THERMALLY-INDUCED UNFOLDING OF THE TRYPTOPHAN SYNTHASE ALPHA(2)BETA-2MULTIENZYME COMPLEX FROM SALMONELLA-TYPHIMURIUM

Tryptophan synthase (TS) from Salmonella typhimurium (144,000 M(r)) is a bifunctional enzyme composed of two heterodimers arranged in an ext ended alpha beta beta alpha geometry [Hyde et al., J. Biol. Chem. 263, 17857, 1988]. The thermal unfolding of TS alpha(2) beta(2) and isolat ed alpha and beta subunits has been studied by differential scanning c alorimetry (DSC), circular dichroism, and UV spectroscopy. DSC profile s of the holo-alpha(2) beta(2) complex containing pyridoxal phosphate (PLP) bound to each beta chain are characterized by two well-resolved endotherms centered at approximate to 52 and approximate to 81 degrees C. Conformational transitions in beta chains (T-m approximate to 46 d egrees C) as well as sequential unfolding reactions in a chains contri bute to the low-temperature endotherm whereas major unfolding of the b eta dimer occurs between 74 and 82 degrees C. The thermally induced un folding disrupts approximate to 70% of the secondary structure with a total enthalpy change of 3600 +/- 100 kJ mol(-1) for holo-alpha(2) bet a(2). Stabilizing interactions between alpha and beta chains are appar ent. The cofactor PLP not only markedly stabilizes beta chains in beta (2) but also increases the cooperativity of unfolding beta chains. Cal orimetric and spectral measurements suggest a sequential unfolding pat hway for the tryptophan synthase alpha(2) beta(2) complex. Thermodynam ic results are correlated with available structural information.