Analysis of cellular function of HBP23 on redox regulation

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HBP23 (Heme Binding Protein, M.W. : 23kDa), a rat cytosolic protein with high binding affinity for heme, is a member of the peroxiredoxin family, which exhibits thioredoxin-dependent peroxidase activity. The mutants, Cys52Ser, Cys173Ser and Cys52-173Ser, did not show the activity, which was different from that of the mutant (Cys83Ser). A 2.6A-resolutin crystal structure of HBP23 (Cys83Ser) in oxidative form revealed a unique dimer structure in which Cys-52 forms a disulfide bond with Cys-173 from another subunit by C-terminal tail swapping. The internal disulfide bond was surrounding by the two arginine residues, Arg-123 and Arg-151. The mutations at the positions showed reduced activity. Thus, the cysteine residues are involved in the peroxidation catalysis and form a disulfide bond as an intermediate. The residues, Arg-123 and Arg-151 may display enhanced reactivity by interacting with Cys-52 due to decrease the level of pK. HBP23 interacts with heme in a 1 : 1 molar ratio of heme/protein. Thioredoxn-dependent peroxidase activity on HBP23 was inhibited in partially by heme. It was suggested that heme bound to the amino acid residue(s) of HBP23 by analysis of Resonance Raman spectra. Although the role of heme is not clear at the moment, it is of interest, because this protein is also induced by heme. The Western-blot analysis showed that HBP23 is a mixture of dimer and decamer in the rat liver cytosolic fraction.