THERMODYNAMICS OF LIGAND-BINDING TO ACYL-COENZYME A BINDING-PROTEIN STUDIED BY TITRATION CALORIMETRY

Ligand binding to recombinant bovine acyl-CoA binding protein (ACBP) w as examined using isothermal microcalorimetry. Microcalorimetric measu rements confirm that the binding affinity of acyl CoA esters for ACBP is strongly dependent on the length of the acyl chain with a clear pre ference for acyl-CoA esters containing more than eight carbon atoms an d that the 3'-phosphate of the ribose accounts for almost half of the binding energy, Binding of acyl-CoA esters, with increasing chain leng th, to ACBP was clearly enthalpically driven with a slightly unfavorab le entropic contribution. Accessible surface areas derived from the me asured enthalpies were compared to those calculated from sets of three -dimensional solution structures and showed reasonable correlation, co nfirming the enthalphically driven binding. Binding of dodecanoyl-CoA to ACBP was studied at various temperatures and was characterized by a weak temperature dependence on Delta G(o) and a strong enthalpy-entro py compensation. This was a direct consequence of a large heat capacit y Delta C-p, caused by the presence of strong hydrophobic interactions . Furthermore, the binding of dodecanoyl-CoA was studied at various pH values and ionic strengths. The data presented here state that ACBP b inds long-chain acyl-CoA esters with very high affinity and suggest th at ACBP acts as a housekeeping protein with no pronounced built-in spe cificity.