Enzyme cofactors and coenzymes | Biology | Khan Academy

Summary

This educational video delves into the world of enzymes, focusing on the crucial role of cofactors in their function. Enzymes, primarily proteins, are known to accelerate reactions by lowering activation energy. The video clarifies that while enzymes are often depicted as simple structures in textbooks, they are complex, folded proteins. It introduces cofactors—non-protein components essential for enzyme activity, including inorganic ions and organic molecules like coenzymes. The video uses hexokinase and NAD as examples, illustrating how cofactors like magnesium ions and organic coenzymes facilitate enzyme reactions, often by stabilizing charges or acting as electron acceptors. It also hints at the significance of vitamins and minerals, which often serve as cofactors, emphasizing their role in biological processes.

Takeaways

• 🔬 Enzymes are proteins that catalyze reactions by lowering activation energies, making reactions proceed more frequently or faster.
• 🧬 Enzymes are often depicted abstractly in textbooks, but in biological systems, they are complex, folded proteins with specific active sites for substrates.
• 🌟 Coenzymes and cofactors are non-protein components that play crucial roles in enzyme function, often assisting in stabilizing charges or facilitating electron transfers.
• 💧 Magnesium ions are an example of inorganic cofactors that can help lower activation energy by interacting with the substrate.
• 🍋 Vitamins and minerals are essential for the body because many of them function as cofactors in enzymatic reactions.
• 🔄 Coenzymes are organic molecules that are not part of the protein structure but are integral to the enzyme's activity, such as NAD+ in the transfer of hydride ions.
• 🔬 The structure of NAD+ is composed of nicotinamide and adenine dinucleotide, which can accept or release a hydride ion, playing a key role in many biological reactions.
• 🌿 Organic cofactors like vitamin C and folic acid are examples of coenzymes that are involved in various enzymatic reactions, highlighting the importance of a balanced diet for enzyme function.
• 🔬 The function of cofactors and coenzymes can vary widely, including electron transfer, charge stabilization, and direct participation in the reaction mechanism.
• 📚 Understanding the roles of cofactors and coenzymes is crucial for grasping how enzymes facilitate the myriad of biochemical reactions within living organisms.

Q & A

What is the primary function of enzymes?

Enzymes primarily function to help reactions proceed by lowering their activation energies, making the reactions happen more frequently or faster.

What is the typical representation of an enzyme in textbooks?

In textbooks, enzymes are often represented as abstract drawings where the enzyme is depicted as a shape that a substrate locks into for a reaction to occur.

What is the actual biological form of an enzyme?

In biological systems, enzymes are proteins with complex structures, including polypeptides and various amino acid side chains that fold in different ways, such as alpha helices and beta sheets.

What is a cofactor and why is it important for enzymes?

A cofactor is a non-protein part of an enzyme, which can be a molecule, ion, or atom that plays a crucial role in facilitating the enzyme's function and catalyzing reactions.

What are the two main categories of cofactors?

Cofactors can be divided into organic cofactors and inorganic cofactors. Organic cofactors are molecules that contain carbon, while inorganic cofactors are typically ions.

What is a coenzyme and how does it differ from other cofactors?

A coenzyme is an organic cofactor, which means it is a non-protein molecule that contains carbon and is involved in helping enzymes catalyze reactions.

What is the role of magnesium ions in the enzyme hexokinase?

Magnesium ions act as cofactors in the enzyme hexokinase by helping to lower the activation energy of the reaction by keeping electrons in the phosphate groups busy, facilitating the phosphorylation of glucose.

What is NAD and how does it function as a coenzyme?

NAD (Nicotinamide adenine dinucleotide) is a coenzyme that facilitates the transfer of hydride ions. It can accept a hydride anion and become NADH, playing a crucial role in many biological reactions.

Why are vitamins and minerals important in biological reactions?

Vitamins and minerals are important because many of them act as cofactors for enzymes, helping to catalyze various biochemical reactions within the body.

Can you provide an example of a vitamin that functions as a cofactor?

Vitamin C is an example of a vitamin that functions as a cofactor, participating in several enzymatic reactions, although the specific roles are not detailed in the script.

What is the significance of the structure of NAD in its function?

The structure of NAD, which includes a nicotinamide group for hydride transfer and an adenine group linked to a ribose and phosphate, allows it to participate in redox reactions by accepting or donating electrons in the form of hydride ions.

