Chirality and Handedness of Protein Structures

In proteins, the polypeptide chain forms a number of right- and left-handed helices and superhelices, right- and left-turned hairpins, and some other structures that are nonsuperimposable, although they are not mirror images of each other as the L-amino acids are not converted to the D-amino acids. This property of protein structures will be referred to here as pseudo-chirality - or handedness. It has been shown that there are two kinds of handedness in proteins - helical handedness and handedness of arrangement. Some protein structures exhibit both the kinds of handedness. Handedness is observed at all levels of protein structural organization - from α-helices, β-strands, hairpins, βαβ-units up to complex structural motifs, superhelices, and supramolecular structures in fibrous and polymer proteins. There are several structures that have unique handedness in proteins, for example, α-helices, αα-corners, βαβ-units, abcd-units, and so on. This property of the polypeptide chain is of particular value in protein folding and protein modeling, because it drastically reduces the number of possible folds.