Direct Observation of Enzyme Activity with the Atomic Force Microscope

The height fluctuations on top of the protein lysozyme adsorbed on mica were measured locally with an atomic force microscope operated in tapping mode in liquid. Height fluctuations of an apparent size of 1 nanometer that lasted for about 50 milliseconds were observed over lysozyme molecules when a substrate (oligoglycoside) was present. In the presence of the inhibitor chitobiose, these height fluctuations decreased to the level without the oligoglycoside. The most straightforward interpretation of these results is that the height fluctuations correspond to the conformational changes of lysozyme during hydrolysis. It is also possible, however, that the height fluctuations are, at least in part, the result of a different height or elasticity of the transient complex of lysozyme plus the substrate.