Myosin and actin | Circulatory system physiology | NCLEX-RN | Khan Academy

Summary

This video script delves into the intricate process of how proteins, specifically myosin and actin, interact with ATP to generate mechanical motion. It illustrates the myosin's role as an ATPase enzyme, converting ATP into ADP and phosphate groups, and the subsequent steps of energy release and protein conformation change that lead to muscle contraction. The script offers a foundational understanding of muscle function, setting the stage for further exploration of muscle activation by nerves.

Takeaways

• 🔬 The video aims to explain the interaction between two proteins, myosin and actin, and ATP to produce mechanical motion.
• 🏋️‍♂️ The process described is fundamental to understanding how muscles work, including outside of muscle cells.
• 🔬 Myosin is a complex enzyme that is involved in the reaction of ATP into ADP and phosphate groups, classifying it as an ATPase enzyme.
• 🧬 Actin is depicted as a filament that myosin 'crawls' along, which is essential for creating mechanical energy.
• 🤝 The myosin head's initial interaction with ATP leads to its detachment from the actin filament.
• 💥 ATP hydrolysis releases energy that changes the conformation of the myosin protein, preparing it for the power stroke.
• 🔄 The release of the phosphate group from myosin triggers the power stroke, pushing the myosin against the actin and creating motion.
• 🔄 The power stroke is the mechanical movement phase where chemical energy from ATP is converted into mechanical energy.
• 🔄 After the power stroke, ADP is released, and the myosin head returns to its original position, ready for the next cycle.
• 🔄 The process is cyclical, with myosin moving one 'rung' further along the actin filament with each cycle.
• 🤔 The script emphasizes the fascination with the conversion of chemical energy into mechanical energy within biological systems.

Q & A

What is the main topic of the video?

The main topic of the video is to understand how two proteins, myosin and actin, interact with ATP to produce mechanical motion, which is fundamental to muscle function.

Why is the video focusing on myosin and actin proteins?

The video focuses on myosin and actin because they are the key proteins involved in muscle contraction and the conversion of chemical energy from ATP into mechanical energy.

What is myosin and why is it significant in muscle contraction?

Myosin is a motor protein that interacts with actin filaments. It is significant because it uses energy from ATP hydrolysis to 'crawl' along actin, creating the mechanical motion necessary for muscle contraction.

What is the role of ATP in the interaction between myosin and actin?

ATP binds to myosin, causing it to release from actin. After ATP hydrolysis, the energy released changes the conformation of myosin, allowing it to attach to actin and perform a power stroke, which results in muscle movement.

What is the function of the myosin head in the process described?

The myosin head is the part of the myosin protein that binds to ATP and interacts with the actin filament. It undergoes a conformational change upon ATP hydrolysis, enabling it to perform the power stroke that moves the actin filament.

What is the significance of ATP hydrolysis in muscle contraction?

ATP hydrolysis is crucial as it provides the energy required to change the shape of the myosin protein, enabling it to detach from actin, move into a high-energy state, and then perform the power stroke that generates motion.

What is the term used to describe the conformational change in myosin after ATP hydrolysis?

The conformational change in myosin after ATP hydrolysis is often referred to as 'cocking' the myosin protein into a high-energy state, preparing it for the power stroke.

What is the power stroke in the context of muscle contraction?

The power stroke is the movement of the myosin head after the release of the phosphate group from ATP, which uses the stored energy to push against the actin filament, resulting in muscle contraction.

How does the release of the phosphate group from myosin lead to muscle movement?

The release of the phosphate group from myosin triggers the power stroke, causing the myosin head to move and pull the actin filament, which results in the sliding of filaments and muscle contraction.

What happens after the power stroke in the cycle of muscle contraction?

After the power stroke, ADP is released from the myosin head, and the cycle resets with a new ATP molecule binding to the myosin, allowing for another cycle of contraction to occur.

What is the process by which chemical energy from ATP is converted into mechanical energy in muscles?

