Beta sheet

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This page is under construction, --Karsten Theis 09:52, 7 August 2021 (UTC)

A beta sheet is a type of secondary structure, i.e. a description of how the main chain of a protein is arranged in space. It is composed of at least two beta strands. Beta strands have repetitive regular secondary structure (just like the alpha helix), i.e. all residues have similar conformation and hydrogen bonding, and it can be of arbitrary length.

Contents

Structure, hydrogen bonding and composition

Connectedness or topology of beta sheets

In a beta sheet, neighboring strands can either have a parallel or an antiparallel orientation, resulting in different hydrogen bonding pattern. The strands, if part of the same subunit, will all be connected as part of a single polypeptide. The parallel or antiparallel orientations and how strands are connected in their primary sequence is shown in topology ; these are useful to compare different protein folds. One example of such a beta sheet topology is the so-called greek key - an all-antiparallel sheet with four strands. Image:251px-Anthrax toxin protein key motif.svg.png

Some topologies occur very often while others have not yet been observed. There are some rules of thumb about preferred topologies. [2]

Types of proteins and folds that contain sheets

Beta sheets in soluble (globular) proteins

Beta sheets in transmembrane proteins

Beta sheets in amyloid fibrils

History

Alpha helices and beta sheets are named after two conformations of keratin, a fiber occuring in mammals (wool, hair, quills) [3]. Alpha keratin is composed of coiled coils of alpha helices, whereas hard stretching these fibers in water changes the conformation to beta sheets. The two conformations show different diffraction data under X-ray illumination.

Experimental evidence

Apart from the historical fiber diffraction data, various spectroscopic techniques may be used to show the presence of beta sheets. Circular dichroism (CD) or infrared (IR) spectroscopy allows an estimate of the beta sheet content of a protein sample. NMR spectroscopy, after resonance assignment, allows secondary structure assignment residue by residue based on chemical shifts of the alpha carbon and beta carbon resonances.

Quiz

1. What level of structure does a beta sheet refer to?

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

2. What are beta sheets composed of?

A. Amino acid side chains
B. Beta strands
C. Disulfide bridges
D. Carbohydrates

3. The beta sheet is a repetitive structure.

A. TRUE.
B. FALSE.

4. Amino acid side chains tend to be more hydrophobic in beta sheets than alpha helices

A. TRUE.
B. FALSE.

5. The following amino acids are rarely found in the center of a beta sheet (more than one answer)

A. Proline
B. Valine.
C. Glycine.
D. Isoleucine.

6. Which atoms/groups are involved in forming hydrogen bonds in beta sheets?

A. the alpha carbons.
B. the beta carbons.
C. the carbonyl oxygen.
D. the amide group.

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See Also

Sheets in Proteins

TOPS diagrams

Physical model of a beta sheet for teaching

References

  1. Tsutsumi M, Otaki JM. Parallel and antiparallel beta-strands differ in amino acid composition and availability of short constituent sequences. J Chem Inf Model. 2011 Jun 27;51(6):1457-64. doi: 10.1021/ci200027d. Epub 2011, May 11. PMID:21520893 doi:http://dx.doi.org/10.1021/ci200027d
  2. Minami S, Chikenji G, Ota M. Rules for connectivity of secondary structure elements in protein: Two-layer alphabeta sandwiches. Protein Sci. 2017 Nov;26(11):2257-2267. doi: 10.1002/pro.3285. Epub 2017 Sep 19. PMID:28856751 doi:http://dx.doi.org/10.1002/pro.3285
  3. Kreplak L, Doucet J, Dumas P, Briki F. New aspects of the alpha-helix to beta-sheet transition in stretched hard alpha-keratin fibers. Biophys J. 2004 Jul;87(1):640-7. doi: 10.1529/biophysj.103.036749. PMID:15240497 doi:http://dx.doi.org/10.1529/biophysj.103.036749

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Karsten Theis

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