Cofactor - Online Tutor, Practice Problems & Exam Prep

Cofactors are essential non-protein components of enzymes that influence catalytic activity. They are categorized into coenzymes, which are organic molecules derived from vitamins, and metal ions, which can be tightly or loosely bound. Coenzymes include prosthetic groups and cosubstrates, with examples like ATP and NADH, which are recyclable. Metal ions, such as transition metals (e.g., iron, zinc) and alkali metals (e.g., sodium, magnesium), play crucial roles in enzyme function. Understanding these components is vital for grasping enzyme catalysis and metabolic pathways.

Cofactor

In this video, we're going to begin our discussion on cofactors. So it turns out that the catalytic activity of many enzymes is actually dependent on the presence of relatively small molecules called cofactors. And so what are cofactors? Well, it turns out that cofactors are just non-protein portions of an enzyme, and they are in some way and somehow involved with proper enzyme catalysis. And so that brings us to two important terms that you guys need to be familiar with when it comes to cofactors, and the first is an apoenzyme, and the second is a holoenzyme. And so apoenzymes are enzymes that are inactive, and that's because this is the enzyme without its cofactor. And so because it doesn't have its cofactor, apoenzymes don't work. Now, on the other hand, holoenzymes are enzymes that are catalytically active, and that's because this is the enzyme with its cofactor. And so this holoenzyme actually does work. And so down below in our example image, I'll share with you guys something that helps me distinguish between apoenzymes and holoenzymes.

But first, I want you guys to know that cofactors tend to bind to the active sites of enzymes. But the functional roles that cofactors actually have vary greatly from enzyme to enzyme. We'll be able to talk about that in more depth when we talk about different types of enzyme catalysis later in our course. Now down below in our example image, notice in our central blue box here that this blue dot right here, notice that the enzyme is referred to as an apoenzyme, and the apoenzyme is inactive, meaning it does not work. And again, that's because it's not bound to its cofactor, which is the red dot. Now when the cofactor binds to the apoenzyme, it becomes a holoenzyme. And so you can see that the cofactor tends to bind to the active site of the enzyme, which is this shape, right here. And so the holoenzyme is actually an active form of the enzyme.

And so something that helps me distinguish between apoenzymes and holoenzymes is that I know that apoenzymes are always trying to apologize. And so here we have a sign and it's saying, "I apologize." And the reason that apoenzymes are always trying to apologize is that they are always inactive. They are always saying, "I'm inactive right now." And so the reason that they are inactive is again because they are not bound to their cofactor.

Now, on the other hand, holoenzymes are enzymes that we know are active. And so if you need an active enzyme, all you need to do is hola at your boy, and your boy is holoenzyme. And so holoenzymes always like, "If you need an active enzyme, all you gotta do is hola at me." And so hopefully that'll help you guys remember that holoenzymes are the active version of the enzyme that is bound to the cofactor. And so that concludes our lesson on the introduction to cofactors, and I'll see you guys in our next video.

Aldoase requires Zn²⁺ for catalysis. Under cellular conditions of zinc deficiency, aldoase is referred to as:

• A. Apoenzyme.
• B. Coenzyme.
• C. Holoenzyme.
• D. Prosthetic group.

Cofactor

So now that we've introduced cofactors, in this video we're going to talk about how cofactors can be broken up into 2 main groups. The first are coenzymes, and the second are metal ions. Coenzymes are small organic molecules that are derived from vitamins. These include both prosthetic groups and cosubstrates. We've mentioned prosthetic groups in some of our previous lesson videos, and recall that these are tightly bound, or covalently attached coenzymes, whereas cosubstrates, on the other hand, are loosely bound and are recyclable substrate coenzymes. Cosubstrates are very similar to substrates because they are both taken by the enzyme and chemically converted into something different. The main difference between a normal substrate and a cosubstrate is that cosubstrates are recyclable, meaning that after they are utilized by an enzyme, they are quickly regenerated back into their normal state. Examples of cosubstrates include both ATP and NADH. We know that ATP can be utilized by an enzyme as a form of energy, and it can be converted into ADP, which has one less phosphate group. We know from our previous bio courses that there are many processes that allow for the regeneration of ATP, converting ADP back into ATP. Because ATP is recyclable in this fashion and can be quickly regenerated, it's considered a cosubstrate. Similarly, NADH is also a cosubstrate.

