Enzymes | Energy and enzymes | Biology | Khan Academy

Summary

The video script delves into the intricacies of biological reactions, focusing on the phosphorylation of glucose as a key example. It explains how, despite being energetically favorable, certain reactions require an input of energy, known as activation energy, to proceed. Enzymes, particularly hexokinase, are highlighted as catalysts that facilitate these reactions by providing an optimal environment and necessary charges. The script also emphasizes the beauty and complexity of these biological systems, with a visual representation of hexokinase and its interaction with glucose and ATP, illustrating the enzyme's role in the phosphorylation process.

Takeaways

• 🔬 **Phosphorylation of Glucose**: The process of adding a phosphate group to glucose is energetically favorable but requires activation energy to proceed.
• 🌐 **Coupled Reactions**: The phosphorylation of glucose is detailed in a video on coupled reactions, emphasizing its importance in biological systems.
• 🏷️ **Glucose Tagging**: Phosphorylation allows glucose to be 'tagged' for cellular processes, preventing it from escaping the cell.
• 🔬 **Mechanism Insight**: The mechanism involves a lone pair of electrons on the glucose's hydroxyl group forming a bond with the phosphorus in the phosphate group.
• ⚡ **Activation Energy**: Despite being energetically favorable, the reaction requires an input of energy to overcome the activation energy barrier.
• 🔄 **Catalytic Role of Enzymes**: Enzymes, such as hexokinase, catalyze the phosphorylation reaction by providing a suitable environment and reducing the activation energy.
• 🧬 **Protein Complexes**: Enzymes are typically protein complexes that may include multiple polypeptide chains and associated ions.
• 🤝 **Substrate Interaction**: Substrates like ATP and glucose bind to the enzyme at the active site, where the reaction takes place.
• 🔄 **Induced Fit**: The enzyme's active site may change shape to better accommodate the substrates, enhancing the likelihood of a successful reaction.
• 🔋 **Enzyme Reusability**: Once the reaction is complete, the enzyme releases the products and can catalyze the reaction multiple times.

Q & A

Why is the phosphorylation of glucose an important reaction in biological systems?

The phosphorylation of glucose is crucial because it prepares glucose to be the input for a series of biological mechanisms, effectively tagging it to prevent it from escaping the cell and making it available for energy production.

What is the role of the phosphate group in the phosphorylation of glucose?

The phosphate group acts as a tag on the glucose molecule, making it less likely to leave the cell and more readily available for cellular processes.

Why doesn't the phosphorylation of glucose occur spontaneously even though it's energetically favorable?

Although the phosphorylation of glucose is energetically favorable, it requires an input of energy known as activation energy to overcome the repulsion between the negatively charged oxygen atoms and the electrons trying to form a bond with the phosphorus.

What is the activation energy in the context of biochemical reactions?

Activation energy is the minimum amount of energy needed to initiate a chemical reaction, allowing the reactants to overcome the energy barrier and proceed to form products.

How do enzymes facilitate biochemical reactions like the phosphorylation of glucose?

Enzymes facilitate reactions by lowering the activation energy required, providing an alternative reaction pathway with a lower energy barrier, and sometimes by providing a more favorable environment for the reaction to occur.

What is the term for the protein that helps phosphorylate sugar molecules like glucose?

The enzyme that helps phosphorylate sugar molecules such as glucose is generally referred to as hexokinase.

What is the active site in the context of enzymes?

The active site is the specific region on an enzyme where substrates bind and the chemical reaction occurs.

What is the induced fit model in enzyme-substrate interactions?

The induced fit model describes how the enzyme's active site changes its shape upon substrate binding to better accommodate the substrate, enhancing the likelihood of a successful reaction.

Why are enzymes sensitive to changes in temperature and pH?

Enzymes are sensitive to temperature and pH because these factors can affect their three-dimensional structure, which is essential for their function. Extreme temperatures or pH levels can cause denaturation, leading to loss of enzyme activity.

How do enzymes maintain their activity after catalyzing a reaction?

After catalyzing a reaction, enzymes release the products and return to their original state, allowing them to participate in multiple rounds of catalysis without being consumed in the reaction.

What is the significance of the visualizations of hexokinase and its interaction with ATP and glucose?

The visualizations of hexokinase interacting with ATP and glucose provide an appreciation for the complexity and specificity of enzyme-substrate interactions, highlighting the beauty and intricacy of biological systems.

