Changing activity of ribonuclease A during adsorption: a molecular explanation

The activity of ribonuclease A (RNase A) during adsorption onto molecular smooth mica increases from 16% to 78% in a period of 24 h when compared to its activity in free solution at pH 5 and 20 +/- 0.5 degrees C. From electropotential plots, the tertiary structure of RNase A, the characteristics of the mica surface, and direct measurements of the intermolecular forces between two adsorbed enzyme layers, a molecular explanation is offered for the changing activity with time. Initially, the RNase A molecules lie flat-on the mica with their smallest axis perpendicular to and their active site facing the surface. As adsorption proceeds, the molecules slowly reorient until at long times they lie end-on with their largest axis perpendicular to the surface and their active site partially exposed to the free solution. A translational diffusion process is indicated for the phase transition and molecular reorientation of the RNase A molecules.