Moving on to our second main group of cofactors, we have metal ions, which are pretty much exactly what they sound like. They are metal atoms with a net charge. Just like coenzymes can be further broken up into prosthetic groups, metal ions can be further broken up into metalloenzymes and activator ions. Metalloenzymes are comparable to prosthetic groups because they are both tightly bound. Metalloenzymes are enzymes with tightly bound metal ions. Activator ions are comparable to cosubstrates because they are both loosely bound, and activator ions are loosely bound metal ions. It turns out that metalloenzymes tend to be transition metals, such as iron, copper, zinc, and manganese, whereas activator ions, on the other hand, tend to be alkali and alkaline earth metals, such as sodium, potassium, magnesium, and calcium.

Notice that we have two different charts that are color coded. We have a blue chart, which shows all of our coenzymes, and a yellow chart, which shows our metal ions. These are certainly not all of the coenzymes that exist, this is just a small portion of relevant coenzymes that we're going to encounter moving forward. However, I do not expect you guys to memorize all of these coenzymes and all of these metal ions at this stage. I just want to show you some examples of coenzymes and metal ions. One thing to note is that the coenzyme chart on the left has an additional column that the metal ion chart does not have, which is the vitamin precursor. This is important to note as coenzymes are derived from vitamins. You can see that we have vitamins B7, B5, B2, B3, and B6 here, and note that all of these vitamins also have alternative names. This is something important to keep in mind moving forward.

We're going to talk about each of these coenzymes in more detail as we move forward in our course and encounter enzymes that utilize these coenzymes. For instance, we know that biotin is a prosthetic group of an enzyme pyruvate carboxylase, which is very important for gluconeogenesis, which we're going to talk a lot more about later in our course. Essentially, we have a bunch of prosthetic groups, and really the only cosubstrates in this list are coenzyme A here and this right here. These are the only cosubstrates. The others in the list are prosthetic groups. Over here on the right, notice we have the metal ions, and remember, the metal ions tend to be transition metals, whereas activator ions tend to be alkali and alkaline earth metals. This concludes our lesson, and I'll see you guys in our next video.

Which of the following options is false?

• A. Metal ions can bind directly to enzymes or coenzymes.
• B. Metal ions often act as cofactors.
• C. Organic cofactors can be called coenzymes.
• D. Metal ions can be prosthetic groups.

Here’s what students ask on this topic:

What are cofactors and why are they important for enzyme activity?

Cofactors are non-protein components essential for the catalytic activity of many enzymes. They can be organic molecules, known as coenzymes, or metal ions. Cofactors bind to the active site of enzymes and are crucial for proper enzyme function. Without their specific cofactors, enzymes (apoenzymes) remain inactive. When bound to their cofactors, they become holoenzymes, which are catalytically active. Cofactors play various roles, such as stabilizing enzyme structure, participating in the catalytic process, or aiding in substrate binding. Understanding cofactors is vital for comprehending enzyme catalysis and metabolic pathways.

What is the difference between an apoenzyme and a holoenzyme?

An apoenzyme is an inactive enzyme that lacks its necessary cofactor. Without the cofactor, the enzyme cannot perform its catalytic function. In contrast, a holoenzyme is the active form of the enzyme, which includes the apoenzyme bound to its cofactor. The binding of the cofactor to the apoenzyme enables the enzyme to catalyze reactions effectively. This distinction is crucial for understanding how enzymes function and the role of cofactors in enzyme activity.

What are the two main types of cofactors?

The two main types of cofactors are coenzymes and metal ions. Coenzymes are small organic molecules derived from vitamins and can be further classified into prosthetic groups (tightly bound) and cosubstrates (loosely bound and recyclable). Examples include ATP and NADH. Metal ions, on the other hand, are metal atoms with a net charge and can be categorized into metalloenzymes (tightly bound) and activator ions (loosely bound). Transition metals like iron and zinc often serve as metalloenzymes, while alkali metals like sodium and potassium act as activator ions.

How do coenzymes differ from metal ions as cofactors?

Coenzymes are organic molecules derived from vitamins and can be either tightly bound (prosthetic groups) or loosely bound and recyclable (cosubstrates). Examples include ATP and NADH. Metal ions, however, are inorganic and can be either tightly bound (metalloenzymes) or loosely bound (activator ions). Transition metals like iron and zinc typically serve as metalloenzymes, while alkali metals like sodium and potassium act as activator ions. Both types of cofactors are essential for enzyme activity but differ in their chemical nature and binding properties.

What are some examples of coenzymes and their roles?

Examples of coenzymes include ATP, NADH, and coenzyme A. ATP serves as an energy source for various biochemical reactions, converting to ADP and then being regenerated back to ATP. NADH acts as an electron carrier in redox reactions, playing a crucial role in cellular respiration and energy production. Coenzyme A is involved in the synthesis and oxidation of fatty acids and the Krebs cycle. These coenzymes are essential for various metabolic pathways and enzyme functions.