The process involves the binding and hydrolysis of ATP by myosin, which leads to conformational changes in the myosin protein, enabling it to perform a power stroke against the actin filament, thus converting chemical energy into mechanical motion.

Outlines

00:00

🔬 Understanding Protein Interaction and ATP in Muscle Motion

The video script begins with an exploration of how two proteins, myosin and actin, interact with ATP to generate mechanical motion. The narrator explains that this process is not only confined to muscle cells but is also the foundation for understanding muscle function. The script introduces images of myosin II, a complex enzyme that acts as an ATPase, converting ATP to ADP and phosphate groups. The actin is described as a filament that myosin 'crawls' along, powered by ATP, to create mechanical energy. The process is illustrated through a step-by-step explanation of how ATP binding and release cause myosin to detach from and interact with actin, highlighting the biochemical mechanism behind muscle movement.

05:02

🚀 The Power Stroke: ATP Hydrolysis and Mechanical Work

Continuing from the previous explanation, this paragraph delves into the specifics of the ATP hydrolysis process and its role in the power stroke of muscle contraction. The narrator describes how ATP binds to the myosin head, leading to its detachment from actin, and then hydrolyzes into ADP and a phosphate group, which energizes myosin into a 'cocked' position. The subsequent release of the phosphate group triggers the power stroke, where myosin pushes against the actin filament, generating motion. This step is likened to the power stroke in an engine, emphasizing the conversion of chemical energy stored in ATP bonds into mechanical energy. The paragraph concludes with the release of ADP, resetting the myosin for another cycle of contraction, thereby illustrating the fundamental mechanism of muscle action.

Keywords

💡Proteins

Proteins are large molecules composed of amino acids that perform a vast array of functions within organisms, including catalyzing metabolic reactions, replicating DNA, responding to stimuli, and transporting molecules from one location to another. In the context of this video, proteins, specifically myosin and actin, are central to the process of muscle contraction. The script describes how myosin, a motor protein, interacts with actin to produce mechanical motion.

💡ATP

ATP, or adenosine triphosphate, is the primary energy currency of the cell, used to power a wide range of metabolic processes. It stores energy in the form of a high-energy phosphate bond. In the video script, ATP is depicted as a key molecule that binds to the myosin head, which then leads to a series of reactions that result in muscle movement.

💡Myosin

Myosin is a motor protein that plays a crucial role in muscle contraction. It has a 'head' and a 'tail' structure, with the head binding to actin and the tail providing the force for movement. The script explains that myosin is an ATPase enzyme, which means it can hydrolyze ATP into ADP and phosphate, releasing energy for muscle contraction.

💡Actin

Actin is a globular protein that forms microfilaments, providing structural support for cells and playing a key role in cell movement and muscle contraction. In the script, actin is described as the 'rope' along which myosin 'crawls' to generate mechanical energy.

💡ATPase

An ATPase is an enzyme that catalyzes the hydrolysis of ATP into ADP and a free phosphate, releasing energy in the process. Myosin, as mentioned in the script, is a type of ATPase enzyme, which is essential for the conversion of chemical energy stored in ATP into mechanical work during muscle contraction.

💡Mechanical motion

Mechanical motion refers to the movement of an object in space, typically involving a change in position or orientation. In the context of the video, mechanical motion is the result of the interaction between myosin and actin proteins, powered by the energy released from ATP hydrolysis.

💡Hydrolysis

Hydrolysis is a chemical reaction in which a molecule of water breaks a bond in another molecule, resulting in the formation of new molecules. In the script, ATP hydrolysis is the process where ATP is broken down into ADP and a phosphate group, which is a crucial step in the myosin's ability to perform work.

💡Conformation

Conformation refers to the specific three-dimensional shape that a molecule, such as a protein, assumes. The script explains that the conformation of myosin changes during the process of ATP hydrolysis, which is essential for its ability to interact with actin and generate force.

💡Power stroke

The power stroke is the step in the myosin cycle where the energy stored in the myosin head is released, causing it to move or 'push' along the actin filament. The script describes this as the step where the phosphate group is released from myosin, resulting in the mechanical movement of the muscle.