Outlines

00:00

🧬 Enzymes and Their Helper Molecules

This paragraph delves into the world of enzymes, emphasizing their role in facilitating chemical reactions by lowering activation energies. It clarifies that while enzymes are often depicted as simple structures in textbooks, they are actually complex protein molecules with various folding patterns. The paragraph introduces the concept of cofactors, which are non-protein components that are crucial for the enzyme's function. Cofactors can be inorganic, like magnesium ions, or organic, such as coenzymes. The discussion uses hexokinase as an example to illustrate how cofactors like magnesium ions assist in the enzyme's catalytic activity. The importance of vitamins and minerals as cofactors in enzyme function is also highlighted.

05:01

🌿 Coenzymes: The Organic Cofactors in Enzyme Reactions

The second paragraph focuses on coenzymes, which are organic molecules that act as cofactors in enzymatic reactions. It uses NAD (Nicotinamide Adenine Dinucleotide) as a prime example, explaining its structure and function in facilitating the transfer of hydride ions. The paragraph describes how NAD can accept a hydride anion, transforming into NADH, and emphasizes its recurring patterns in various biological molecules. The structure of NAD is broken down into its components, including nicotinamide and adenine, and the significance of its dinucleotide nature is discussed. The paragraph also touches on other coenzymes like vitamin C and folic acid, illustrating their roles in enzyme reactions and their importance in maintaining health.

Keywords

💡Enzymes

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They work by lowering the activation energy required for a reaction to occur, making the reaction proceed more frequently or faster. In the video, enzymes are the central theme, with a focus on how they function and the components that assist them. An example given is hexokinase, which facilitates the phosphorylation of glucose.

💡Activation Energy

Activation energy is the minimum amount of energy required to initiate a chemical reaction. Enzymes help reactions proceed by lowering this energy barrier, making it easier for reactions to occur. The video script mentions that enzymes are around to help reactions to proceed by lowering their activation energies.

💡Substrate

A substrate is the molecule upon which an enzyme acts to catalyze a chemical reaction. In the context of the video, the substrate is the molecule that gets embedded in the enzyme's active site, where the reaction takes place. The script uses ATP and glucose as examples of substrates in the hexokinase model.

💡Proteins

Proteins are large biomolecules composed of amino acids and are essential for the structure, function, and regulation of the body's cells, tissues, and organs. The video emphasizes that when discussing enzymes, we are often referring to proteins, which fold into complex structures to catalyze reactions.

💡Cofactors

Cofactors are non-protein components that work in conjunction with enzymes to assist in catalyzing reactions. They can be either inorganic or organic and are essential for the enzyme's activity. The video script explains that cofactors like magnesium ions are crucial for the function of enzymes like hexokinase.

💡Inorganic Cofactors

Inorganic cofactors are small inorganic molecules or ions that assist enzymes in catalyzing reactions. They often help stabilize the enzyme-substrate complex or participate directly in the reaction mechanism. The script mentions magnesium ions as an example of inorganic cofactors.

💡Organic Cofactors

Organic cofactors, also known as coenzymes, are organic molecules that assist enzymes in catalyzing reactions. They are distinct from the protein part of the enzyme and often play a direct role in the chemical transformation of substrates. An example from the script is NAD, which facilitates the transfer of hydride ions.

💡Coenzymes

Coenzymes are a type of organic cofactor that is not a protein but is required for an enzyme's activity. They often participate in the transfer of chemical groups between substrates. The video uses NAD as a prominent example of a coenzyme involved in many biological reactions.

💡Hydride Ions

A hydride ion is a hydrogen ion (H-) that carries two electrons, giving it a negative charge. In the context of the video, coenzymes like NAD can accept hydride ions, which is crucial for their role in enzyme-catalyzed reactions. The script describes how NAD can become NADH by accepting a hydride anion.

💡Vitamins and Minerals

Vitamins and minerals are essential nutrients required in small amounts for various bodily functions, including their role as cofactors in enzymes. The video script connects the importance of vitamins and minerals to their function as cofactors, emphasizing their role in enzyme catalysis.

💡NAD (Nicotinamide Adenine Dinucleotide)

NAD is a coenzyme that plays a critical role in cellular metabolism, particularly in redox reactions. The video script delves into the structure of NAD, explaining how it can accept a hydride ion and become NADH, which is vital for many enzymatic reactions.

Highlights

Enzymes help reactions proceed by lowering activation energies.

Enzymes are proteins that facilitate biochemical reactions.

Enzymes are often depicted in textbooks as abstract entities, but in reality, they are complex protein structures.

Substrates are molecules that get embedded in the protein structure of an enzyme.

