title: Cytochrome b5 from Giardia lamblia.
abstract: 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 b(5) 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 b(5), 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 b(5) in Giardia is evidence of uncharacterized cytochrome-mediated metabolic processes within this medically important protist.
authors.affiliation: Environmental and Life Sciences Graduate Program, Trent University, Peterborough, ON, Canada.
authors.forename: Samiah
authors.initials: S
authors.lastname: Alam
authors.forename: Janet
authors.initials: J
authors.lastname: Yee
authors.forename: Manon
authors.initials: M
authors.lastname: Couture
authors.forename: Shin-ichi J
authors.initials: SJ
authors.lastname: Takayama
authors.forename: Wan-Hsin
authors.initials: WH
authors.lastname: Tseng
authors.forename: A Grant
authors.initials: AG
authors.lastname: Mauk
authors.forename: Steven
authors.initials: S
authors.lastname: Rafferty
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D003001:Cloning, Molecular; D015786:Cytochromes b5 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016054:DNA, Protozoan / Q000235:genetics; D055664:Electrochemical Techniques; D017125:Genes, Protozoan; D016829:Giardia lamblia / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
chemical_list: D016054:DNA, Protozoan; D015800:Protozoan Proteins; D011994:Recombinant Proteins; D015786:Cytochromes b5