Heme proteins of Giardia intestinalis.

Among the few organisms that cannot make the iron cofactor heme, some nonetheless possess heme proteins. This includes the protozoan parasite Giardia intestinalis, which encodes five known heme proteins: a flavohemoglobin and four members of the cytochrome b5 family. Giardia flavohemoglobin closely resembles those of the Enterobacteriaceae in structure and function, acting as a nitric oxide dioxygenase that is induced when trophozoites are exposed to reactive nitrogen species. The Giardia cytochromes b5 are soluble proteins having relatively low reduction potentials and lack several features that are expected to promote rapid electron transfer with redox partners. Only one potential electron donor, and no electron acceptors, have yet been identified in the Giardia genome, and the roles of these cytochromes are presently unknown. The answer may lie in the sequences that flank the heme-binding core of these proteins which could serve to localize them within the cell through reversible post-translational modifications and to promote specific protein-protein interactions.