Hexokinase is an enzyme that facilitates the phosphorylation of glucose with the help of ATP.

Cofactors are non-protein parts of enzymes that play a crucial role in catalyzing reactions.

Magnesium ions are an example of inorganic cofactors that help lower activation energy by interacting with substrates.

Vitamins and minerals are often cofactors necessary for enzyme function.

Cofactors can be further divided into organic and inorganic types.

Coenzymes are organic cofactors that are not part of the protein structure but are essential for enzyme function.

NAD is a coenzyme that facilitates the transfer of hydride ions in biochemical reactions.

NAD can accept a hydride anion and transform into NADH, playing a role in many biological processes.

The structure of NAD includes nicotinamide, adenine, and two nucleotides, reflecting its role in various biochemical processes.

Vitamin C acts as a cofactor in enzymes, highlighting the importance of vitamins in biological reactions.

Folic acid is another coenzyme that plays a role in enzyme function, particularly in one-carbon transfer reactions.

Cofactors can help stabilize charges, act as electron acceptors or donors, or be part of the reaction mechanism.

Transcripts

- [Voiceover] We've already spent a couple of videos

talking about enzymes, and what I want to do

in this video is dig a little bit deeper and focus

on some actors that actually help enzymes.

And just as a reminder, enzymes

are around to help reactions to proceed,

to lower their activation energies, to make the reactions

happen more frequently or to happen faster.

Now, we've already seen examples of enzymes,

and just to frame things in our brain properly,

sometimes in a textbook you'll see an enzyme like this,

you'll see a drawing like this.

And people will call this the enzyme,

they'll call this the enzyme,

and then they'll call this right,

they'll say okay, and it's acting on

some kind of a substrate right over here,

it's going to do something to that.

And this is nice for a very abstract, textbook idea

of a substrate locking into an enzyme like this,

but this isn't actually what it

looks like in a biological system.

We have to remind ourselves, when people talk about enzymes

they're talking about proteins.

Now there are these kind of RNA enzymes called ribozymes

but the great majority, when we're talking about enzymes,

we tend to be talking about proteins.

And we spent a lot of time talking about how proteins

are these structures, there's polypeptides,

and all the side chains of the various amino acids

fold the proteins in all sorts of different ways.

So a better drawing for something like this

would be this protein that's all folded in different ways,

maybe has some alpha helices here,

maybe it has some beta sheets right over here.

It's all this kinda crazy stuff right over there.

And then the substrate might be some type

of a molecule, that is it gets embedded in the protein.

And you see some examples right over here.

This is actually a hexokinase model and you see,

at least you can see a little bit of the ATP

right over there, and it's a little harder to see

the glucose that's going to be phosphorylated.

And this reaction is being facilitated by this big

protein structure, the hexokinase.

Now, what we're going to focus on in this video

is that, when we talk about an enzyme, and we're talking

about proteins, we're talking about a chain of amino acids,

but there's often other parts of the enzyme

that aren't officially proteins.

And we even saw that when we talked about hexokinases,

when we talked about the phosphorylation of glucose,

we said hey, the way that it lowers the activation energy

is you have these positive magnesium ions,

these positive magnesium ions,

that can keep the electrons in the phosphate groups

a little bit busy, draw them away, so that

this hydroxyl group right over here can bond

with this phosphate and not be

interfered with these electrons.

Well these magnesium ions right over here,

they aren't officially part of the protein.

These are what we call cofactors.

So you might have a cofactor right over there

that latches onto the broader protein to become

part of the enzyme, and you actually need that

for the reaction to proceed, it plays a crucial role here.

So another drawing in the textbook, you'll see

something like this, or even, they'll draw, they'll say

okay, in order for this reaction to proceed, yes,

you need the substrate, but you also need the cofactor.

The cofactor.

And once again, it sounds like a fancy word,

but all it means is a non-protein part of an enzyme.

It's another molecule or ion or atom that is involved

in letting the enzyme perform its function

that it's not formally a part of an amino acid

or part of a side chain or part of the protein,

but it's another thing that needs

to be there to help catalyze the reaction.

We saw that with hexokinase, you had

magnesium ions that the complex picks up.

And this is why, when people talk about your vitamins

and minerals, a lot of the vitamins and minerals

that you need, they actually act as cofactors for enzymes.

And so you could even see it in this drawing over here,

at least based on what I read these are the magnesium ions

in green right over here, and these are cofactors.

These are cofactors.

So cofactor, non-protein part of your actual enzyme.

Now, we can subdivide cofactors even more.

