A Genome-Scale Metabolic Model of Cryptosporidium hominis by Niti Vanee a ), Seth B. Roberts b ), Stephen S. Fong b ), Patricio Manque a ) c ), and Gregory A. Buck* a ) c ) a ) Center for the Study of Biological Complexity, VCU Life Sciences, Virginia Commonwealth University, P.O. Box 980678, 1101 E. Marshall St., 5-036 Sanger Hall, Richmond, VA 23298-0678, USA (phone: þ 1-804-8282318; fax: þ 1-804-8281397; e-mail: gabuck@vcu.edu) b ) Chemical and Life Sciences Engineering, School of Engineering, Virginia Commonwealth University, Richmond, VA, USA c )Microbiology and Immunology, School of Medicine, Virginia Commonwealth University, Richmond, VA, USA The apicomplexan Cryptosporidium is a protozoan parasite of humans and other mammals. Cryptosporidium species cause acute gastroenteritis and diarrheal disease in healthy humans and animals, and cause life-threatening infection in immunocompromised individuals such as people with AIDS. The parasite has a one-host life cycle and commonly invades intestinal epithelial cells. The current genome annotation of C. hominis , the most serious human pathogen, predicts 3884 genes of which ca. 1581 have predicted functional annotations. Using a combination of bioinformatics analysis, biochemical evidence, and high-throughput data, we have constructed a genome-scale metabolic model of C. hominis . The model is comprised of 213 gene-associated enzymes involved in 540 reactions among the major metabolic pathways and provides a link between the genotype and the phenotype of the organism, making it possible to study and predict behavior based upon genome content. This model was also used to analyze the two life stages of the parasite by integrating the stage-specific proteomic data for oocyst and sporozoite stages. Overall, this model provides a computational framework to systematically study and analyze various functional behaviors of C. hominis with respect to its life cycle and pathogenicity. Introduction. – Cryptosporidium is a protozoan parasite that belongs to the family of apicomplexans. It causes acute gastroenteritis and diarrhea. In developing countries, diarrhea is the one of the leading causes of death in young children (age less than five years), and Cryptosporidium is responsible for ca. 30 – 50% of those deaths [1]. In developed countries, due to its resistance to standard water treatment, cryptospor- idiosis is a threat to water supplies and is a major cause of disease related to recreational water [2 – 4] . It is a serious threat to the immunocompromised individuals, including people with AIDS. In Milwaukee, Wisconsin, in 1993, over 400,000 people were affected in an outbreak of cryptosporidiosis from contaminated water supplies [5]. The two most studied species, C. hominis and C. parvum, which differ in host range, genotype, and pathogenicity, are most relevant to humans. C. hominis infects only humans, whereas C. parvum also infects other mammals [6] [7]. For both species, oocysts are transmitted via the fecal-oral route. The thick-walled oocyst stage of the parasite excysts in the gastrointestinal system of the host, and each cyst releases 4 – 8 sporozoites, which invade the gastrointestinal epithelium. In healthy humans, Cryptosporidium causes a normally self-limiting watery diarrhea, abdominal cramps or pain, dehydration, nausea, vomiting, fever, and weight loss. In immunocompromised individuals (people with HIV, AIDS, or transplant recipients), the parasites cause a CHEMISTRY & BIODIVERSITY – Vol. 7 (2010) 1026  2010 Verlag Helvetica Chimica Acta AG, Zürich