Outlines

00:00

🔬 The Role of Enzymes in Biological Reactions

This paragraph discusses the necessity of enzymes in facilitating biological reactions that are energetically favorable but do not occur spontaneously due to activation energy barriers. The phosphorylation of glucose serves as a key example, where the addition of a phosphate group to glucose is essential for initiating a series of biological processes. Despite the reaction being energetically favorable, it requires a catalyst to overcome the activation energy. Enzymes, typically proteins, play this catalytic role by providing an environment conducive to the reaction, such as through induced fit and the provision of necessary charges. The paragraph also introduces hexokinase, an enzyme that catalyzes the phosphorylation of glucose, and highlights how enzymes are proteins that can bind substrates at their active sites, facilitating reactions through various mechanisms, including charge manipulation and structural changes.

05:00

🌡️ Enzyme Function and Structural Complexity

The second paragraph delves into the operational conditions of enzymes, noting that they function optimally within specific pH and temperature ranges. It explains that while higher temperatures can increase molecular interactions, excessively high temperatures can lead to enzyme denaturation and loss of function. The paragraph emphasizes the intricate and beautiful structures of enzymes, which are composed of protein complexes and may include additional components like metal ions. It uses the example of hexokinase to illustrate the enzyme's structural complexity and its role in the phosphorylation of glucose. The visualization of hexokinase and its interaction with ATP and glucose substrates at the active site is highlighted, showcasing the enzyme's ability to induce fit and facilitate the desired reaction. The paragraph concludes with a three-dimensional visualization of hexokinase, emphasizing the enzyme's structural features and its role in biological systems.

Keywords

💡Phosphorylation

Phosphorylation is the process of adding a phosphate group to a molecule, typically a protein or a carbohydrate. In the context of the video, phosphorylation of glucose is highlighted as a crucial step in cellular metabolism. The video explains that even though this reaction is energetically favorable, it requires an input of energy, known as activation energy, to proceed. This process is essential for the glucose molecule to be utilized within the cell, as it 'tags' the glucose, preventing it from escaping the cell.

💡Activation Energy

Activation energy is the minimum energy required to initiate a chemical reaction. The video script describes how, despite being energetically favorable, the phosphorylation of glucose does not occur spontaneously due to the presence of activation energy. This concept is pivotal in understanding why certain biological reactions, even those that release energy, need a catalyst to proceed.

💡Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. In the video, enzymes are introduced as biological catalysts that facilitate reactions like the phosphorylation of glucose. The script explains that catalysts help overcome the activation energy barrier, making reactions more likely to occur by providing an alternative reaction pathway with a lower energy requirement.

💡Enzymes

Enzymes are biological catalysts that are typically proteins. They play a crucial role in many biochemical reactions, including the phosphorylation of glucose as mentioned in the video. The script describes how enzymes can provide a positive charge to counteract the negative charges in the reactants, thereby facilitating the reaction. They are also depicted as highly complex and beautiful structures that are integral to the functioning of biological systems.

💡Glucose 6-Phosphate

Glucose 6-phosphate is a compound formed when a phosphate group is added to glucose. The video script discusses this compound as a product of the phosphorylation of glucose, which is an important intermediate in cellular metabolism. The formation of glucose 6-phosphate is significant because it marks the glucose molecule as ready for further metabolic processes within the cell.

💡Substrates

Substrates are the reactants upon which enzymes act to catalyze a reaction. In the video, ATP and glucose are identified as substrates in the phosphorylation reaction. The script explains how these substrates bind to the enzyme at the active site, which is the region of the enzyme where the reaction takes place.

💡Active Site

The active site is the specific region of an enzyme where the substrates bind and the chemical reaction occurs. The video script uses the active site to illustrate where ATP and glucose bind in the phosphorylation process. It is a critical concept in understanding enzyme function, as it is where the enzyme's catalytic activity is focused.

💡Induced Fit

Induced fit is a model that describes how the binding of substrates to an enzyme can cause a conformational change in the enzyme, making the active site a better fit for the substrates. The video script mentions this concept to explain how the enzyme's shape can change to accommodate the substrates, thereby facilitating the reaction.

💡Hexokinase

Hexokinase is a specific type of enzyme that catalyzes the first step of glycolysis by phosphorylating hexoses, such as glucose. The video script refers to hexokinase as the enzyme responsible for the phosphorylation of glucose, highlighting its role in preparing glucose for further metabolic processing.

