Conformational Changes in Aspartate Transcarbamylase: I. STUDIES OF LIGAND BINDING AND OF SUBUNIT INTERACTIONS BY CIRCULAR DICHROISM SPECTROSCOPY

Abstract

Abstract In native aspartate transcarbamylase, the binding of carbamyl phosphate and succinate perturbs certain tyrosine residues which are not perturbed under the same conditions in the isolated catalytic subunit. These results, obtained using circular dichroism difference spectroscopy, suggest that the participation of tyrosyl residues in the allosteric transition relates to homotropic interactions between substrate binding sites. Association of isolated regulatory and catalytic subunits renders tyrosyl residues inaccessible to solvent, as judged by ultraviolet absorption difference spectra. Thus, since this association process regenerates cooperativity, both the introduction of conformational constraints by subunit association and the expression of allosteric interactions by ligand binding produce tyrosyl perturbations. It is concluded, therefore, that tyrosyl residues may be involved in the stereochemical mechanism of allosteric interactions in this enzyme. Circular dichroism features which are identical for ligand binding both with aspartate transcarbamylase and with the isolated catalytic subunit are carbamyl phosphate and succinate perturbations of tryptophanyl transitions. These tryptophanyl perturbations are, therefore, inherent features of ligand binding in the presence or absence of allosteric interactions and may result from the presence of tryptophan in the active site region.