Effect of Tb(III) on activity and stability of nattokinase

Nattokinase, is an effective fibrinolytic enzyme with the potential for fighting cardiovascular disease. The aim of study was to investigate the interaction of Tb(III) with nattokinase and the impact of Tb(III) on the enzyme activity and protein stability. The binding of Tb(III) with nattokinase was studied by fluorescence spectrum in 100 mmol/L Tris-HCl (pH 8.0). It could be seen that the protein bound one Tb(III) with low affinity, and the binding constants K were 2.90×104 L/mol at 288 K. Although the activity of nattokinase determined by tetra-peptide substrate method at proper pH and temperature was not influenced for the binding of Tb(III), the transformation rate of substrate was increased to 113%. To better assess the stability of protease in the absence and presence of Tb(III), nattokinase was unfolded through continuous concentrations urea. Based on the model of structural element, the results showed that Tb(III) could not change the average structural element free energy <ΔG0element (H2O)> of nattokinase by the measurement of enzyme activity, but it could improve the stability of the global protein by the fluorescence spectral measurement.