors to function. Cofactors can be inorganic ions (like metal zinc) or non-protein, organic (carbon-containing) molecules, known as coenzymes. Just as co-pilots can help fly many different planes, the same cofactor can bind to many different enzymes.

What do cofactors have to do with holoenzymes? An enzyme that needs a cofactor but does not have one is referred to as an apoenzyme. An apoenzyme with a cofactor attached is a holoenzyme. Holoenzymes, then, are enzymes with a bound cofactor, like a plane with its strapped-in co-pilot, ready for take-off.

Function of a Holoenzyme

An apoenzyme is inactive; that is, it will not catalyze substrates into products. An apoenzyme needs to become a holoenzyme (apoenzyme + cofactor) in order to function. The function of a holoenzyme is to change substrate into product, just like an enzyme does, but holoenzymes require a cofactor to be present. Additionally, holoenzymes are often made up of smaller protein parts called subunits.

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