So now that we've talked about alpha helices, we can move on and talk about our next type of secondary structure, the beta strand. So the beta strand, again, is a type of secondary structure and this is a structure where the protein backbone takes an extended zigzag conformation that is periodic and repeats. And so, by zigzag, what you'll see is, if you take a quick look at our example down below, that the R groups are zigzagging. And so when you look at the r groups which are in green, they go from being down to going up to going down to going up to going down, and so what you'll see is that the r groups are literally zigzagging, and that's what we mean by the zigzag confirmation. And so the extended periodic zigzag confirmation or structure repeats as the extended periodic zigzag conformation or structure repeats every 2 amino acid residues. And so, the rise, so if we take a look at the rise and remember that the rise is just the length or the distance covered per amino acid residue, it's actually about about 3.5 angstroms and if we compare this to the rise of an alpha helix, that's actually more than double because the rise of an alpha helix is just 1.5 angstroms. So, basically, what that's saying is that the beta strand is much more extended. It's more extended than the alpha helix, which is much more coiled together. And so the pitch of a beta strand is going to be 7 angstroms. And again, that's much longer than the pitch of a alpha helix which is just 5.4 0.4 angstroms. So again, all that's saying is that the beta strand is more extended than the alpha helix which is more coiled. So in our example below, what we're gonna do is compare the rise, pitch, and length of 5 amino acid residues in a beta strand conformation and then 5 amino acid residues in an alpha helix conformation. So, in our example down below, what we have is the beta strand on the left over here and the alpha helix on the right. And so on the far left over here, what we have is our key, the green represent the r groups, the red balls represent oxygens, blue are the nitrogens, black are carbons, and white are hydrogens. And so looking at the beta strand and comparing it to the alpha helix, again, they both have exactly 5 amino acid residues. And one of the first things that you'll notice is that the beta strand conformation on the left over here is much much more extended. It's just a beta strand is literally just much more extended. It's just way longer than the alpha helix which is much coiled together. And so the backbone for our beta strand is literally just much more extended, whereas with the alpha helix, it's much much more coiled together. And so, if you look at the rise, so remember that the rise again is just that length or distance per amino acid residue, the rise for a beta strand is 3.5 angstroms, whereas the rise for an alpha helix is just 1.5 angstroms. So notice that the distance here, the distance of the arrow, this arrow here and this arrow over here, it's just much, much more extended. One single amino acid residue stretches out much much further and extends much more in the beta strand conformation. Now, looking at the pitch, remember the pitch is when, for an alpha helix is when you have one turn of the alpha helix backbone. And so here, what we have is the pitch is referring to this blue line here and this blue line over here, and that's exactly where there's one turn of the backbone. And so, that pitch comes out to 5.4 angstroms in the alpha helix. Now, for the, pitch in a beta strand, it's referring to these lines over here which is, showing the repeated, structure of the beta strand. So notice that the r groups are both going down here and the distance between this, periodic repeat structure is just 2 amino acid residues. But this the length turns out to be 7 angstroms and that's what the pitch actually is. And so, the way that we calculate the length of a beta strand is gonna be the same way that we calculated the length for the alpha helix. And remember, all we needed to do was take the number of amino acid residues and multiply it by the rise. And so, for this beta strand here which has 5 amino acid residues in it, all we need to do is take the 5 amino acid residues, so take 5 and multiply it by the rise of 3.5 angstroms. And that comes out to 17.5 angstroms and that's the length of this alpha he of this beta strand over here on the left. Now, if we do the same for the alpha helix, which again has 5 amino acid residues, We'll take the 5 amino acid residues and multiply it by 1.5 angstroms because that's the rise for the alpha helix. And so this ends up coming out to just 7.5 angstroms. And so what you can see is that when you're comparing, 5 amino acids in a beta strand, notice that the rise, I'm sorry, the length is 17.5 angstroms, which is much much more longer and extended than, 5 amino acids and an alpha helix conformation which is much more coiled. And so, that's really the biggest thing that you wanna take away from this, along with the pitch and the rise and the ability to be able to calculate the lengths here. And so that concludes our lesson on beta strands, and I'll see you guys in our practice
2
Problem
Problem
What is the approximate length of a β-strand containing 27 amino acids?
A
94.5 Å
B
189 Å
C
75.4 Å
D
40.5 Å
3
concept
Beta Strand
Video duration:
2m
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Video transcript
So now that we know that the beta strand is just an extended zigzag conformation of the protein backbone, we can talk about beta strand depictions. And so beta strands are commonly depicted as extended broad arrows and these arrows can actually twist and and they point towards the c terminal end of the protein. And so similar to alpha helices, the beta strands are stabilizing the beta strands. Now, unlike alpha helices, the hydrogen bonds are actually not parallel. They are actually perpendicular to the direction of the, beta strands. And so recall that perpendicular just means that when they intersect, that they form these, little 90 degree angles. And so, if we look at our example, what we're gonna do is is label the terminals of the beta strand. And over here on the left, what we have is just a single beta strand, just one beta strand, and that's why we just have one arrow. And again, this arrow is pointing towards the c terminal end, which means that this must be the c terminal end over here, and that must mean that this other end over here has to be the n terminal end. And this is pretty easy because we always consider sequences from the n terminal end to the c terminal end anyways. And so, having the arrow point towards the c terminal end seems like it's pretty natural. So hopefully, that'll be easy for you guys to remember. Now, over here on the right, what we have are 2 beta strands, and so notice that these two beta strands have 2 arrows. There's 1 arrow here and another arrow over here. And also notice that these two arrows are connected together into a single chain. And so, these two arrows, we know that they're pointing towards their c terminal end. And so, because this is the tip of the arrow over here, this has to be the c terminal end of this protein and that must mean that this other end up here, which has the back of the arrow has to be the n terminal end. And again, when, beta strands are together like this, they're stabilized by hydrogen bonds of the peptide backbone, which we'll talk more about the hydrogen bonds in more detail a little later in our course. But for now, what I want you to know is that these hydrogen bonds are actually perpendicular to the directions of these strands. So notice that this these strands are going in this direction and this direction, and the hydrogen bonds are going almost perpendicular, moving in this direction, which is a perpendicular direction to the direction of the strand. So that's something that's important to keep in mind and something that's much different than alpha helices. And so, this concludes our lesson on Beta Strand Depictions, and we'll get a little bit of practice in our next video, so I'll see you guys there.
4
Problem
Problem
Which phrase best describes the hydrogen bonds of a β-strand in silk fibroin, a protein with β-conformations?
A
They occur mainly near the amino and carboxyl termini of the β-strands.
B
They are perpendicular to the plane of the two β-strands.
C
They occur mainly between the atoms of the R groups.
D
They occur between backbone atoms of adjacent β-strands.