Antiparallel and Parallel Beta Sheets - Video Tutorials & Practice Problems
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Antiparallel And Parallel Beta Sheets
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Video transcript
In this video, we're gonna begin distinguishing between antiparallel and parallel beta sheets. Now you might be wondering why talk about anti parallel beta sheets before we talk about parallel beta sheets? Well, it turns out that the anti parallel beta sheets are actually easier than the parallel beta sheets. So we'll talk about the easy stuff first, and then we'll talk about the harder stuff later. And so with the anti parallel beta sheets, it it turns out that the beta strands are going to be aligned in opposite directions in terms of their n and c terminal ends. And the rise per residue for an anti parallel beta sheet is, to no surprise, 3.5 angstroms since we said in our previous lesson videos that beta sheets have a rise per residue of 3.5 angstroms. And so down below in our example, it's pretty clear to see that this image on the left here is an anti parallel beta sheet. And so we can see that we have a single polypeptide chain that's all connected to one another, and these arrows here represent beta strands. And the beta strands that are adjacent to one another, right next to another, are pointing in opposite directions. And so, when they're pointing in opposite directions, in terms of their n and c terminal ends, that identifies them as being anti parallel beta sheets. And so, we know that the arrows are pointing towards their c terminal end. And because this is the beginning of the chain, this is the end terminal end, and the arrows pointing towards the c terminal end are gonna be, makes this the c terminal end over here. And again, because this is one continuous polypeptide chain, that makes this an intra chain anti parallel beta sheet. And so moving on to the parallel beta sheets, in contrast, the beta strands are actually aligned in the same direction in terms of their n and c terminal ends instead of the opposite direction. And the rise per residue for a parallel beta sheet is actually a little bit more condensed. It's 3.2 angstroms. Angstroms. And so, even though 3.2 angstroms is a little bit more condensed, it's still, in comparison to an alpha helix, a lot more extended. Because an alpha helix has a rise per residue of 1.5 angstroms. So that's really really condensed. 3.2 angstroms is all very extended in comparison to the alpha helix. It's just that this anti parallel beta sheet is a little bit more extended than the parallel beta sheet. So, something interesting to keep in mind. And so down below on the right, we have an image of the parallel beta sheet and we can tell because the beta strands are all pointing in the same direction in terms of their n and c terminal ends. And so, this is one continuous polypeptide chain here, so we know that this is an intra chain parallel beta sheet. And this is the back of the arrow over here, so this will be the end terminal end. And this is the, what the arrows are pointing to, so this is the c terminal end. And so this concludes our initial lesson on the differences between antiparallel and parallel beta sheets. And in our next lesson video, we're gonna be able to talk even more differences between these 2, specifically the hydrogen bonding differences between the 2. But before we get there, let's get some practice. So I'll see you guys in that practice video.
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Problem
Problem
Silk fibroin contains predominantly β sheet conformation. Which of the following is true regarding its β sheets?
A
Its antiparallel β sheets are oriented in the same direction.
B
Its parallel β sheets are oriented in opposite directions.
C
Its antiparallel β sheets are slightly more extended.
D
Its parallel β sheets are slightly more extended.
