Analysis of two-residue turns in proteins.

The conformational properties of tight two-residue beta-turns in proteins are examined by empirical energy function calculations. Twenty-five tight turns are studied in isolation, in the presence of the protein, and in the presence of the protein and crystal water molecules. The conformational properties are subdivided into those that are intrinsic to the turn and those that depend on the protein and water environment. Two factors are shown to determine the conformation of a tight beta-turn. One is the twist of the beta-sheet (responsible for selecting either a type I' or II' conformation as opposed to the more common types I or II) and the other is a local electrostatic effect (responsible for distinguishing between the type I' and II' conformations). In the rare cases where a two-residue turn is found in a type I conformation, there exists a stabilizing feature (turn-protein interaction, a side-chain in a conformation that stabilizes type I, etc.) which compensates for the unfavorable twist of the turn relative to the beta-sheet.