Thermodynamical properties of protein kinase with adenine inhibitors

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 k i and high k d ) and the smallest negative value of free energy change (ΔG) 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.