Beta Turns - Online Tutor, Practice Problems & Exam Prep

Beta turns and loops are non-repetitive secondary structures in proteins that facilitate changes in the peptide backbone direction, allowing for compact protein shapes. Loops consist of long amino acid sequences without fixed hydrogen bonds, while beta turns, stabilized by internal hydrogen bonds, contain fewer than four amino acids. Type 1 beta turns are more common and feature proline at position 2, whereas type 2 beta turns include glycine at position 3. Glycine's small R group enables it to adopt diverse phi and psi bond angles, making it prevalent in type 2 beta turns, which occupy multiple regions of the Ramachandran plot.

Beta Turns

So now that we've talked about alpha helix and beta sheet secondary structures, in this video, we're going to talk about another type of secondary structure: the beta turns and loops. Beta turns and loops are a type of non-repetitive secondary structure. By non-repetitive, all we mean is that they don't have that same repetitive periodic structure that alpha helices and beta sheets have. However, beta turns and loops do cause the peptide backbone to change directions. These structures are usually found on the surface of proteins with hydrophilic amino acid residues. Being on the surface of proteins allows the hydrophilic amino acid residues to easily interact with their aqueous environment in which most proteins are situated. Moreover, beta turns and loops enable the proteins to take on a folded, compact shape. You can imagine a protein that has a folded compact shape must change the backbone direction frequently. Otherwise, it would continue in one direction and would not have a compact shape. Thankfully, beta turns and loops that allow for changes in backbone direction enable us to have proteins with folded compact shapes.

Now, we are going to distinguish between the loops and beta turns. Loops are essentially large links of amino acids that cause changes in backbone direction without fixed internal hydrogen bonds; they do not utilize fixed internal hydrogen bonds. Beta turns, on the other hand, also known as reverse turns, are small loops that have less than four amino acid residues and cause abrupt changes in backbone direction. They are able to do this because they are stabilized by fixed internal hydrogen bonds unlike the loops.

In our example below, we will identify all the loops and beta turns in the figures. Notice in our first figure over here on the left, we have a single polypeptide chain with an N-terminal and a C-terminal. You'll see we have beta sheets, alpha helices, and more beta sheets, or beta strands. Notice that this beta strand is connected to this alpha helix here via a loop. We know that it's a loop because it contains 20 amino acids, which is greater than four amino acids. This is a large link of amino acids causing a change in the backbone direction. Observe how the backbone direction changes through this loop.

In our next example, notice what we have is a beta turn, and we know this is a beta turn because the backbone is abruptly changing directions, doing a quick U-turn. It incorporates a small link of amino acids, less than four amino acids, and it's stabilized by this internal hydrogen bond. That's a way to identify it as a beta turn. Notice in this image that with the loop, there are no internal hydrogen bonds.

In the image on the far right, we have a single polypeptide chain that exhibits several secondary structures. It has the yellow structures, which are beta sheets, the pink structures, which are alpha helices, and it includes beta turns and loops. The grayish links of long amino acids are the loops, causing changes in the backbone direction. The shorter blue structures throughout are our beta sheets and are labeled as beta turns because they cause abrupt changes in the backbone direction with a small amount of amino acid residues. Moving forward, we'll be able to practice more with beta turns and discuss different types of beta turns that exist.

I'll see you guys in those videos.

Which of the following options contains a true statement about protein turns & loops?

• A. Loops & turns can interact with other proteins & the environment.
• B. Loops are short links causing abrupt changes in direction & extend only from β strands.
• C. Loops and turns usually contain hydrophilic residues located on the interior of proteins.
• D. Loops exposed to an aqueous environment are usually composed of hydrophobic amino acids.

