The protein-based molecular recognition of the adenine ring is essential to understand protein function and drug design as well.In this paper,a variety of the adenine-based inhibitors with modified groups of amino groups,nitrogen and oxygen atoms in the aromatic ring are designed,and the binding capability of these adenosine analogues with an aminoglycoside antibiotic kinase [APH(3')-IIIa] are investigated with activity assays and isothermal titration calorimetry(ITC) experiments.1-aminoisoquinoline is one of the weakest substrates bound to APH(3')-IIIa with the lowest affinity(high ki and high kd) and the smallest negative value of free energy change(G) among the inhibitors tested.The binding process of adenine and 5-nitroisoquinoline to APH(3')-IIIa is an enthalpy-driven event with unfavorable entropy,which is consistent with the energy change induced by the binding of ATP to the enzyme.However,the reverse is true for 1-aminoisoquinoline,3-amino-5-nitrobenzisothiazole,5-aminoisoquinoline binding to the enzyme because the entropy is more favorable and the enthalpy makes a lower contribution to the binding process.These results suggest that small changes of the adenine ring can lead to significant influence on the ability of these analogues to occupy the adenine-binding region of the enzyme,which can be the potential inhibitors as drug candidates against the bacterial resistance.
