How tropomyosin and troponin regulate muscle contraction | NCLEX-RN | Khan Academy

How tropomyosin and troponin regulate muscle contraction | NCLEX-RN | Khan Academy

In the last video, we learned
how myosin– and myosin II in particular– when we say myosin
II it actually has two of these myosin heads and their
tails are inter-wound with each other– how myosin
II can use ATP to essentially– you can almost
imagine either pulling an actin filament or walking
up an actin filament. It starts attached. ATP comes and bonds onto it. That causes it to be released. Then the ATP hydrolyzes into
ADP and a phosphate group. And when that happens, that
energy’s released. It puts this into a higher
energy state. It kind of spring-loads the
protein and then it attaches up another notch on the actual
actin filament and then the phosphate group leaves and
that’s where the confirmation change in this protein
is enough. It generates the power stroke to
actually push on the actin filament– and you could
imagine, either move the myosin– whatever the myosin is
connected to– to the left or whatever the actin is
connected to to the right. We’re going to talk a lot more
about what they’re connected to in future videos. Now, a couple of questions
might have been raising in your head. This guy had so much effort to
pull on this thing, right? There’s some tension pulling in
the other direction, right? I said this is what happens in
muscles, so there must be some weight or some other
resistance. So what happens when
this releases? At the first step when ATP
joined and this released, wouldn’t the actin filament
just go back to where it was before? Especially if there’s
some tension on it going in that direction. And the simple answer to that
is, this isn’t the only myosin protein that’s acting
on this actin. You have others all
along the chain. Maybe you have one
right there. Maybe you have one
right there. They’re all working at their own
pace at different times. So you have so many of these
that when one of them is disengaged, another one of them
might be in their power stroke or another one
might be engaged. So it’s not like you have this
notion of, if all of a sudden one lets go, that the actin
filament will recoil back to where it was. Now the next question that you
might be thinking is, how do I turn on and off this
situation? We have command over
our muscles. What can turn on or off this
system of the myosin essentially crawling
up the actin? And to understand that, there’s
two other proteins that come into effect. That’s tropomyosin
and troponin. And so I’m going to redraw the
actin– I’ll do a very rough drawing of the actin filament. Let’s say that that’s my actin
filament right there with its little grooves. It’s actually a helical
structure. And actually, these grooves–
it’s kind of a helical– but we won’t worry too
much about that. What we drew so far, at least
in the last video, you had these little myosin. You can view them as feet or
head or whatever that keep attaching to it and then based
on where they are in that ATP cycle, they can keep getting
cranked back up or spring-loaded and go to the
next one and push back. Now, on top of this actin,
you actually have this tropomyosin protein. And this tropomyosin protein,
it coils around the actin. So this is our actin
right here. This is one of the two heads
of the myosin II. And then we have our
tropomyosin. Tropomyosin is coiled around. It’s a very rough sketch, but
you can imagine it’s coiled around and it goes back behind
it, then it goes like that, and then it goes back behind
it, then it goes like that. So it’s coiled around it and the
important thing about it is, if there’s– let me
take a step back. It’s coiled around and it’s
attached to the actin by another protein called
troponin. Let’s say it’s attached there
and– this isn’t exact, but let’s say it’s attached there,
and there, and there, and there, and there by
the troponin. So let me write this down. So you can imagine, the troponin
is kind of like the nails into the actin. So it dictates where
the tropomyosin is. So when a muscle is not
contracting, it turns out that the tropomyosin is blocking
the myosin from being able to– and I’ve read a bunch of
accounts on this and I think this is still an area
of research. It’s not 100% clear one
way or the other. Tropomyosin is– or maybe both–
blocking the myosin from being able to attach to
the actin where it normally attaches so it won’t be able
to crawl up the actin– or sometimes the myosin is attached
to the actin, but it keeps it from releasing and
sliding up the actin to keep that walking procedure. So the bottom line is that
this tropomyosin kind of blocks the myosin head– this
is the myosin head right there– from crawling up the
actin, either by physically blocking its actual binding site
or if it’s already bound, keeping it from being able to
keep sliding up the actin. Either way, it’s blocking it
and the only way to make it unblocked is for the troponins
to actually change their confirmation, for them to
change their shape. And the only way for them to
change their shape is if we have a high calcium
ion concentration. So if you have a bunch of
calcium ions, if you have a high enough concentration, these
calcium ions are going to bond to the troponin and
then that changes the confirmation of the troponin
enough to move the configuration of the
tropomyosin. So let me write this down. So normally, tropomyosin blocks,
but then when you have a high calcium ion
concentration, they bind to troponin and then the troponin,
they change their confirmation so it moves the
tropomyosin out of the way. So when it moves out of the way,
you have a high calcium concentration, bonds troponin,
moves tropomyosin out of the way, then all of a sudden what
we talked about in the last video– these guys can start
walking up the actin or pushing the actin to the
right, however you want to view it. But then if the calcium
concentration goes low, then the calciums get released
from the troponin. You need to have enough to
always hang around here. If the concentration becomes
really low here, these guys will start to leave. So then the
troponin goes back to, I guess, standard confirmation. That makes the tropomyosin
block the myosin again. So it’s actually– I
mean, I can’t say anything here is simple. This was only discovered maybe
50 or 60 years ago and you can imagine to actually observe
these things or to create experiments to definitively
know what’s happening– nothing is simple, but
the idea is simple. Without calcium, the tropomyosin
is blocking the ability of the myosin to attach
where it needs to attach or slide up the actin so
it can keep pushing on it. But if the calcium concentration
is high enough, they will bond to the troponin–
which essentially nails down the tropomyosin
that’s wound around the actin and when they change their
confirmation with the calcium ions, it moves the tropomyosin
out of the way so that the myosin can do what it does. So you can imagine already,
we’re building up a way for– one, for muscles to contract,
but even better, for us to control muscles to contract. So if we have a high calcium
concentration within the cell, the muscle will contract. If we have a low calcium
concentration again, then all of a sudden, these
will release. They’ll be blocked, and then the
muscle will relax again.

