Analysis and Design of Turns in alpha-Helical Hairpins

Although the anal. and design of turns that connect the strands in antiparallel b-hairpins has reached an advanced state, much less is known concerning turns between antiparallel helixes in helical hairpins. We have conducted an anal. of the structures and sequence preferences of two types of interhelical turns, each of which connects the two helixes by a two-residue linker in an aL-b conformation. Based on this anal., it became apparent that the turn introduced into a designed four-helix bundle protein, DF1, did not occur within an optimal structural context. DF1 is a dimeric model for the diiron class of proteins. A longer loop with a b-aR-b conformation was inserted between two helixes in the protein, and a sequence was chosen to stabilize its conformation. X-ray crystallog. and NMR anal. of the protein showed the structure to be in excellent agreement with design.