Issue 12, 2012

Cytochrome b5 from Giardia lamblia


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 haem-binding 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.

Graphical abstract: Cytochrome b5 from Giardia lamblia

Supplementary files

Article information

Article type
26 Jul 2012
29 Oct 2012
First published
30 Oct 2012

Metallomics, 2012,4, 1255-1261