The Finding of Nondissolving Lysozyme Crystals and Its Significance for the Study of Hard-to-Crystallize Biological Macromolecules

Major discoveries in structural biology depend on obtaining well-diffracting macromolecular crystals. This necessity has motivated many fundamental studies on protein crystallization using lysozyme as a model system. In the present contribution, we report the unprecedented observation of lysozyme crystals that stop dissolving under undersaturated conditions imposed to sub-microliter crystallization drops at mild temperatures. Subsequent growth of the same crystals is apparently undisturbed after the drops are cooled below the saturation temperature. The succession of heating/cooling cycles only partially recovers crystal dissolution while crystal growth becomes gradually slower. Ultimately, increasing and decreasing the temperature between 10 and 37 °C has no visible effect on the size of the crystals. We ascribe this phenomenon to the partial denaturation of the soluble protein in the drop as evidenced by the decreasing glycoside hydrolase activity of lysozyme over the incubation time. The disturbances in the phase transition processes are explained as the result of the changed chemical potential due to different folding states. In a time when the high-hanging fruits in structural biology have to be picked, the present findings call attention to interfacial phenomena as an important, though often imperceptible, aspect that affects protein stability and justifies further optimization of current crystallization methods.