Abstract
γ-Glutamylcysteine synthetase (GCS) catalyzes the first and rate-limiting step of biosynthesis of a ubiquitous tripeptide glutathione and is a target for development of potential therapeutic agents such as drugs that can suppress multidrug resistance of cancer cells. The optimization of the GCS crystallization using the High Density Protein Crystal Growth (HDPCG) cells was carried out on the ground for the microgravity crystal growth on Space Shuttle Mission STS-107. The crystals were grown using sodium formate as a precipitant and their nucleation and growth rate were analyzed. The crystallization experiments, the pH measurement and the ion chromatography suggested that formic acid was transported from the reservoir solution to the enzyme solution by vapor diffusion. Therefore, a local concentration of the precipitant on the free surface of the enzyme solution may cause the earlier nucleation as a trigger of crystallization. The final conditions of GCS crystallization were determined based on this new trigger model.
