state three features that alpha helices and beta sheets have in common.
Answers
Explanation:
Protein Secondary Structure: α-Helices and β-Sheets
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 figure below. This type of representation of a protein structure is called sticks representation. To give you a better impression of how a helix looks like, only the main chain of the polypeptide is show in the figure, 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, observed as mentioned earlier in the text, as clustering of the corresponding torsion angles within the helical region of the Ramachandran plot. The α-helix is the major structural element in proteins. When looking at the helix in the figure below, we notice how the carbonyl oxygen atoms C=O (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.
The α-helix is not the only helical structure in proteins. Other helical structures include the 3_10 helix, which is stabilized by hydrogen bonds of the type (i, i+3) and the π-helix, which is stabilized by hydrogen bonds of the type (i, i+5). The 3_10 helix has a smaller radius, compared to the α-helix, while the π-helix has a larger radius. A paper describing the occurrence of the π-helix in proteins, which is based on the analysis of entries in the Protein Data Bank (PDB) has been published by Fodje & Al-Karadaghi, 2002.
The second major type of secondary structure in proteins is the β-sheet. β-sheets consist of several β-strands, stretched segments of the polypeptide chain kept together by a network of hydrogen bonds. An example of a β-sheet with the stabilizing hydrogen bonds shown as dashed lines is shown on the figure below:
