Thermodynamics of the Interaction between O-Acetylserine Sulfhydrylase and the C-Terminus of Serine Acetyltransferase^†

Cysteine biosynthesis in plants is partly regulated by the physical association of O-acetylserine sulfhydrylase (OASS) and serine acetyltransferase (SAT). Interaction of OASS and SAT requires only the 10 C-terminal residues of SAT. Here we analyze the thermodynamics of formation of a complex of Arabidopsis thaliana OASS (AtOASS) and the C-terminal ligand of AtSAT (C10 peptide) as a function of temperature and salt concentration using fluorescence spectroscopy and isothermal titration calorimetry (ITC). Our results suggest that the C-terminus of AtSAT provides the major contribution to the total binding energy in the plant cysteine synthase complex. The C10 peptide binds to the AtOASS homodimer in a 2:1 complex. Interaction between AtOASS and the C10 peptide is tight (K_d = 5−100 nM) over a range of temperatures (10−35 °C) and NaCl concentrations (0.02−1.3 M). AtOASS binding of the C10 peptide displays negative cooperativity at higher temperatures. ITC studies reveal compensating changes in the enthalpy and entropy of binding that also depend on temperature. The enthalpy of interaction has a significant temperature dependence (ΔC_p = −401 cal mol^-1 K^-1). The heat capacity change and salt dependence studies suggest that hydrophobic interactions drive formation of the AtOASS·C10 peptide complex. The potential regulatory effect of temperature on the plant cysteine synthase complex is discussed.