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-acet yl-N'-methylamide of 5-hydroxytryptophan (5OH-Trp) using ECEPP/3. As o bserved with tryptophan (Trp), the most preferred conformation about t he 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 r esult has been obtained with Trp [Piela et al. (1987), Biopolymers 198 7, 1273-1286]. These results suggest that replacement of Trp by its an alog 5-hydroxyTrp may be tolerated in an alpha-helix. To test this hyp othesis, we have replaced Trp by 5OH-Trp in the fifth helices of two f unctionally active mutants of the N-terminal domain of the bacteriopha ge lambda repressor, Computations on the packing of these helices have shown that no significant structural changes result from the replacem ent of Trp by 5OH-Trp. The DNA-binding activity of these mutants, as a ssessed indirectly through geometrical parameters, is also unaltered.