Beta Turns

So now that we know a little bit about beta turns and loops, in this video we're going to distinguish between 2 different types of beta turns. Type 1 beta turns and type 2 beta turns. Now, it turns out that there's actually many different types of beta turns, but these are 2 of the more common types of beta turns that your professor is likely going to want you guys to know. And so both type 1 and type 2 beta turns produce abrupt changes in the backbone direction. They both also contain a total of 4 amino acid residues and they are both stabilized by hydrogen bonding. And so really the main differences between type 1 and type 2 beta turns is that type 1 beta turns are even more common than type 2 beta turns, and they usually contain a proline amino acid residue specifically at position number 2 of the turn. And then type 2 beta turns, on the other hand, they're going to be less common and they contain a glycine amino acid residue, but specifically at position number 3 of the turn. And so down below in our example, we can visually distinguish between type 1 beta turn on the left and type 2 beta turn on the right. And what you'll notice is that both type 1 and type 2 beta turns produce an abrupt change in the backbone direction. And so over here, you can see that the backbone direction is starting off in this direction and then it very quickly changes direction and does a U-turn and now it's going in the opposite direction. And the same applies for type 2 beta turns, so they both produce these abrupt changes in backbone direction. And also notice that they both contain 4 amino acid residues, as shown by these 4 blue squares in the back of each of the structures, numbered 1, 2, 3, and 4, and they are both also stabilized by hydrogen bonds between the carbonyl group and the amino group in the polypeptide backbone. Since we know that only backbone hydrogen bonds stabilize secondary structures and type 1 and type 2 beta turns are a type of secondary structure. So we can see the hydrogen bonds forming in type 1 and the hydrogen bond forming in type 2. And so again, the major differences between type 1 and type 2 beta turns is that type 1 beta turn over here on the left is more common and it has a proline amino acid residue specifically at position number 2 of the turn as shown. So the pro is the 3 letter code for proline and the 2 is for position number 2. And then for type 2 beta turns over here, they specifically have a glycine amino acid residue at position number 3 of the turn. And so you might be wondering, how am I supposed to remember the differences between type 1 and type 2 beta turns? That's really, really specific. And so I can tell you something that I came up with that helps me remember the differences between type 1 and type 2 beta turns, is that type 1 beta turn, because it has a proline at position 2, and proline's 3 letter code is pro, pro. Whenever I think of pro, I think of a pro boxer. And what do pro boxers do? They throw 1, 2 punches. And so, when I think of a type 1 beta turn, I literally think of 1, 2 p. And so when I think of 12 p, I'm really thinking of the pro boxer 12 punching. And so, the one here is, a reminder for it's a type 1 beta turn. The 2 is specifically for at position number 2, and the p here is for the proline amino acid residue, since we know that p is the one letter code for proline. And so if you can remember 12 p, then you'll remember type 1, beta turn at position number 2 have a proline amino acid residue and that's exactly what we're seeing over here. Now, for type 2 beta turns on the other hand, all you need to remember is 23g. And, really, what this is referring to is that the g of basketball is like the GOAT of basketball. The greatest of all time. And so when you think of the g of basketball, you think about either LeBron James or Michael Jordan. And their jerseys had the number 23 on them. And so, essentially when you think of 23 gs, you think of the GOAT, who's Michael Jordan or LeBron James with the number 23 and that tells us that at, with a type 2 with a type 2 beta turn, at position number 3, there is a glycine amino acid residue. And so, essentially, if you can remember 12p and 23 gs, then you'll be able to remember the differences between type 1 and type 2 beta turns. So you can see that with this type, 23 gs here, that type 2 beta turns, these over here, at position number 3, up here, they have a glycine amino acid residue. So, hopefully that'll help you guys remember the differences between type 1 and type 2 beta turns, and you guys can now go tell all of your friends, hey, I know a great way to remember the differences between type type 1 and type 2. And so we'll be able to get some practice utilizing these concepts and memory tools in our next couple of practice videos. So I'll see you guys there.

