Another example, (PDB code 1abb), contains a pi helix in addition to the above structures. illustrates the relationship of the protein backbone to the secondary structure representation. The structure of a human transferrin n-lobe mutant (PDB code 1dtg) shows the presence of alpha helices, 3 10 helices, beta-sheets, and beta-turns. More detail and illustrations of these turns are at Turns in Proteins. γ-turns are made up of only three amino acids and are therefore a much tighter turn. β-turns are composed of four amino acids and can have several difference conformations. β-turn and γ-turn are the two types of turns. These structural differences and other characteristics of β-sheets can be seen at Sheets in Proteins. The strands making up the sheets can be parallel or antiparallel and the pleats in the sheet can be twisted as well as being parallel. Jmol colors them alpha helix, 3 10 helix and pi helix as shown in Helices in Proteins. The characteristics of these three helices are given at Helices in Proteins. Alpha helix, pi helix and 3 10 helix are the three types of helices with the alpha helix being the most important. There are three common secondary structures - helices, β-pleated sheets and turns, and there are several variations of each one of them. For this reason, on a Ramachandran plot, the values for phi and psi are located at a particular area of the plot for each secondary structure. Each type of secondary structure has segments that have a repeating conformational pattern which is produced by a repeating pattern of values for the phi and psi torsional angles. Secondary structure of a protein refers to the three-dimensional structure of local segments of a protein.
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