3
concept
Antiparallel And Parallel Beta Sheets
Video duration:
5m
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Video transcript
So now that we understand the fundamental differences between anti parallel and parallel beta sheets, let's talk about the differences between hydrogen bonding of the anti parallel and parallel beta sheets. And so, what's important to note is that both anti parallel and parallel beta sheets are stabilized by hydrogen bonds that form between the carbonyl group and the amino groups on the backbones of adjacent beta strands. And so again, what this means is that the r groups are not involved in stabilizing the beta sheet and so that's something that's very important and this is also true for alpha helices. So this is something that's true for really secondary structures. Our groups are not involved. It's all about the backbone. And so, really, this is how, beta sheet hydrogen bonding works. So each amino acid residue in both anti parallel and parallel beta sheets only forms 2 hydrogen bonds, but the hydrogen bonding is still slightly different. And so here's how it works. With anti parallel beta sheets, they actually have stronger bonds that are actually pretty much perfectly perpendicular. So they have stronger hydrogen bonds that are pretty much perfectly perpendicular, and because because they're stronger hydrogen bonds, those bonds are going to be more stable. And because they're more stable hydrogen bonds, anti parallel beta sheets are going to be more prevalent and more common in proteins found in nature than parallel beta sheets, but that's not to say that parallel beta sheets are never found because they are. And so with parallel beta sheets, they actually have weaker hydrogen bonds and their hydrogen bonds are actually slightly distorted. So what that means is that they are not perfectly perpendicular. They are not perfectly perpendicular. And again, because parallel beta sheets have weaker distorted hydrogen bonds that are not perfectly perpendicular, they're going to be less stable and because they're less stable, they're gonna be less commonly found in proteins found in nature. So that's something important to keep in mind. So in our example below, we're gonna distinguish between anti parallel and parallel beta sheet hydrogen bonding or H bonding. And so, notice on the left here, what we have is our anti parallel beta sheets, so these are anti parallel beta sheets, and then on the right, what we have are our parallel beta sheet. And so, what you'll see is that the anti parallel beta sheet, remember, it is slightly more extended because it has a rise per residue of 3 point 5 angstroms, so it's a little bit more extended. Whereas, the parallel beta sheet only has a rise per residue of just, 3.2 angstroms, so it's slightly less extended. So, really what you'll notice is, both residues, again, and both anti parallel and parallel beta sheets are gonna form 2 hydrogen bonds. So there's nothing different about the amount of hydrogen bonds, still the same amount, but what you'll notice is that there's gonna be some slight differences. So, for antiparallel beta sheets, both of the hydrogen bonds on a residue actually link to just one residue on the other strand. And so if we were to highlight a single residue, so let's highlight this residue which is boxed in this dotted line here, notice that the 2 hydrogen bonds of this single residue right here, the 2 hydrogen bonds are right down here in these, highlighted in red down below with these dotted lines, so those 2 hydrogen bonds only connect to 1 single residue on the opposite strand here. And so, really, it's just the hydrogen bonds link from one residue to just one residue on the other strand and that's the way anti parallel beta sheets work, so it's a little bit easier to think about a little bit, more straightforward. Now, with parallel beta sheets on the other hand, notice that we have one residue over here and, dotted like this. And again, it still has 2 hydrogen bonds, but both hydrogen bonds of a residue linked to 2 different residues, so they linked to 2 residues on the other strand. And so notice that we've got 2 hydrogen bonds, 1 over here and one over here, and they're slightly distorted. They're not perfectly perpendicular like they were over here. They're perfectly perpendicular to the direction of the beta strand, but these are slightly distorted. Notice that they're going off at slight different angles. And so also notice that they're connecting to 2 different, residues on the opposite strand. So it's connecting to one residue over here with this hydrogen bond, and then the other hydrogen bond, which is over here is connecting to another residue, which is this one over here. And so, that is just both hydrogen bonds of a residue connect to 1 residue on just, both hydrogen bonds of a residue connect to one residue on the other strand, which is what allows for the hydrogen bonds to be stronger, perpendicular, and, that's why antiparallel beta sheets again are more commonly found and more stable. So that concludes our lesson here on the differences between anti parallel and parallel beta sheet hydrogen bonding, and in our next couple of videos, we'll be able to get some practice. So I'll see you guys in those practice videos.
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Problem
Problem
The diagram illustrates:
A
The polypeptide chain of an alpha helix.
B
Two polypeptides of a β-sheet running in a parallel fashion.
C
Two polypeptides of a β-sheet running in an antiparallel fashion.
D
Two polypeptides of a coiled coil.
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Problem
Problem
The major reason that antiparallel β-sheets are more stable than parallel β-sheets is that the latter:
A
Are in a slightly less extended configuration than antiparallel strands.
B
Do not have as many disulfide crosslinks between adjacent strands.
C
Do not stack in sheets as well as antiparallel strands.
D
Have fewer lateral hydrogen bonds than antiparallel strands.
E
Have weaker hydrogen bonds laterally between adjacent strands.
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Problem
Problem
Which (phi, psi) pair of bond angles is closest to those of the residues shown in the figure below?
A
(-90, -90).
B
(-90, 90).
C
(90, -90).
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Problem
Problem
What type of β-sheet is presented in the figure below? Draw all hydrogen bonds between appropriate groups.
A
Antiparallel β-sheet.
B
Parallel β-sheet.
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Problem
Problem
Draw a two-stranded antiparallel β-sheet with appropriate hydrogen bonding between the following peptides: