The oxidation and correct formation of disulfide bonds is an important biochemical modification of many proteins. Early observations showed that disulfide bond formation proceeds much faster in vivo than in vitro, suggesting the existence of a catalyst for oxidative protein folding in living cells [1]. Protein disulfide isomerase (PDI, EC 5.3.4.1) is a eukaryotic oxidoreductase that catalyzes the oxidation, reduction, and isomerization of disulfide bonds in nascent polypeptides [2]. The subcellular localization and its function suggest that PDI plays a key role in the folding of proteins delivered to the secretory pathway [3]. For the protozoan parasite Entamoeba histolytica, the causative agent of human amoebiasis, the accurate formation of disulfide bonds is an important biochemical modification necessary for the correct folding of some proteins, including proteins involved in the adhesion and destruction of human tissues. Amoebic proteins such as the inhibitory lectin Gal/GalNAc and pore-forming peptides have disulfide bonds that are crucial for the acquisition of their active conformation [4, 5]. Therefore, the identification and characterization of amoebic enzymes that play a key role in protein folding is essential to understand this biochemical process and gain in-depth knowledge of the cellular biology of this parasite. An entamoebal PDI enzyme (EhPDI) that exhibits oxidative folding activity in vivo as well as oxidative and reductive activities in vitro have been identified [6, 7]. Structurally, EhPDI shares domain architecture with the Dictyostelium discoideum homolog, DdPDI [8], characterized by two active thioredoxin domains and a D domain (also known as the Erp29c domain). Interestingly, both enzymes lack the canonical ER-retentive...... center of paper ......zolides, Exp. Parasitol. (2008) 80-88.[22] T. Kimura, Y. Hosoda, Y. Kitamura, H. Nakamura, T. Horibe, M. Kikuchi, Functional differences between human and yeast protein disulfide isomerase family proteins, Biochem. Biof. Res. Co.320 (2004) 359-365.[23] CW Gruber, M. Cemazar, B. Heras, JL Martin, DJ Craik, Protein disulfide isomerase: the structure of oxidative folding, Trends Biochem. Sci. 31 (2006) 455-464.[24] MA Ramos, RE Mares, PD Magaña, JE Ortega, JM Cornejo-Bravo, In silico identification of the protein disulfide isomerase family from a protozoan parasite, Comput. Biol Chem. 32 (2008) 66-70.[25] DR Boettner, CD Huston, AS Linford, SN Buss, E. Houpt, NE Sherman, WA Petri Jr., 2008. Phagocytosis of human erythrocytes by Entamoeba histolytica involves PATMK, a member of the transmembrane kinase family, PLoS Pathog. 4 (2008) e8.