The Thermodynamics of Ligand Binding to the Aminoglycoside <i>O-</i>Nucleotidyltransferase(4′) and Variants Yields Clues about Thermophilic Properties

The aminoglycoside nucleotidyltransferase(4′) is an enzyme with high substrate promiscuity and catalyzes the transfer of the AMP group from ATP to the 4′-OH site of many structurally diverse aminoglycosides, which results in the elimination of their effectiveness as antibiotics. Two thermostable variants carrying single-site mutations are used to determine the molecular properties associated with thermophilicity. The thermodynamics of enzyme–ligand interactions showed that one variant (T130K) has properties identical to those of the mesophilic wild type (WT) while the other (D80Y) behaved differently. Differences between D80Y and the T130K/WT pair include the change in heat capacity (ΔCp), which is dependent on temperature for D80Y but not for WT or T130K. The change in ΔCp with temperature (ΔΔCp) with D80Y is dependent on aminoglycoside only in H2O and remains the same with all aminoglycosides in D2O. Furthermore, the offset temperature (Toff), the temperature difference that yields identical enthalpies in H2O and D2O, becomes larger with an increase in temperature for WT and T130K but remains mostly unchanged for D80Y. Studies in H2O and D2O revealed that solvent reorganization becomes the major contributor to ligand binding with an increase in temperature for WT and T130K, but changes in low-frequency vibrational modes are the main contributors with D80Y. Data presented in this paper suggest that global properties associated with the enzyme–ligand interactions, such as the thermodynamics of ligand binding, may yield clues about thermophilicity and permit us to distinguish those variants that are simply a more thermostable version of the mesophilic protein