💡Protein Complexes

Protein complexes are assemblies of two or more proteins that work together to perform a function. In the context of the video, enzymes, which are often protein complexes, are described as the biological catalysts that facilitate reactions. The script suggests that enzymes can be composed of multiple polypeptide chains and may include other components, such as ions, to function effectively.

💡Denaturation

Denaturation refers to the loss of a protein's native structure, often caused by extreme conditions such as high temperatures or pH changes. The video script mentions that enzymes, being proteins, can denature if the temperature becomes too high, which would inactivate them. This concept is important for understanding the conditions under which enzymes function optimally.

Highlights

Phosphorylation of glucose is energetically favorable but requires activation energy to proceed.

The phosphorylation of glucose 6-phosphate is detailed in a video on coupled reactions.

Phosphate group attachment to glucose tags it for biological mechanisms and prevents it from escaping the cell.

The phosphorylation mechanism involves a lone pair of electrons on a hydroxyl group forming a bond with phosphorus.

Enzymes, typically proteins, catalyze reactions in biological systems by providing necessary conditions.

Hexokinase is an enzyme that helps phosphorylate sugar molecules like glucose.

Enzymes work by providing a suitable environment, bending molecules, or providing necessary ions for reactions.

The active site of an enzyme is where substrates bind and interact to facilitate the reaction.

Induced fit is a process where the enzyme changes shape to better bind and position substrates for the reaction.

Positive charges from enzyme side chains or ions like magnesium help draw away negative charges to facilitate reactions.

Enzymes are protein complexes that can include multiple polypeptide chains and associated ions.

Enzymes operate optimally within specific pH and temperature ranges, with potential denaturation at extreme temperatures.

The complexity and beauty of enzyme structures are highlighted, emphasizing their presence within biological systems.

Hexokinase's visualization shows the scale of glucose and ATP molecules in relation to the enzyme.

Enzymes can facilitate reactions by bending molecules and bringing substrates close together.

Enzymes are reusable, capable of catalyzing the same reaction multiple times throughout their lifespan.

Three-dimensional visualizations of hexokinase provide insight into the binding sites and molecular interactions.

Transcripts

- There are all sorts of reactions in biological systems

that are energetically favorable,

but they're still not going to happen quickly

or even happen on their own,

and the phosphorylation of glucose is an example of that.

We go into some detail into that

on the video on coupled reactions,

and I think we actually called that

The Phosphorylation of Glucose 6-Phosphate,

but it's super important because

by putting the phosphate group on a glucose,

it's ready to be the input

to a whole series of biological mechanisms,

it allows the glucose to be tagged

so it's going to be hard for it to escape the cell again,

and it's fairly straightforward mechanism,

where you have a lone pair of electrons

on this hydroxyl group right over here,

and then it attempts to, if it's in the right configuration,

it could form a bond with the phosphorus

in the phosphate group.

Now, the reason why it doesn't happen on its own,

even though it's energetically favorable,

once you form the bond, you have,

electrons are gonna be able to go to a lower energy state.

So it has a negative delta G.

If this is the molecules before the reaction,

this is how much free energy they have before the reaction,

after the reaction, they have less free energy,

they have been able to release energy,

so this is something that we would consider

to be spontaneous, but for the reaction to happen,

you need a little bit of energy to be put into the system.

We call this our activation energy.

You might say, "Well, why is that?"

Well, we have electrons that want to form a bond

with this phosphorus,

but this phosphorus is surrounded by negative charges.

This oxygen right over here has a negative charge.

This oxygen right over here has a negative charge,

and as you can imagine, electrons don't like

being around other electrons, like charges repel each other,

so in order for this reaction to occur,

or for it to occur more frequently,

it has to be catalyzed.

A catalyst is anything that makes a reaction happen faster,

or even allows the reaction to happen at all,

and when we talk about catalysts in biological systems,

we're typically talking about,

we're typically talking about Enzymes.

Enzymes.

And the way that an Enzyme might catalyze this reaction,

we actually talk about it,

and when we talk about coupled reactions,

it'll maybe can provide some positive charges.

It could provide some positive charges

around these negative charges to pull them further away

to create space

so that you can actually have the reaction proceed,

and so what an Enzyme would do,

it would make this curve, instead of having this hump on it,

the curve would more like this,

so that the reaction can just proceed.

But what are these Enzymes?

