Secondary structure turn helix and coils, strands denoted by
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In the following we will focus on the general aspects of protein secondary structure. Many of the features discussed here are essential for practical applications − for example in sequence alignment and analysis, homology modelling and analysis of model quality, in planning mutations or when analyzing protein-ligand interactions.
The most common type of secondary structure in proteins is the α-helix. Linus Pauling was the first to predict the existence of α-helices. The prediction was confirmed when the first three-dimensional structure of a protein, myoglobin (by Max Perutz and John Kendrew) was determined by X-ray crystallography. An example of an α-helix is shown on the image below. This type of representation of a protein structure is called “sticks representation”. To get a better impression of how a helix looks like, only the main chain of the polypeptide is shown, no side chains. There are 3.6 residues/turn in an α-helix, which means that there is one residue every 100 degrees of rotation (360/3.6). Each residue is translated 1.5 Å along the helix axis, which gives a vertical distance of 5.4 Å between structurally equivalent atoms in a turn (pitch of a turn). The repeating structural pattern in helices is a result of repeating φ values and ψ values, which is reflected in the clustering of the torsion angles within the helical region of the Ramachandran plot. When looking at the helix in the figure below, notice how the carbonyl (C=O) oxygen atoms (shown in red) point in one direction, towards the amide NH groups 4 residues away (i, i+4). Together these groups form a hydrogen bond, one of the main forces in the stabilization of secondary structure in proteins. The hydrogen bonds are shown on the right figure as dashed lines.
Protein which contains at least one coiled coil domain, a type of secondary structure composed of two or more alpha helices which entwine to form a cable structure. In proteins, the helical cables serve a mechanical role in forming stiff bundles of fibres.