Now that we know about the beta strand, we can talk about beta sheets. So beta sheets just consist of 2 or more beta strands arranged side by side. And beta sheets are also known as beta pleated sheets. And if you Google pleated anything, pleated cups, pleated pants, pleated shorts, whatever, pleated just means zigzag. And if we take a quick look at our example down below of our beta sheets over here, what you'll see is that we've got this zigzag type of structure to the beta sheets. Notice that the r groups, which are in yellow here, the r groups which are in yellow, these yellow balls, they go down, then up, down, then up, down, then up, and they create this zigzag type of structure. And that's really what the pleated sheets are referring to, the pleated is referring to. And so, these r groups that go down, up and down, up and down, they're actually perpendicular to the beta sheets themselves. So they're perpendicular to the plane of the beta sheets. So notice they're they're going straight up and straight down, up above and down below the beta sheet plane. Whereas, the hydrogen bonds, remember, they're perpendicular, but they're perpendicular to the directions of the beta strands. And so, if we were to consider the directions, remember that the, beta strands can be portrayed as arrows, and the arrows always point towards the c terminal end. And so if this were an arrow here, it would be pointing this direction like this. This one would be pointing in the opposite direction. This one would be pointing this way, and then this one would be pointing this way like that. And so, what you can see is that the hydrogen bond, which are in red here, that they are going perpendicular to the directions of these, beta strands. So it's something important to keep in mind. So unlike alpha helices, something that's also important to keep in mind is that the hydrogen bonds in the beta sheets can actually form between either separate protein chains, which would mean they are inter chain hydrogen bonds, or they could form between the same chain, and that would mean that they are intra chain hydrogen bonds. And we'll be able to distinguish that in our example down below. Now, typically, beta sheets have only between 25 beta strands, but it's possible for them to have up to as many as 10 or even more beta strands arranged side by side. So let's take a look at our example where we can distinguish between inter and intra chain pleated sheets, beta pleated sheets. So, again, over here on our left side, beta sheets are also known as beta pleated sheets, and that's because of their zigzag structure, which is, seen here with the r groups. Again, they go down and up, down and up, down and up. So that's what it means by the pleated part. Now, over here in this image, what you'll notice is that the, 2 beta strands that are being shown here with these arrows, so this arrow here and this arrow over here, notice that they are not connected. So this one curves this way and the other one curves this way, and these 2 are separate, polypeptide chains. These chains are not connected. And so because these are separate protein chains, what this means is that this is an inter this is an interchain, so an interchain beta sheet. And so interchain beta sheets just means that the hydrogen bonds that form between the beta strands are on opposite polypeptide chains. And so we've got this polypeptide chain up here and this one down below. And the hydrogen bonds, again, are here, in these dotted black lines that, form between the separate polypeptide chains. Now Now over here, what you'll notice is that we've got one continuous polypeptide chain that has, 3 beta strands. So here's 1 beta strand, and then it's connected. So this is a continuous one polypeptide chain. Here's our second beta strand, and then it's connected. And then here's our 3rd beta strand. And so these beta strands, because they're all part of the same polypeptide chain, this is an intra chain beta sheet. Intra chain. And so, intra chain just means that it's all part of a single one polypeptide chain. And, this is something that's incredibly different than alpha helices because alpha helices cannot form inter chained hydrogen bonds, and that's where you have separate polypeptide chains. So, because beta sheets can form both interchain beta sheets and intra chain beta sheets, that makes them uniquely different than the alpha helices, which can only form intra chain hydrogen bonds. So that concludes this, lesson here on beta sheets. We'll get a little bit of practice and then we'll talk about beta sheet bond angles in a Ramachandran plot. So I'll see you guys in that practice video.
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Problem
Problem
Which of the following is true about interchain β-sheets?
A
Only have two β-strands.
B
Backbone H-bonding between same chain β-strands.
C
Backbone H-bonding between separate chain β-strands.
D
R-group H-bonding between separate chain β-strands.
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concept
Beta Sheet
Video duration:
1m
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Video transcript
So as you guys already know, beta sheets require a very particular set of phi and psi bond angles, And it turns out that when you plot these beta sheet phi and psi bond angles on a Ramachandran plot, that they actually fall into the upper left quadrant of the Ramachandran plot. And so if we take a look at our example down below, what you'll see is that we've got this Ramachandran plot on the left over here and notice that it's broken up into these quadrants. And again, on the x axis, we have the phi bond angles and on the y axis on the side of the Ramachandran plot, we have the psi angles. And so again, the beta sheet phi and psi, bond angles and the beta strand phi and psi bond angles fall into the upper left quadrant right up here. And so again, because the beta strands fall into the upper left, that means that the beta sheets also fall into the upper left quadrant. So taking a look at this Ramachandran plot over here, again, if we break it up into quadrants, what you'll see is that the beta sheets, again, are falling into the upper left quadrant. And so, notice that we have actually 2 different types of beta sheets that we need to distinguish between, and those are anti parallel beta sheets and parallel beta sheets. And notice that they have slightly different locations that they fall in within this upper left hand quadrant. So if you analyze a Ram Machandran plot close enough, you can actually distinguish between these different types of beta sheets. And again, you can also distinguish between, all types of secondary structures. And so, we'll talk a lot more about the differences between anti parallel beta sheets and parallel beta sheets in our next couple of videos. So, before we get there, we're actually gonna get some practice with beta sheet bond angles. Angles, so I'll see you guys in that practice video.
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Problem
Problem
Which set of φ and ψ bond angles is best for β-sheet secondary structure?