Crystal structures of threonine synthase from Thermus thermophilus HB8: conformational change, substrate recognition, and mechanism.

Threonine synthase, which is a PLP-dependent enzyme, catalyzes the beta,gamma-replacement reaction of l-homoserine phosphate to yield threonine and inorganic phosphate. The three-dimensional structures of the enzyme from Thermus thermophilus HB8 in its unliganded form and complexed with the substrate analogue 2-amino-5-phosphonopentanoic acid have been determined at 2.15 and 2.0 A resolution, respectively. The complexed form, assigned as an enamine, uncovered the interactions of the cofactor-analogue conjugate with the active site residues. The binding of the substrate analogue induces a large conformational change at the domain level. The small domain rotates by about 25 degrees and approaches the large domain to close the active site. The complicated catalytic process of the enzyme has been elucidated based on the complex structure to reveal the stereochemistry of the reaction and to present the released inorganic phosphate as a possible catalyst to carry a proton to the Cgamma atom of the substrate.