Effects on Protein Structure and Function of Replacing Tryptophan with 5-Hydroxytryptophan: Single-Tryptophan Mutants of the N-Terminal Domain of the Bacteriophage A. 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^α-C^β 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) α-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 a-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 λ 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.