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

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Abstract
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 theC α −C β bond of the side chain isg + ort. This preference is reduced to only thet 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 [Pielaet al. (1987),Biopolymers 1987, 1273–1286]. These results suggest that replacement of Trp by its analog 5-hydroxyTrp may be tolerated in anα-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.