Cytochrome B5 from Giardia Lambliaw

The protozoan intestinal parasite Giardia lamblia lacks mitochondria and the ability to make haem yet encodes several putative haem-binding proteins, including three of the cytochrome b5 family. We cloned one of these (gCYTb5-I) and expressed it within Escherichia coli as a soluble holoprotein. UV-visible and resonance Raman spectra of gCYTb5-I resemble those of microsomal cytochrome b5, and homology modelling supports a structure in which a pair of invariant histidine residues act as axial ligands to the haem iron. The reduction potential of gCYTb5-I is 165 mV vs. SHE and is relatively low compared to most values (110 to +80 mV) for this class of protein. The amino- and carboxy-terminal sequences that flank the central haembinding core of the Giardia cytochromes are highly charged and differ from those of other family members. A core gCYTb5-I variant lacking these flanking sequences was also able to bind haem.
The presence of one actual and two probable functional cytochromes b5 in Giardia is evidence of uncharacterized cytochrome-mediated metabolic processes within this medically important protist.
Introduction
The protozoan parasite Giardia lamblia is one of the most common causes of infectious diarrhoea, known as giardiasis.
Infection of the host begins when dormant Giardia cysts are ingested through contaminated water, food or faecal–oral contact. Exposure of the cysts to the acidic environment of the host’s stomach triggers their excystation, and the resulting flagellated trophozoites adhere to the epithelial cells of the upper small intestine where the trophozoites reproduce by asexual binary fission. As trophozoites pass through the intestinal tract, they convert to cysts, which are excreted with the faeces to become the source for new infections. Giardiasis occurs worldwide but is a particular concern in underdeveloped regions, and it is a target of the World Health Organization’s
Neglected Diseases Initiative.1
Apart from its

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