Thermodynamics of nucleotide binding to NBS-I of the Bacillus subtilis preprotein translocase subunit SecA

SecA is the dissociatable nucleotide and preprotein binding subunit of the bacterial translocase, The thermodynamics of nucleotide binding to soluble SecA at nucleotide binding site I were determined by isothermal titration c alorimetry. Binding of ADP and non-hydrolyzable ATP gamma S is enthalpy-dri ven (Delta H-0 of -14.44 and -5.56 kcal/mol, respectively), but is accompan ied by opposite entropic contributions (Delta S-0 of -18.25 and 9.55 cal/mo l/K, respectively). ADP binding results in a large change in the heat capac ity of SecA (Delta C-p = -780 cal/mol/K), It is suggested that ADP binding promotes the interaction between the two thermodynamically discernible doma ins of SecA which is accompanied by a shielding of hydrophobic surface from solvent. (C) 1999 Federation of European Biochemical Societies.