CYTOCHROMES: Of the five known heme proteins of Giardia four are paralogs of cytochrome b5, a small electron transfer protein that occurs in all kingdoms of life. Humans have two genes for membrane-anchored cytochrome b5 that participate in redox reactions on the endoplasmic reticulum and the mitochondrial inner membrane. However, the four Giardia cytochromes (gCYTB5 I-IV) are soluble proteins. Working with the laboratory of Professor Janet Yee of Trent University we have used immunofluorescence microscopy to locate these proteins (gCYTB5 I-IV) in different regions of the Giardia trophozoite: I is associated with the peripheral vesicles, II with the nucleolus, III with the nucleus and IV with the axonemes.
A further difference between the Giardia cytochromes and those of other organisms is their unusually low reduction potentials, which means that their equilibrium favours the oxidation state to a greater extent. For example the reduction potential of isotype-III is ~ 180 mV lower than mammalian CYTB5A at pH 7 and 25 C°.
These differences in i) localization and ii) reduction potential mean that the biological roles of the cytochromes in Giardia are different from other known organisms. But what these roles are is unknown.
A further difference between the Giardia cytochromes and those of other organisms is their unusually low reduction potentials, which means that their equilibrium favours the oxidation state to a greater extent. For example the reduction potential of isotype-III is ~ 180 mV lower than mammalian CYTB5A at pH 7 and 25 C°.
These differences in i) localization and ii) reduction potential mean that the biological roles of the cytochromes in Giardia are different from other known organisms. But what these roles are is unknown.
Our instruments for measuring the reduction potentials of cytochromes by spectroelectrochemistry, which takes advantage of the difference in the uv-visible spectrum of cytochromes in their oxidized and reduced states. The cell on the left contains a solution of the protein and an optically-transparent electrode that is used to control the cell potential.