In the peptide below, circle the individual amino acid residues indicating the most likely positions for β-turns:

Alright. So this practice problem says, in the peptide below, circle the individual amino acid residues indicating the most likely positions for beta turns. And down below, we have a peptide with 28 amino acid residues. And so recall from our previous lesson videos that we covered two different types of beta turns, type 1 beta turns and type 2 beta turns. And recall that type 1 beta turns are more common than type 2 beta turns. And because we're looking for the most likely positions for beta turns, we're really looking for indications of type 1 beta turns. And so recall the mnemonic that helps us memorize the nature of type 1 beta turns is "one two p." And so recall with "12p," the 1 is referring to type 1 beta turns, the 2 is referring to position number 2, and the p is of course referring to the proline amino acid residue that's found at position 2. And because the problem is specifically asking us to circle individual amino acid residues, we're really looking to circle the proline amino acid residues. And so scanning our problem, notice that we have two proline amino acid residues, one at position 7 and one at position 19. And so technically, the correct answer for this problem is to circle these two proline amino acid residues. Now recall that beta turns actually have a total of 4 amino acid residues, but the problem's not asking us to circle those 4 amino acid residues. However, if it was asking us to circle those 4, all we'd need to do is make sure that the proline is at position number 2 of those 4 residues. And so if we were to circle these 4 residues, that would put proline at position 2. And if we were to circle these 4 residues, that would also put proline at position 2. So, those would be the correct answers if it was asking us again to circle the 4 residues of those beta turns. But again, it's only asking us to circle individual amino acid residues. So this is the correct answer, and I'll see you guys in our next video.

Which of the following statements is true regarding β-turns?

• A. Only Type I β-turns are stabilized by hydrogen bonds, not Type II β-turns.
• B. Type II β-turns have a Pro residue at position #2 of the turn.
• C. Type I β-turns have a Gly residue at position #4 of the turn.
• D. Type II β-turns have a Gly residue at position #3 of the turn.

Beta Turns

So now that we've distinguished between type 1 and type 2 beta turns, in this video we're going to talk about beta turn bond angles. And so the bond angles for loops and turns are actually found in multiple regions of the Ramachandran plot. And that's actually a unique feature of loops and turns because we know that with alpha helices and beta sheets, we're able to pinpoint those phi and psi bond angles into very specific regions in the Ramachandran plot. So let's take a look at our example down below to refresh our memories. And so we know that we can split a Ramachandran plot into four different quadrants. Amino acid residues that are part of a beta sheet will have phi and psi bond angles that fall into the upper left-hand quadrant of the Ramachandran plot, whereas amino acids that are part of an alpha helix will have phi and psi bond angles that fall into the lower left-hand quadrant of the Ramachandran plot. And so we're able to pinpoint beta sheets and alpha helices to very specific regions within the Ramachandran plot. But we're not able to do the same with loops and turns because loops and turns are found in multiple regions of the Ramachandran plot. So let's take a look at an example of a type 2 beta turn. And so it turns out that some of the phi and psi bond angles of a type 2 beta turn actually lie outside of the expected permissible bond angles for most of the amino acid residues. But we know that glycine is an exception to this because glycine can actually adopt a wide range of phi and psi bond angles. Because it has such a small R group. It's literally just a hydrogen atom. And that small R group is able to avoid steric hindrance, which allows it to adopt a wide range of phi and psi bond angles. And so it can easily adopt those phi and psi bond angles that lie outside of the expected permissible angles for most amino acids. And from our mnemonic, 23 3G, we know that type 2 beta turns, specifically at position number 3, have a glycine amino acid residue. And so, glycine is often the residue that's found in type 2 beta turns and that's because it's one of the only amino acids that can actually adopt those phi and psi bond angles that lie outside of the expected permissible angles. And so if we take a look at our example down below, what we'll see is that type 2 beta turns have phi and psi bond angles that fall into the upper left-hand quadrant, as well as phi and psi bond angles that fall into the lower right-hand quadrant. And so we know that most amino acids are able to easily adopt the phi and psi bond angles required for type 2 beta turns that fall into the upper left, but not many amino acids have phi and psi bond angles that are able to form in the lower right quadrant of the Ramachandran plot, but glycine is the exception and so, glycine has a Ramachandran plot that is very unique because it essentially has permissible bond angles in every region of the Ramachandran plot except for the middle region here. And so you can see that glycine is pretty easily able to adopt the type 2, phi and psi bond angles in the upper left-hand quadrant, and it's also able to pretty easily adopt the type 2 beta turn bond angles in the lower right-hand quadrant as well. And so again, that's why glycine is often the residue found in type 2 beta turns. And so really the major takeaway from this video is the fact that the phi and psi bond angles for residues that fall into loops and turns are found in multiple regions of the Ramachandran plot. And so we'll be able to get some practice, applying these concepts in our next practice video. So I'll see you guys there.

