Could a polyAla sequence form a beta sheet How about a colla

Could a poly-Ala sequence form a beta sheet? How about a collagen triple helix? Why or why not? Provide a clear rationale for each answer and cite any resources you consulted to write your response.

Please explain, thank you for your help!

Solution

As we all know that Alanine and leucine are strong helix-favoring residues, but it has been reported that expanded polyalanine stretch was found to be forming the core -sheet structure which is involved in mediating the intermolecular association of mutant proteins into fibrillar inclusions in case of human pathologies [1].

Yes, Poly-Ala sequences can form the beta sheet.

No, poly-Ala sequence cannot form collagen triple helix [2]. Repetitious amino acid sequence glycine - X - Y will form the triple helix, here the X and Y amino acids will be frequently proline or hydroxyproline. Here triple helix is stabilized by the steric repulsion exerted by pyrrolidine rings of proline and hydroxyproline residues. Gly-Ala-Ala-Ala can form triple helix, but Ala alone cannot form the collagen triple helix. It has been reported that collagen like triple helix is easily formed by Gly-imino acid-imino acid.

References:

[1] Leonid M. Shinchuk,1† Deepak Sharma,1 Sylvie E. Blondelle,2 Natalia Reixach,2 Hideyo Inouye,1 and Daniel A. Kirschner1. Poly-(L-Alanine) Expansions Form Core -Sheets that Nucleate Amyloid Assembly. PROTEINS: Structure, Function, and Bioinformatics 61:579 –589 (2005)

[2] Matthew D. Shoulders1 and Ronald T. Raines1,2. Collagen structure and stability. Annu Rev Biochem. 2009; 78: 929–958.