Crystal structure of the W35A mutant thioredoxin h from Chlamydomonas reinhardtii: The substitution of the conserved active site Trp leads to modifications in the environment of the two catalytic cysteines

The conformational analysis of W35A thioredoxin h from the eukaryotic green alga Chlamydomonas reinhardtii in the solid state has been carried out by x-ray diffraction, with the aim to clarify the role of Trp in the catalysis . Comparative analysis of W35A mutant with wild-type (WT) thioredoxin shows that, even if the structural motif of thioredoxin is not perturbed, the su bstitution of Trp35 by an Ala leads to significant changes in protein confo rmation near the active site. This rearrangement increases its solvent expo sure and explains the change of the pKa values of the catalytic cysteines. The substitution of the Trp residue also influences the crystal packing as well as the recognition ability of thioredoxin. The solid state analysis su ggests that the Trp residue has a structural function both to force the act ive site in the bioactive conformation, and to mediate the protein-protein recognition. (C) 2001 John Wiley & Sons, Inc.