We can divide them into organic cofactors

and inorganic cofactors.

So if you have cofactors, we've seen an inorganic cofactor,

a lot of these ions, you'll see magnesium ions,

you'll see sodium ions, you'll see calcium ions,

you'll see all sorts of things acting as cofactors,

often times to distract electrons, or to keep them busy

so that electrons can proceed.

But you can also have organic ones,

you can also have organic molecules.

Remember, organic molecules, these are just,

they'll involve carbon, they have

chains of carbons and other things.

And cofactors that are organic molecules,

we call them coenzymes.

Coenzymes.

And there's a bunch of examples of coenzymes.

This right over here is

the enzyme lactate dehydrogenase

and it has a coenzyme, and this coenzyme

you are going to see a lot in your biological careers,

NAD, right over here.

Notice, this isn't just an ion, it is an entire molecule.

It has carbon in it, that's why we call it organic.

And it is not formally protein, it's not part

of the amino acids that make up the protein,

so that's what makes it a cofactor,

and since it's an entire organic molecule,

we call this a coenzyme.

Coenzyme.

But like any cofactor, it plays a role

in actually allowing the enzyme

to do its function, to facilitate a reaction.

And this particular coenzyme, NAD,

which you're going to see a lot,

it helps facilitate the transfer of hydride ions.

Hydride ions never, or very seldom, exist by themselves,

but it's a hydrogen with an extra

electron, so it has a negative charge.

So it allows the transfer of this group

from a substrate or to a substrate,

and that's because NAD can accept

a hydride anion right over here and become NADH.

And if you want to see its broader structure,

it's actually quite fascinating.

I'll probably do a whole video on NAD

because in so many textbooks growing up

I just saw NAD and NADH and I'm like what is this thing?

And it's a fascinating molecule.

So what it can do is it can actually pick up

the hydride anion right over here

at this carbon, you can actually form another bond

with the hydrogen, and I'll do that in a future video,

I'll show the mechanism for it.

But it's a pretty cool molecule

and I like to actually look at this molecule

and remember, the whole focus of this is coenzymes,

but we see these patterns throughout biology

because the name, nicotinamide adenine dinucleotide

exactly describes what it is.

Nicotinamide, right down here, that is

this piece of the molecule, and this is the part

that can accept a hydride or let go of a hydride,

so you could say this is the active part of the molecule.

Adenine, our good old friend, we've seen adenine in DNA,

in RNA, in ATP, so this is our

good old friend adenine, right over here.

And it says dinucleotide, cause we actually have

two nucleotides paired together,

their phosphate groups are tied together.

And there's a couple cool ways to think about this.

You have an adenine right over here,

you have a ribose, you have a phosphate group.

If you just looked at this piece,

right over here, if you looked at this right over here,

this is your building block,

or this could be a building block, of RNA,

if you have an adenine right over there.

And if you include, let me undo this,

if you include all of this, this right over here,

this is ADP, well the reason why it's called dinucleotide

is you can also divide it the other way.

You can say, alright you have one nucleotide

that has nicotinamide right over here,

so that's one of the nucleotides,

and then the other nucleotide is right over here,

the one that involves adenine,

that's why it's called dinucleotide.

So hopefully this makes NAD less of a mysterious molecule,

we'll see it in the future, but I like to look at it

because it's got all these patterns,

it's got all these components that you see

over and over again, and you see it in ATP,

you see it in RNA, over and over again.

But this isn't the only cofactor or coenzyme.

There are many many others, in fact when people say

take your vitamins and your minerals,

that tends to be because they are cofactors.

Vitamin C is a very important cofactor

to be involved in enzymes that,

well I won't go into all of the

different things that it can do.

These are two different views of vitamin C,

a space-filling model and this is a ball-and-stick

model right over here of vitamin C.

Folic acid, once again, two different views,

but these are all coenzymes, they all work,

you know if you have a protein right over here

that you know it's all this really complex structure,

maybe you have some substrates, but to help facilitate,

let me do the substrates in a different color,

so maybe you have some substrates,

so these are the things that the enzyme

is trying to catalyze the reactions for.

But then you could have some ions,

which would, you know, you could kind of view these

as you would view these, you would view the ions

as cofactors, and you could have organic cofactors,

like the vitamin C, or other things that we talked about

that are also involved and help facilitating the mechanism,

or help facilitate the reaction.

And once again, sometimes it might be to help

stabilize some charge, sometimes it might be

to be an electron acceptor or donor,

or a whole series of different things.

They can actually act as part of the reaction mechanism.