Modulation of activity of NADH oxidase from Thermus thermophilus through change in flexibility in the enzyme active site induced by Hofmeister series anions

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Abstract
The conformational dynamics of NADH oxidase from Thermus thermophilus was modulated by the Hofmeister series of anions (H2PO4–, SO42–, CH3COO–, Cl–, Br–, I–, ClO4–, SCN–) in the concentration range 0–3 M. Both chaotropic and kosmotropic anions, at high concentration, inhibit the enzyme by different mechanisms. Chaotropic anions increase the apparent Michaelis constant and decrease the activation barrier of the reaction. Kosmotropic anions have the opposite effect. Anions from the middle of the Hofmeister series do not significantly affect the enzyme activity even at high concentration. We detected no significant changes in ellipticity of the aromatic region in the presence of the anions studied. There is a decreased Stern–Volmer quenching constant for FAD fluorescence quenching in the presence of kosmotropic anions and an increased quenching constant in the presence of chaotropic anions. All of this indicates that active site flexibility is important in the function of the enzyme. The data demonstrate that both the high rigidity of the active site in the presence of kosmotropic anions, and its high flexibility in the presence of chaotropic anions have a decelerating effect on enzyme activity. The Hofmeister series of anions proved to be suitable agents for altering enzyme activity through changes in flexibility of the polypeptide chain, with potential importance in modulating extremozyme activity at room temperature.

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