If the phi & psi angles of loop regions are plotted, where do they tend to fall on the Ramachandran plot below?

• A. The area labeled in green.
• B. The area labeled in blue.
• C. The area labeled in grey.
• D. All the above.

Which of the following statements is correct?

• A. Loops and turns are usually found tucked away on the interior of folded proteins.
• B. An α-helix peptide backbone located in the interior of a protein will H-bond to R-groups of other residues.
• C. In extended fibrous proteins that are elongated, we would expect to find numerous β-turns & loops.
• D. Tightly compact spherical/globular proteins tend to have more β-turns than elongated fibrous proteins.
• E. A membrane-embedded α-helix is likely rich in Asp residues.

Here’s what students ask on this topic:

What are beta turns in protein structures?

Beta turns, also known as reverse turns, are a type of non-repetitive secondary structure in proteins. They consist of fewer than four amino acids and cause abrupt changes in the direction of the peptide backbone. Beta turns are stabilized by internal hydrogen bonds, which help maintain the protein's compact and folded shape. They are commonly found on the surface of proteins, allowing hydrophilic amino acid residues to interact with the aqueous environment. Beta turns are crucial for the overall 3D structure and function of proteins.

What is the difference between type 1 and type 2 beta turns?

Type 1 and type 2 beta turns both cause abrupt changes in the peptide backbone direction and contain four amino acid residues stabilized by hydrogen bonds. The main difference lies in their amino acid composition: Type 1 beta turns are more common and typically have a proline residue at position 2, while type 2 beta turns are less common and usually feature a glycine residue at position 3. These differences affect the specific phi (ϕ) and psi (ψ) bond angles that the turns can adopt, influencing their placement on the Ramachandran plot.

How do beta turns and loops differ in protein structures?

Beta turns and loops are both non-repetitive secondary structures that facilitate changes in the peptide backbone direction. However, they differ in size and stabilization. Beta turns are small, containing fewer than four amino acids, and are stabilized by internal hydrogen bonds. Loops, on the other hand, are larger, consisting of more than four amino acids, and do not have fixed internal hydrogen bonds. Both structures are essential for the compact and folded shapes of proteins, but their different stabilization mechanisms and sizes distinguish them.

Why is glycine often found in type 2 beta turns?

Glycine is often found in type 2 beta turns because of its small R group, which is just a hydrogen atom. This small size allows glycine to adopt a wide range of phi (ϕ) and psi (ψ) bond angles, avoiding steric hindrance. This flexibility enables glycine to fit into the specific bond angles required for type 2 beta turns, which often lie outside the permissible angles for most other amino acids. As a result, glycine is commonly found at position 3 in type 2 beta turns, facilitating the abrupt change in backbone direction.

What role do beta turns play in protein folding?

Beta turns play a crucial role in protein folding by allowing the peptide backbone to change direction abruptly. This ability to change direction helps proteins achieve their compact and folded 3D structures, which are essential for their biological functions. Beta turns are stabilized by internal hydrogen bonds, which maintain the integrity of the turn and contribute to the overall stability of the protein. By facilitating these directional changes, beta turns enable proteins to form complex shapes necessary for their specific roles in biological processes.