Affinity properties of phosvitin: Interaction of phosvitin with serine hydroxymethyl transferase

The affinity of phosvitin with serine hydroxymethyl transferase (SHMT), an acidic multi-subunit protein, was evaluated by measurements of enzyme activ ity, sedimentation velocity, steady-state fluorescence, circular dichroism and kinetic thermal stability. While the presence of phosvitin had no effec t on the SHMT activity, the sedimentation coefficient of SHMT increased fro m 8.7 S to 12.5 S suggesting the formation of a complex at a SHMT:phosvitin molar ratio of 2:1. Based on steady-state fluorescence quenching measureme nts an association constant of 2.4 +/- 0.2 x 10(5) M-1 at 25 degrees C was obtained for the interaction of phosvitin with SHMT. The temperature depend ency of the association constant in the range 15-35 degrees C suggests the involvement of ionic forces in the interaction. The thermal inactivation of SHMT followed first order kinetics. In the presence of phosvitin the rate constant decreased and half time increased. The circular dichroism measurem ents suggest that phosvitin interaction does not involve pyridoxal phosphat e binding domain of the enzyme. Although minor changes in the secondary str ucture of the enzyme were observed, the environment around aromatic amino a cids did not change significantly.