Crystallization and preliminary crystallographic analysis of cysteine synthase from Entamoeba histolytica

Entamoeba histolytica , the causative agent of human amoebiasis, is essentially anaerobic, requiring a small amount of oxygen for growth. It cannot tolerate the higher concentration of oxygen present in human tissues or blood. However, during tissue invasion it is exposed to a higher level of oxygen, leading to oxygen stress. Cysteine, which is a vital thiol in E. histolytica , plays an essential role in its oxygen‐defence mechanisms. The major route of cysteine biosynthesis in this parasite is the condensation of O ‐acetylserine with sulfide by the de novo cysteine‐biosynthetic pathway, which involves cysteine synthase (EhCS) as a key enzyme. In this study, EhCS was cloned, expressed in Escherichia coli and purified by affinity and size‐exclusion chromatography. The purified protein was crystallized in space group P 4 1 with two molecules per asymmetric unit and a complete data set was collected to a resolution of 1.86 Å. A molecular‐replacement solution was obtained using the Salmonella typhimurium O ‐acetylserine sulfhydrylase structure as a probe and had a correlation coefficient of 37.7% and an R factor of 48.8%.