タンパク質の結晶化

Protein crystallization provides three-dimensional structural information that other techniques cannot. Mass spectrometry and SDS-PAGE only reveal one-dimensional protein structure, whereas crystallization combined with x-ray diffraction produces a complete 3D model showing atomic arrangement with typical resolution of 2 angstroms, enabling detailed study of protein function and mechanism.

The first step is obtaining milligram quantities of very pure protein, typically using recombinant protein expression. The gene of interest is cloned into an expression vector, and the expressed protein is fused to an affinity tag like poly-histidine. This tag assists in purification by chromatography-based biomolecule purification methods, ensuring the protein is sufficiently pure for crystallization.

In vapor diffusion, a droplet containing protein, buffer, and precipitant is placed above a reservoir with higher precipitant concentration. Water vaporizes from the droplet into the reservoir, decreasing water availability and increasing protein concentration. This creates a supersaturated solution where nucleation occurs, allowing protein molecules to arrange into an ordered crystal lattice.

Successful crystallization requires careful control of pH, ionic strength, precipitant concentration, protein concentration, temperature, and rate of equilibration. The proper combination of these factors determines whether crystals form. Too-low initial concentrations prevent crystallization, while the system must reach equilibrium gradually to allow ordered crystal growth rather than amorphous precipitation.

A crystal is exposed to a monochromatic x-ray beam at all angles, producing diffraction patterns registered by a detector. Each spot represents a diffracted x-ray emerging from the crystal. Software converts two-dimensional images taken at different orientations into a three-dimensional electron density model, determining atomic positions and revealing the complete protein structure.

Integral membrane proteins like G-protein coupled receptors have limited polar surface area available for forming crystal lattice contacts, making crystallization extremely difficult. This limitation led to fusion-protein-assisted crystallization, where a hydrophilic protein like lysozyme is fused to the membrane protein, increasing extracellular hydrophilic surface and enabling the packing interactions necessary for crystal formation.

Protein crystallization enables in silico drug design by revealing three-dimensional binding sites. For example, the Influenza virus polymerase basic protein 2 structure was determined by crystallization and x-ray diffraction, allowing researchers to visualize potential binding sites and use docking programs to design molecules that insert into protein clefts for therapeutic targeting.