Effects on protein structure and function of replacing tryptophan with 5-hydroxytryptophan: Single-tryptophan mutants of the N-terminal domain of the bacteriophage lambda repressor

Conformational energy computations have been carried out on the N-acetyl-N'-methylamide of 5-hydroxytryptophan (5OH-Trp) using ECEPP/3. As observed with tryptophan (Trp), the most preferred conformation about the C-alpha-C-beta bond of the side chain is g(+) or t. This preference is reduced to only the t conformational state when 5-hydroxyTrp is in the middle of a right-handed poly(L-alanine) alpha-helix. A similar result has been obtained with Trp [Piela et al. (1987), Biopolymers 1987, 1273-1286]. These results suggest that replacement of Trp by its analog 5-hydroxyTrp may be tolerated in an alpha-helix. To test this hypothesis, we have replaced Trp by 5OH-Trp in the fifth helices of two functionally active mutants of the N-terminal domain of the bacteriophage lambda repressor, Computations on the packing of these helices have shown that no significant structural changes result from the replacement of Trp by 5OH-Trp. The DNA-binding activity of these mutants, as assessed indirectly through geometrical parameters, is also unaltered.