Nonrepetitive Structure Connection Loop and Turn

The right-handed helices and extended P strands are the only protein conformations in which the same ( ), / angles repeat for each consecntive residne as regular, repetitive structures. The remaiiung portions of protein structures are made up of well-ordered but nonrepetitive conformations referred to as coils. Nonrepetitive structure involve both backbone hydrogen bonds and freqnent side chain-to-main chain hydrogen bonds. The reliance on specific side chain interactions is characteristic for nonrepetitive structures. The nonrepetitive stmctnre consists of two general types connection or loop and turn. 

The conformations of turn regions depend primarily on the positions of certain amino acid residnes (usually Gly, Pro or Asn) in the loop (Hutchinson and Thornton, 1994). The type I P turn can accommodate any amino acid at positions i through i + 3, except that Pro cannot be at position i -i- 2. For the type I and II P turns, Gly and Pro predominate at positions i + 3 and i + 1 respectively, whereas Asn, Asp, Cys and Ser occur fie-quently at position i. The type II P turn shows preference for Gly and Asn at position i + 2. Gly occurs at positions i + 1 and i -i- 2 in type T as well as type IIT turns, and i + 1 in type IT turns. The conformation of the type HI turn corresponds to that of one turn of the 3.0io-helix, while Type Via and VIb have a cis peptide bond. 

Bend type Dihedral angles of central residue(s) (deg)  

IV Any bend with two or more angles differing by 40° from those given  

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