Giardia intestinalis is a protozoan parasite that causes waterborne diarrheal disease in animals and humans. It is an unusual eukaryote as it lacks the capacity for heme biosynthesis; nonetheless it encodes heme proteins, including three cytochrome b5 isotypes (gCYTB5s) of similar size. Homology modelling of their structures predicts increased heme pocket polarity compared to mammalian isotypes, which would favour the oxidized state
and lower their reduction potentials (E°'). This was confirmed by spectroelectrochemical experiments, which measured E°' of -171 mV, -140 mV and -157 mV for gCYTB5-I, II, III respectively, compared to +7 mV for bovine microsomal cytochrome b5. To explore the influence of heme pocket polarity in more detail, five gCYTB5-I mutants in which polar residues were replaced by nonpolar residues at one of three positions were investigated.
While these substitutions all increased the reduction potential, replacement of a conserved tyrosine residue at position-61 with phenylalanine had the most significant effect, raising E°' by 106 mV. This tyrosine residue occurs in all gCYTB5s and is likely the greatest contributor to their low reduction potentials. Finally, complementary substitutions were made into a bovine microsomal cytochrome b5 triple mutant to lower its reduction potential. These not only lowered the E°' by more than 140 mV but also weakened the interaction of heme with the protein. The lower reduction potentials of the gCYTB5s may indicate that these proteins have different roles from their more well-known mammalian counterparts.