💡ADP

ADP, or adenosine diphosphate, is a molecule formed when ATP is hydrolyzed. It is a product of the energy-releasing reaction that powers many cellular processes, including muscle contraction. In the script, ADP remains bound to myosin after the hydrolysis of ATP, until the subsequent release of the phosphate group.

💡Muscle contraction

Muscle contraction is the process by which muscle tissue shortens or stiffens to produce movement or maintain posture. The script details the biochemical process involving myosin and actin that leads to muscle contraction, emphasizing the conversion of chemical energy from ATP into mechanical energy.

Highlights

Exploring the interaction between two proteins and ATP to produce mechanical motion.

Introduction to myosin II, a complex protein with two intertwined strands.

Myosin's role as an ATPase enzyme, catalyzing the reaction of ATP to ADP and phosphate groups.

Actin's depiction as an 'actin rope' for myosin to crawl along, generating mechanical energy.

Illustration of the myosin head's initial position and its interaction with ATP.

ATP binding causing myosin to release from actin, initiating the motion process.

ATP hydrolysis and the release of energy to prepare myosin for the power stroke.

The transformation of myosin's conformation due to ATP hydrolysis, setting it in a high-energy state.

The release of the phosphate group from myosin, triggering the power stroke and mechanical movement.

The conversion of chemical energy in ATP to mechanical energy through protein interactions.

The importance of protein shape changes in the energy release and muscle contraction process.

The four-step process of myosin-actin interaction, from ATP binding to ADP release.

The significance of the power stroke in muscle action and the generation of mechanical energy.

The myosin's return to its original position after the power stroke, ready for the next cycle.

The marvel of ATP's role in converting bond energy to mechanical energy in everyday biological functions.

Building a foundation for understanding muscle function through the actin-myosin interaction.

Transcripts

What I want to do in this video is try to understand how

two proteins can interact with each other in conjunction with

ATP to actually produce mechanical motion.

And the reason why I want to do this-- one, it occurs

outside of muscle cells as well, but this is really going

to be the first video on really how muscles work.

And then we'll talk about how nerves actually stimulate

muscles to work.

So it'll all build up from this video.

So what I've done here is I've copy and pasted two images of

proteins from Wikipedia.

This is myosin.

It's actually myosin II because you actually have two

strands of the myosin protein.

They're interwound around each other so you can see it's this

very complex looking protein or enzyme, however you want to

talk about it.

I'll tell you why it's called an enzyme-- because it

actually helps react ATP into ADP and phosphate groups.

So that's why it's called an ATPase.

It's a subclass of the ATPase enzymes.

This right here is actin.

What we're going to see in this video is how myosin

essentially uses the ATP to essentially crawl along.

You can almost view it as an actin rope and that's what

creates mechanical energy.

So let me draw it.

I'll actually draw it on this actin right here.

So let's say we have one of these myosin heads.

So when I say a myosin head, this is one of the myosin

heads right here and then it's connected, it's interwound,

it's woven around.

This is the other one and it winds around that way.

Now let's just say we're just dealing with one

of the myosin heads.

Let's say it's in this position.

Let me see how well I can draw it.

Let's say it starts off in a position that looks like that

and then this is kind of the tail part that connects to

some other structural and we'll talk about that in more

detail, but this is my myosin head right there in its

starting position, not doing anything.

Now, ATP can come along and bond to this myosin head, this

enzyme, this protein, this ATPase enzyme.

So let me draw some ATP.

So ATP comes along and bonds to this guy right here.

Let's say that's the-- and it's not going to be this big

relative to the protein, but this is just to

give you the idea.

So soon as the ATP binds to its appropriate site on this

enzyme or protein, the enzyme, it detaches from the actin.

So let me write this down.

So one, ATP binds to myosin head and as soon as that

happens, that causes the myosin to release actin.

So that's step one.

So I start it off with this guy just touching the actin,

the ATP comes, and it gets released.

