Pretransition and progressive softening of bovine carbonic anhydrase II as probed by single molecule atomic force microscopy.

To develop a simple method for probing the physical state of surface adsorbed proteins, we adopted the force curve mode of an atomic force microscope (AFM) to extract information on the mechanical properties of surface immobilized bovine carbonic anhydrase II under native conditions and in the course of guanidinium chloride-induced denaturation. A progressive increase in the population of individually softened molecules was probed under mildly to fully denaturing conditions. The use of the approach regime of force curves gave information regarding the height and rigidity of the molecule under compressive stress, whereas use of the retracting regime of the curves gave information about the tensile characteristics of the protein. The results showed that protein molecules at the beginning of the transition region possessed slightly more flattened and significantly more softened conformations compared with that of native molecules, but were still not fully denatured, in agreement with results based on solution studies. Thus the force curve mode of an AFM was shown to be sensitive enough to provide information concerning the different physical states of single molecules of globular proteins.