Low reduction potential cytochrome b5 isotypes of Giardia intestinalis

Despite lacking mitochondria and a known pathway for heme biosynthesis the micro-aerotolerant anaerobic protozoan parasite Giardia intestinalis encodes four members of the cytochrome b5 family of electron transfer proteins, three of which are small, single-domain proteins. While these are similar in size and fold to their better-known mammalian counterparts the Giardia proteins have distinctly lower reduction potentials, ranging from -140 to -171 mV compared to +6 mV for the bovine microsomal protein. This difference is accounted for by a more polar heme environment in the Giardia proteins, as mutation of a conserved heme pocket tyrosine residue to phenylalanine in the Giardia cytochrome b5 isotype-I (gCYTb5-I Y61F) raises its reduction potential by nearly 100 mV. All three isotypes have UV-visible spectra consistent with axial coordination of the heme by a pair of histidine residues, but electron paramagnetic spectroscopy indicates that the planes of their imidazole rings are nearly perpendicular rather than coplanar as observed in mammalian cytochrome b5, which may be due to geometrical constraints imposed by a one-residue shorter spacing between the ligand pair in the Giardia proteins. Although no function has yet to be ascribed to any Giardia cytochrome b5, the presence of similar sequences in many other eukaryotes indicates that these represent an under-characterized class of low reduction potential family members.