So in the next step-- so after that step, it's going to look

something like this-- and I want to draw

it in the same place.

After the next step, it's going to look

something like this.

It will have released.

So now it looks something like that and you have the ATP

attached to it still.

I know it might be a little bit convoluted when I keep

writing over the same thing, but you have the

ATP attached to it.

Now the next step-- the ATP hydrolizes, the phosphate gets

pulled off of it.

This is an ATPase enzyme.

That's what it does.

Let me write that down.

And what that does, that releases the energy to cock

this myosin protein into kind of a high energy state.

So let me do step two.

This thing-- it gets hydrolized.

It releases energy.

We know that ATP is the energy currency of biological

systems. So it releases energy.

I'm drawing it as a little spark or explosion, but you

can really imagine it's changing the conformation of--

it kind of spring-loads this protein right here to go into

a state so it's ready to crawl along the myosin.

So in step two-- plus energy, energy and then this-- you can

say it cocks the myosin protein or

enzyme to high energy.

You can imagine it winds the spring, or loads the spring.

And conformation for proteins just mean shape.

So step two-- what happens is the phosphate group gets--

they're still attached, but it gets detached from

the rest of the ATP.

So that becomes ADP and that energy changes the

conformation so that this protein now goes into a

position that looks like this.

So this is where we end up at the end of step two.

Let me make sure I do it right.

So at the end of step two, it might look

something like this.

So the end of step two, the protein looks

something like this.

This is in its cocked position.

It has a lot of energy right now.

It's wound up in this position.

You still have your ADP.

You still have your-- that's your adenosine and let's say

you have your two phosphate groups on the ADP and you

still have one phosphate group right there.

Now, when that phosphate group releases-- so let me write

this as step three.

Remember, when we started, we were just sitting here.

The ATP binds on step one-- actually, it does definitely

bind, at the end of step one, that causes the myosin protein

to get released.

Then after step one, we naturally have step two.

The ATP hydrolyzes into ADP phosphate.

That releases energy and that allows the myosin protein to

get cocked into this high energy position and kind of

attach, you can think of it, to the next rung

of our actin filament.

Now we're in a high energy state.

In step three, the phosphate releases.

The phosphate is released from myosin in step three.

That's step three right there.

That's a phosphate group being released.

And what this does is, this releases that energy of that

cocked position and it causes this myosin protein

to push on the actin.

This is the power stroke, if you imagine in an engine.

This is what's causing the mechanical movement.

So when the phosphate group is actually released-- remember,

the original release is when you take

ATP to ADP in a phosphate.

That put it in this spring-loaded position.

When the phosphate releases it, this releases the spring.

And what that does is it pushes on the actin filament.

So you could view this as the power stroke.

We're actually creating mechanical energy.

So depending on which one you want to view as fixed-- if you

view the actin as fixed, whatever myosin is attached to

it would move to the left.

If you imagine the myosin being fixed, the actin and

whatever it's attached to would move to the right,

either way.

But this is where we fundamentally

get the muscle action.

And then step four-- you have the ADP released.

And then we're exactly where we were before we did step

one, except we're just one rung further to the left on

the actin molecule.

So to me, this is pretty amazing.

We actually are seeing how ATP energy can be used to-- we're

going from chemical energy or bond energy in ATP to

mechanical energy.

For me, that's amazing because when I first learned about

ATP-- people say, you use ATP to do everything in your cells

and contract muscles.

Well, gee, how do you go from bond energy to actually

contracting things, to actually doing what we see in

our everyday world as mechanical energy?

And this is really where it all occurs.

This is really the core issue that's going on here.

And you have to say, well, gee, how this thing change

shape and all that?

And you have to remember, these proteins, based on

what's bonded to it and what's not bonded to

it, they change shape.

And some of those shapes take more energy to attain, and

then if you do the right things, that energy can be

released and then it can push another protein.

But I find this just fascinating.

And now we can build up from this actin and myosin

interactions to understand how muscles actually work.