Influence of Allosteric Effector Substances on the Structure and Catalytic Activity of Tryptophan Oxygenase

Abstract

Abstract Interactions at the allosteric and catalytic sites of Pseudomonas tryptophan oxygenase were explored by measuring the effects of tryptophan, α-methyltryptophan, and indole on the catalytic, spectral, and structural properties of the native enzyme in the presence and in the absence of the anionic detergent sodium dodecyl sulfate (SDS). In the absence of effector substances, SDS treatment leads to an irreversible loss of catalytic activity and a profound hypochromicity in the Soret region of the absorption spectrum of tryptophan oxygenase. Either tryptophan or α-methyltryptophan antagonizes this effect of SDS, i.e. a higher concentration of SDS is required to produce an equivalent response in the presence of either effector substance. The competitive inhibitor indole, however, exerts no such protective effect. Sedimentation studies revealed that SDS promotes dissociation of the tetrameric form of the native enzyme into catalytically inactive subunits. α-Methyltryptophan markedly protects the native enzyme against the SDS-induced disruption of its quaternary structure. The structural changes elicited by the action of SDS correlate well with the catalytic and spectral changes effected by the detergent, indicating that both the catalytic and the chromophoric properties of the native enzyme are dependent upon an intact quaternary structure. These results are compatible with the view that the binding of tryptophan or α-methyltryptophan to allosteric sites mediates a conformational alteration in quaternary structure of such a nature that the noncovalent bonds between subunits are strengthened, thereby requiring a higher concentration of SDS to effect dissociation.