The binding of UDP-N-acetylglucosamine (UDPNAG) to the enzyme UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) was studied in the absence and presence of the antibiotic fosfomycin by isothermal titration calorimetry. Fosfomycin binds covalently to MurA in the presence of UDPNAG and also in its absence as demonstrated by MALDI mass spectrometry. The covalent attachment of fosfomycin affects the thermodynamic parameters of UDPNAG binding significantly: In the absence of fosfomycin the binding of UDPNAG is enthalpically driven (DeltaH = -35.5 kJ mol(-1) at 15 degrees C) and opposed by an unfavorable entropy change (DeltaS = -25 J mol(-1) K(-1)). In the presence of covalently attached fosfomycin the binding of UDPNAG is entropically driven (DeltaS = 187 J mol(-1)K(-1) at 15 degrees C) and associated with unfavorable changes in enthalpy (DeltaH = 28.8 kJ mol(-1)). Heat capacities for UDPNAG binding in the absence or presence of fosfomycin were -1.87 and -2.74 kJ mol(-1) K(-1), respectively, indicating that most ( approximately 70%) of the conformational changes take place upon formation of the UDPNAG-MurA binary complex. The major contribution to the heat capacity of ligand binding is thought to be due to changes in the solvent-accessible surface area. However, associated conformational changes, if any, also contribute to the experimentally measured magnitude of the heat capacity. The changes in solvent-accessible surface area were calculated from available 3D structures, yielding a DeltaC(p) of -1.3 kJ mol(-1) K(-1); i.e., the experimentally determined heat capacity exceeds the calculated one. This implies that other thermodynamic factors exert a large influence on the heat capacity of protein-ligand interactions.