Abstract
Interactions among Cd2+, glutamic acid (Glu), and adenosine 5′-triphosphate (ATP) have been studied by potentiometric pH titration, IR, Raman, fluorescence, and NMR methods. In the Cd2+–ATP binary system, the main interaction sites are the α-, β-, and γ-phosphate groups, N-1, and/or N-7. Cd2+ binds to the N-1 site at relatively low pH and binds to the N-7 site of adenosine ring of ATP with increasing pH. In the Cd2+–Glu–ATP ternary system, ATP mainly binds to Cd2+ by the triphosphate chain. Oxygens of Glu coordinate with Cd2+ to form a complex to catalyze ATP hydrolysis. Hydrolysis of ATP catalyzed by the CdGlu complex was studied at pH 7.0 and 80°C by 31P-NMR spectrometry. Kinetics studies showed that the rate constant of ATP hydrolysis was 0.0199 min−1 in the ternary system, which is 9.9-fold faster than that in the ATP solution (2.01 × 10−3 min−1). Hydrolysis occurs through an addition–elimination reaction mechanism with Cd2+ regulating the recognition and catalytic hydrolysis of ATP; water participates in the hydrolysis reaction of ATP at different steps with different functions in the ternary system.
Acknowledgments
The authors thank Mr Zhiqiang Shen for his technical assistance in obtaining NMR spectra. This study is supported by the 973 Program under the Grant no. 2009CB22003.