Beta Strand

So now that we've talked about Alfa Ulysses, 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 zig zag confirmation 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 are groups are zigzagging. And so when you look at the are 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 are groups are literally zigzagging, and that's what we mean by this zigzag confirmation. And so the extended periodic zigzag confirmation or structure repeats every two amino acid residues and so, uh, the rise. So if we take a look at the rise and remember that the rise is just the length of the distance covered per amino acid residue, it's actually about 3.5 angstrom. And if we compare this to the rise of an Alfa helix, that's actually more than double because the rise of an alfa helix is just 1. angstrom. So basically what that saying is that the Beta strand is much more extended, its more extended than the Alfa Helix, which is much more coiled together. And so the pitch of a beta strand is going to be seven angstrom. And again, that's much longer than the pitch of Alfa Helix, which is just 5.4 angstrom. So again, all that saying is that the beta strand is mawr extended than the Alfa Helix, which is Mork oiled. So in our example below, What we're gonna do is compare the rise, pitch and length of five amino acid residues in a beta strand confirmation and then five amino acid residues in an Alfa Helix confirmation. So in our example, down below, What we have is the beta strand on the left over here and the Alfa helix on the right, and so on the far left. Over here, what we have is our key. The green represent the are groups. The red balls represent oxygen's blue or the nitrogen, black or carbons and white are hydrogen. And so, looking at the beta strand and comparing it to the Alfa Helix again. They both have exactly five amino acid residues, and one of the first things that you'll notice is that the beta strand confirmation on the left over here is much, much more extended. It's just way longer than the Alfa helix, which is much coiled together. And so the backbone for a beta strand is literally just much mawr extended, whereas with the Alfa Helix it's much much Mork oiled 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 Beta Strand is 35 angstrom, whereas the rise for an alfa helix is just 1.5 angstrom. 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 Maurin the beta strained confirmation. Now, looking at the pitch, remember, the pitch is when foreign Alfa Helix is when you have one turn of the Alfa 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 54 angstrom in the Alfa Helix. Now for the pitch and a beta strand. It's referring to these lines over here, which is showing the repeated structure off the beta strand. So notice that they are groups are both going down here, and the distance between this periodic repeat structure is just two amino acid residues. But this the length turns out to be seven angstrom, 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 Alfa 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 five amino acid residues in it, all we need to do is take the five amino acid residues, so take five and multiply it by the rise of 3.5 angstrom, and that comes out to 17.5 Angstrom. And that's the length of this alfa of this beta strand over here on the left. Now, if we do the same for the Alfa Helix, which again has five amino acid residues, we'll take the five amino acid residues and multiply it by 1. angstrom because that's the rise for the Alfa Helix. And so this ends up coming out to just 7.5 angstrom. And so what you can see is that when you're comparing, uh, five amino acids in a beta strand, notice that the rise I'm sorry. The length is 17.5 angstrom, which is much, much more longer and extended than five amino acids and an Alfa Helix confirmation, which is much more coiled. That's really the biggest thing that you want to take away from this, along with the pitch and rise and the ability to be able to calculate the length here. And so that concludes our lesson on Beta Strand, and I'll see you guys in our practice video

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

So now that we know that the beta strain is just an extended zig zag confirmation of the protein backbone, we can talk about beta strand depictions. And so beta strands air commonly depicted as extended broad arrows and these arrows can actually twist. And they point towards the C terminal end of the protein and so similar to Alfa Ulysses, the beta strands are stabilized by hydrogen bonding of the peptide backbone. And so the our group are again not involved in stabilizing the beta strands. Now, unlike Alfa Hillis ease, 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 labeled 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 end 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 will be easy for you guys. Remember? Now, over here on the right, What we have are two beta strands. And so notice that these two beta strands have two arrows. There's one arrow here and another arrow over here and also noticed that these two arrows air connected together into a single chain. And so, uh, 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 end terminal end. And again when beta strands air together like this there stabilized by hydrogen bonds of the peptide backbone which will talk more about the hydrogen bonds and 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, that moving in this direction, which is a perpendicular direction to the direction of the strands. So that's something that's important to keep in mind and something that's much different than Alfa. He'll asses. And so, uh, this concludes our lesson on beta Strand depictions, and we'll get a little bit of practice and our next video, so I'll see you guys there.

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.