Structural and functional analysis of Ascaris suum cytochrome b in the nematode methemoglobin reductase system

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Objectives : This research project was undertaken to elucidate structural properties of Ascaris suum cytochrome b_5, the component of novel NADH-methemoglobin reductase system , by comparing the coding gene and crystal structure with those of cytochromes b_5 from mammalian host and free-living nematode C. elegans. More specifically, 1) To resolve structural properties of A. suum cytochrome b_5 , which have been gained in the course of parasitic adaptation, 2) To analyze its mode of biosynthesis, i. e. the role of presequence of A. suum cytochrome b_5 by expressing the precursor cytochrome b_5 in C. elegans nematode. Achievements 1) The crystal structure of A. suum cytochrome b_5 was resolved at 1.8 A resolution demonstrating the structure different from that of aerobic mammalian cytochrome b_5(erythrocyte type, soluble). Docking models of A. suum hemoglobin and cytochrome b_5 strongly suggest that they are physiological reaction partners. Immunohistochemical and immunoblotting studies … More showed that A. suum cytochrome b_5 was localized in both the hypodermis and perienteric fluid , and that the cytochrome was a secretary protein. We proposed a working hypothesis that the cytochrome b_5 was a protein specialized during adaptation to low-oxygen tension of host intestinal lumen. To test this hypothesis, cytochrome b_5 species were surveyed from gene data bases of C. elegans, of which genome project has been completed. Four species of C. elegans cytochrome _b5 were found although none of them possesses presequence at all. Hydropathy analysis of the four species suggested that two of them were soluble proteins and one of the two exhibited highest homology with A. suum cytochrome b_5. However, none of the two was detected as expressed sequence tag. 2) Experimental conditions are currently examined because of low efficiency of transfection. 3) Affinity analysis using Biacore, employing Ascaris suum and human cytochromes b_5 as ligand, showed that the latter had less affinity with perienteric hemoglobin than the former. These results supported the working hypothesis described above. Less