These things that can maybe,

it could place some interesting charge

that can allow the reaction to happen a certain way,

it might bend the molecules in a certain way

to expose some bonds,

it might have a more acidic or basic environment

that might be more favorable for the reaction.

What are these seemingly magical things?

Well, at a very high level, they tend to be

these protein complexes, plus or minus a few other things,

so you can view them as proteins and

maybe sometimes, they'll be multiple polypeptide chains

put together, they might have some other

ions associated with them, but for the most part,

they are proteins, and the molecules

that are going to react,

that are going to bind to the proteins,

we call these the Substrates.

So these, and this reaction, (mumbles) glucose

and the ATP, these are going to be the Substrates.

So you can imagine

the Enzyme that does this,

and the general term for the Enzyme

that helps phosphorylate a sugar molecule like this,

we call it hexokinase.

So it might be this crazy-looking,

this crazy-looking protein, we're gonna

take better looks at this in a few moments,

but the ATP might bind

to it right over there.

ATP is one of the Substrates,

and then the glucose might bind

to it right over there,

and so these two Substrates bind,

and the area where all of this is going on,

we call that the Active Site.

So the Active Site, because that's where all the action is,

the Active Site.

And often, when you have the Substrates bind,

they're able to interact with the protein

to make the fit even stronger,

to make it even more,

more suitable for the reaction to take place,

and so the whole protein might bend a little bit

to kind of lock these two in place a little bit more,

and we call that Induced fit.

Induced fit.

And so, where would these positive charges come from?

Well these would be things that are the side chains

of the different amino acids on the actual,

on the polypeptide chain on the protein,

and it could even be other ions that get involved,

in fact, in particular,

to facilitate the phosphorylation of glucose,

a magnesium ion might be involved

to help draw some positive charge away,

but there's other positively charged groups

that help draw charge away so that the reaction

is more likely to occur.

So that's what enzymes are, and they tend to be

optimally working in certain pH environments

or certain temperatures.

In general, the higher temperatures allow more interactions,

things are bumping around more,

but if temperatures get a little bit too high,

the protein or the Enzyme might stop working,

it might denature,

it might lose its actual structure.

And what I want now give you an appreciation for

is how beautiful and complex these structures are.

You should appreciate what I'm showing you.

These are in your cells!

These are in your,

look at your hand, look at

everything around you, there's a lot of this stuff

going on inside of you, so hopefully it gives

an appreciation for the complexity

of you as a biological system, but frankly,

all biological systems.

So this right over here, this is a visualization

of a hexokinase, one variety of it,

and just to get a sense of scale,

this is a glucose molecule,

and this right over here is an ATP,

and so they will bind, these are the two Substrates,

they will bind at the Active Site.

You might have the Induced fit, where this fits around it.

It draws some charge away,

it might bend the molecules in a certain way

so that they're more likely to interact,

bring these things close together,

and so you're gonna have the reaction occur

and then once the reaction occurs,

they're not gonna want to bind to the Substrates anymore.

I guess you could say the products, at that point,

and then they're gonna let go of them,

and then the Enzyme has a change,

and that's an important property of an Enzyme.

It's not like it just has one use and it goes away,

it can keep doing this over and over and over again.

One Enzyme will do this many, many, many, many, many times

in its actual life.

And so now what I want to show you is a little

three-dimensional visualization that I got from a website,

so let me go get that.

Go ahead and pause my recording

so I could get to this little simulation or this model,

and this is actually a hexokinase as well,

and hexokinase is come into,

in a bunch of different varieties,

but this is a pretty neat thing to look at

and this has been visualized differently,

and when you look up protein images on the web,

or anywhere, you'll see them sometimes

with these ball and stick models,

sometimes you'll see them in these space-filling model,

sometimes you'll see them with this kind of,

where you the very structures,

and you notice the alpha helices here

that we studied when we talked about protein structures,

and you can also see some beta sheets,

but this gives you an appreciation of

the binding sites and how these things might interact.

This right over here,

that is a molecule of ATP,

and then right next to it, I believe,

if I'm looking at that right, that is a molecule of glucose,

and notice they have bound,

they are the two Substrates,

they have bound at the Active Site,

and now, they can interact with each other,

the Enzyme, the hexokinase in this case,

can help facilitate the reaction that we care about,

the phosphorylation of glucose.

So hopefully, images like this, and like this,

give you an appreciation for how complex

and how beautiful these things actually are.