Protein secondary stucture

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The secondary structure of a protein describes the way in which a polypeptide chain of amino acids are folded. These secondary structures can be separated into 2 section alpha-helicies and a beta-sheet formation, these describe the level of organisation of the specific folding patterns.


An alpha-helix is a right-handed helix that resembles a right-handed spiral staircase. An alpha helix contains hydrogen bonding between the N-H group of every peptide bond and C=O groups that are next to a peptide bond[1]. these are usually located 4 amino acids away from each other but in the same chain. This is what produces the spiralling structure of the alpha-helix. There is 3.6 amino acid residues in each coil turn in the alpha helix[2].

The most common known helix structure is that of DNA, where 2 alpha helices are wound around each other with the complementary base pairing between them holding them together.


The beta-sheet is where one chain of amino acids, form a parallel or antiparallel folded structure with hydrogen bonding between the adjacent strands of the fold, between N-H and C=O groups[3].

The antiparallel structure is stronger due to the almost perfect lining up of the strands to make hydrogen bonds that form directly perpendicular to the running strand. The parallel structure isn't as strong however it is still a very rigid structure. Sometimes, these beta sheets are able to twist, thus they are able to form a barrel shape. This is a useful tool as this means they form cylindrical structures which can, for example, be used as passageways and pores[4].


  1. B. Alberts et al. (2014) Essencial Cell Biology 4th edition, Gartland Science, New York
  2. Berg.J, Stryer.L, Tymoczko J, Biochemistry, seventh edition, WH Freeman, 2012, page 39
  3. B. Alberts et al. (2014) Essential Cell Biology 4th edition, Gartland Science, New York
  4. Petsko.G, Ringe.D, Protein Structure and Function, New Science Press, 2004, page 16
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