Why is alpha helical membrane channels better suited than beta barrell channels?

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Discussion Overview

The discussion revolves around the suitability of alpha helical membrane channels compared to beta barrel channels in transmembrane proteins. Participants explore the structural characteristics and implications of these configurations in the context of membrane biology, particularly in an introductory biology course.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why alpha helical proteins are more common in membrane channels despite the apparent advantages of beta barrel configurations, suggesting that the hydrophilic interior of alpha helices facilitates interactions with molecules.
  • Another participant proposes that the beta barrel's appeal as an "ideal channel" may stem from a simplified human intuition about larger openings allowing easier passage of substances.
  • Discussion includes the importance of side group (R group) positioning in determining the functionality of alpha helices and beta sheets, with a suggestion to verify consistency with their 3D structures.
  • A participant mentions the specific example of gramicidin, an alpha helical protein, highlighting its hydrophilic interior and hydrophobic exterior, and how this configuration interacts with the lipid bilayer.
  • There is a suggestion that the beta barrel might be misunderstood, with a clarification that it may refer to an alpha/beta barrel structure that includes both alpha helices and beta sheets.

Areas of Agreement / Disagreement

Participants express uncertainty about the advantages of beta barrel configurations and whether they are indeed ideal for transmembrane channels. There is no consensus on the reasons for the prevalence of alpha helical structures over beta barrels.

Contextual Notes

The discussion is situated within an introductory biology course context, which may influence the depth of understanding and the complexity of the concepts being discussed. There are references to specific structural representations that may not be universally accessible to all participants.

confusedbyphysics
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I have this question I need to answer and I am confused. I've checked my notes and book and can't find an answer. Here is the question:

"Even though the beta barrel configuration for a transmembrane protein channel would seem an ideal channel, most of the channel proteins have multipass alpha helical domains. Why would this configuration be so well suited to membrane channels?"

Is it just because the alpha helical proteins form a hole, where the inside is hydropholic and the exterior is hydrophobic (thus interacting with the phospholipids), and the hydrophilic interior makes it easier for the molecules to interact with the protein? I don't get why "beta barrell configuration would seem an ideal channel" in the first place. Confused here, lol. :redface:
 
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Do you have a good representation (perhaps a drawing in your text) of where the side groups lie, on an alpha helix and beta sheet (I'm presuming a beta barrel is a beta sheet that curls up into a barrel.)?

I think you're close to the right track with your answer, but I haven't looked at the locations of side groups (R groups) on these structures, in a few years. Double check that and see if your answer is consistent with the 3D structure of each of these motifs.

(Another question for you: Why do we focus on the R groups in these sorts of questions?)

(I'd guess, but it's only a guess, that the phrase the beta barrel configuration for a transmembrane protein channel would seem an ideal channel" is an appeal to the (oversimplified) human intuition that a "big channel" let's "stuff" go through "easier." I could be wrong. Is this an introductory course, or an advanced course?)
 
Hi patty, thanks for the reply! Yes, it is an introductory course, Bio 201 (Cell and Molecular Biology). It's the first class required for Bio majors. The only picture I could find in my book is of a channel protein called gramicidin. And it's basically just a picture of a protein with a hydropholic interior with the hydrophobic exterior around it. It says, "gramicidin as an alpha helix consisting of only 15 amino acids. In top view, the molecule forms a hole or pore. In side view, a green helix traces the peptide bonded backbone of they polypeptide. R gropus hang off the backbone to the outside." So I guess the r groups are just drawn to the nonpolar, hydrophobic lipids inside the lipid bilayer. And so the hydrophilic part of the protein is shoved inward to get away from the nonpolar bilayer and forms a hole This stuff is very confusing! Maybe I am making it too hard and there is a very simple answer.
 
You seem to be on the right track there. You probably aready understand the material well enough to not be sure why a beta barrel would make a good channel in the first place (I think your teacher might be alluding to the apparent "donut" shape of the structure). Here is a site with some illustrations that might help if you're not finding it in your textbook. Take note of the space-filling model shown.
http://www.chembio.uoguelph.ca/educmat/phy456/Protstr4.htm

I'm also assuming by beta barrel that you're really talking about what's known as an alpha/beta barrel, which is a configuration including both alpha helices and beta sheets all "rolled" into a "barrel" shape.
 

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