Describe the differences in the shape of an α helix and a β-pleated sheet.

Question

Accepted Answer

All right. Hello everyone. So this question is asking us which of the following statements correctly describes the shape of the two secondary protein structures, alpha helix and a beta pleated sheet. Now, here we have four different answer. Choices labeled A through D proposing different possible descriptions for these structural elements. But the one thing that I want to draw your attention to before we start talking about these elements in greater detail is the intermolecular forces or IMF that are described in this question. Now, option A for example, brings up sulfide bonds when talking about the beta PTA cheat. Whereas option B talks about dispersion forces and sulfide bands. And option C option D excuse me mentions dile dile interactions in the beta pleated sheet. So the first thing that I want to go ahead and point out here is that the secondary structure of a protein is describing intermolecular forces between the atoms of a peptide bond backbone and secondary structure to be more specific is characterized by hydrogen bonding or h bonding as I'm writing here. So just to go ahead and reiterate that point, hydrogen bonding is characteristic of secondary structure, it's the only IMF that we expect to see at this level. Now, interactions like sulfide bonds, dispersion forces and dile dile interactions are characteristic of tertiary structure, which if you recall describes interactions between the side chains of amino acids. Now, for this reason, we can really eliminate all of the answer choices except for one, but just for the sake of completeness. Let's talk about these structural elements starting off with the alpha helix. Now, the alpha helix can best be described as a cylindrical spiral shape. So it's a right-handed coil and it's a right handed coil in which as mentioned before, the atoms of the backbone of the polypeptide chain are engaging in hydrogen bonding to create that helical structure. So here, these hydrogen bonds can be described as intra chain, meaning that these hydrogen bonds are taking place within backbone atoms of the same polypeptide chain. So that's an important distinction to make because on the other hand, we can talk about the beta pleaded sheet. Now, the reason why it's called the beta pleated sheet is because of its characteristic zigzag pattern, which creates a pleated appearance. And so the difference, the difference between alpha helices and beta sheets in terms of factors, other than how they look is that a beta pleated sheet is stabilized by inter chain h bonds, meaning that h bonding can take place between different polypeptide chains. And so because of these polypeptide chains are extended and lie next to each other it creates a more flexible structure compared to the alpha helix. So here we've arrived at our final answer, which happens to be option C in the multiple choice and it reads as follows. The alpha helix is a compact spiral structure stabilized by intra chain hydrogen bonds. While the beta pleated sheet is an extended sheet like structure stabilized by inter chain hydrogen bonds and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.

Describe the differences in the shape of an α helix and a β-pleated sheet.

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Key Concepts

α Helix Structure

β-Pleated Sheet Structure

Hydrogen Bonding in Protein Structures