85 Replies to “How tropomyosin and troponin regulate muscle contraction | NCLEX-RN | Khan Academy”

  1. i am pretty sure this essay last year got me the 4 on the test. i wrote everything i knew , it covered the 3 pages provided. this will help, learn it well people.

  2. thanks alot for this video, now i understand this 100% ! 5/5
    exam next week from 80 muscles , insertio and origo, action and all that yay -.-

  3. The troponin has three subunits, Troponin T, Troponin I, and Troponin C. Troponin T binds to tropomyosin as well as to the other two troponin components. Troponin I inhibits the
    F-actin-myosin interaction and also binds to the other
    components of troponin. Troponin C is a calcium-
    binding polypeptide that is structurally and functionally
    analogous to calmodulin, an important calcium-
    binding protein widely distributed in nature. (Harper's Illustrated Biochemistry, 26th edition, 2003)

  4. khan academy i want to make love to u so bad, i have an exam tomorrow and i knew nothing about this, the teacher explained it to me but it just went out my head, i can only learn from videos which make stuff interesting so thank u so fricken much

  5. YEAP I just had a simple slide to study on and on thursday I'll have an exam on this. In this depressed moment, I prayed and god let me find your videos to me. Thank you!

  6. my professor is retarded. Glad I paid 1200 bucks to listen to that idiot babble. Sal, come to my university, so you can have my 1200 dollars.

  7. The troponin-i subunit binds to actin (the "i" stands for inhibitory). The -t subunit binds to tropomyosin. The -c subunit binds to Ca2+.

    Tropomyosin will block the active sites of actin (where myosin binds to actin) until troponin undergoes a conformational change due to high [Ca2+].

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  12. I LOVE your videos. You always put it in clear perspective for me! I've been beasting anatomy because of you. Haha
    Keep doing what you do!

  13. that was a question of mine in the previous video…if tropomyosin blocks the myosin how can the myosin release the actin when it binds ATP…it has nothing to release :-??

  14. Wow. Good clip.

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  15. Why am I wasting thousands of dollars on a college degree when I can get better instruction here! I would like a BS from KhanAcademy please

  16. No mention of how many Calcium ions bind to each molecule of troponin
    I believe it is 4 but clarification is much needed.

  17. Molecular machines are very cool!

    My favorite is ATP synthase. But that's because I think proton driven motors fueled by the redox reactions in the electron transport chain and chemiosmosis in general is such a nice conversation piece for picking up chicks. lol

    Not really but hey, feel free to use the concept. lol

  18. Thank you so much Mr Khan. You explain in a way that not only shows us how the problem is solved, but you also show us where it originated from, why we need it and how we go about understanding the whole mechanism behind the problem, hence you cover all angles of answering the question: something we pay thousands for lecturers to do but after 2 weeks we are still clueless, yet you did it in 9 minutes without requesting a cent.

  19. Thank you very much but I have one qustion if the amount of calcum determinante if the myosin moves on the actin why then it need's ATP to move ?

  20. Your video is so much better than how much i have paid for university lectures $$$$. It really helped me get through my exam!!!!

  21. It's amazing how much easier these videos make the topic seem. Lecture notes are nothing compared to this. Great video!

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