Citrate synthase (CS) is a Krebs tricarboxylic acid cycle enzyme that catalyzes the synthesis of citrate from oxaloacetate and acetyl coenzyme A. It is found in nearly all cells capable of oxidative metablism. This protein is nuclear encoded and transported into the mitochondrial matrix, where the mature form is found. Recombinant human CS protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.