Solute transport in orthorhombic lysozyme crystals: a molecular simulation study

Guest-protein incorporation into solvent channels of a protein host crystal (hostal)

Soaking small molecules into the solvent channels of protein crystals is the most common method of obtaining crystalline complexes with ligands such as substrates or inhibitors. The solvent channels of some protein crystals are large enough to allow the incorporation of macromolecules, but soaking of protein guests into protein crystals has not been reported. Such protein host crystals (here given the name hostals) incorporating guest proteins may be useful for a wide range of applications in biotechnology, for example as cargo systems or for diffraction studies analogous to the crystal sponge method. The present study takes advantage of crystals of the Escherichia coli tryptophan repressor protein (ds-TrpR) that are extensively domain-swapped and suitable for incorporating guest proteins by diffusion, as they are robust and have large solvent channels. Confocal fluorescence microscopy is used to follow the migration of cytochrome c and fluorophore-labeled calmodulin into the solvent channels of ds-TrpR crystals. The guest proteins become uniformly distributed in the crystal within weeks and enriched within the solvent channels. X-ray diffraction studies on host crystals with high concentrations of incorporated guests demonstrate that diffraction limits of ∼2.5 Å can still be achieved. Weak electron density is observed in the solvent channels, but the guest-protein structures could not be determined by conventional crystallographic methods. Additional approaches that increase the ordering of guests in the host crystal are discussed that may support protein structure determination using the hostal system in the future. This host system may also be useful for biotechnological applications where crystallographic order of the guest is not required.

Fabrication of polypyrrole nano-arrays in lysozyme single crystals

A template-directed method for the synthesis and organization of partially oxidized polypyrrole (PPy) nanoscale arrays within the solvent channels of glutaraldehyde-cross-linked lysozyme single crystals is presented. Macroscopic single crystals of the periodically arranged protein-polymer superstructure are electrically conductive, insoluble in water and organic solvents, and display increased levels of mechanical plasticity compared with native cross-linked lysozyme crystals.

Modeling the mechanical response of tetragonal lysozyme crystals

We investigate the temperature- and humidity-dependent mechanical response of tetragonal lysozyme crystals using a continuum-based crystal plasticity model calibrated with existing experimental data. The effects of temperature and intracrystalline water are incorporated in the elastic constant of the protein crystal as well as the critical resolved shear stress on the slip planes in the crystal plasticity model. The critical resolved shear stresses have been deduced from Vickers microhardness test data corresponding to different temperatures and states of hydration. Compression analyses have then been carried out along different crystallographic directions of lysozyme crystals which reveal that their mechanical response is highly anisotropic and orientation dependent, being purely elastic along the [110] direction but elastoplastic along the [100] and [212] directions. An interesting observation is that an increase in temperature and the amount of intracrystalline water molecules leads to a decrease in the critical resolved shear stress of the slip systems resulting in softening of the crystal. The analysis presented in this paper may be applied to the study of other protein crystal systems as well as their optimal design for biotechnological applications.