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Skręty zwrotne w peptydach i białkach. Mimetyki skrętów zwrotnych. Część 1

Owińska M.

Wiadomości Chemiczne

2016

[Z] 70, 9-10

598--611

The secondary structure of the proteins can be divided into α-helix, βsheet and reverse turns. A reverse turn is defined as a site where a polypeptide chain reverses its overall direction. It leads the chain to fold back on itself by almost 180°. Another condition for defining this motif is the amount of amino acid residues involved in forming the turn that can not be greater then six and polypeptide chain in this region can not be in helical conformation [1]. Turns are classified as: γ-turn, β-turn, α-turn and π-turn, which are formed by 3-, 4-, 5- and 6- amino acid residues, respectively. Turns defined as “closed” are stabilized by intramolecular hydrogen bonds between the main chain carbonyl group from the first residue and the main chain amide group from the last residue in the turn. This results in formation of 7-, 10-, 13- and 16-membered pseudo-rings, respectively (Fig. 1, Fig. 2). The other group of turns also stabilized by hydrogen bond, but conversely between the main chain amide group of the first residue and the carbonyl group of the last residue, are δ-turn (2-residues, 8-membered pseudo-ring) and ε-turn (3-residues, 11-membered pseudo-ring) (Fig. 2) [10–13]. These are seldom found in proteins. Turns are considered irregular structures due to the lack of clearly defined torsion angle preferences. because of that, each of the turn types can be further divided into several different subtypes (Table 1, 2 and 3) [1, 15, 31]. The “open” type structures are not stabilized by hydrogen bonds, but Cα-Cα distance between the first and the last residue in main chain is up to 10 A [10]. As turns can be found mostly on the surface of the proteins they play the important role in folding processes, thus enabling the formation of the tertiary structure [2]. The turns are also responsible for the interactions between proteins, recognition processes and ligand-receptor interactions [3–8]. In the following article, the classification and characterization of the turn types is described. The particular attention was given to the γ and β turns, as these are most commonly found in proteins structure.