Conformational Changes in the Tryptophan Synthase from a Hyperthermophile upon α2β2 Complex Formation: Crystal Structure of the Complex,

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Abstract
The three-dimensional structure of the bifunctional tryptophan synthase α2β2 complex from Pyrococcus furiosus was determined by crystallographic analysis. This crystal structure, with the structures of an α subunit monomer and a β2 subunit dimer that have already been reported, is the first structural set in which changes in structure that occur upon the association of the individual tryptophan synthase subunits were observed. To elucidate the structural basis of the stimulation of the enzymatic activity of each of the α and β2 subunits upon α2β2 complex formation, the conformational changes due to complex formation were analyzed in detail compared with the structures of the α monomer and β2 subunit dimer. The major conformational changes due to complex formation occurred in the region correlated with the catalytic function of the enzyme as follows. (1) Structural changes in the β subunit were greater than those in the α subunit. (2) Large movements of A46 and L165 in the α subunit due to complex formation caused a more open conformation favoring the entry of the substrate at the α active site. (3) The major changes in the β subunit were the broadening of a long tunnel through which the α subunit product (indole) is transferred to the β active site and the opening of an entrance at the β active site. (4) The changes in the conformations of both the α and β subunits due to complex formation contributed to the stabilization of the subunit association, which is critical for the stimulation of the enzymatic activities.

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