Characterization of two Protein Disulfide Oxidoreductases from Thermophilic Organisms Pyrococcus furiosus and Aquifex aeolicus : Characterization of two Protein Disulfide Oxidoreductases

Members of the thioredoxin superfamily of proteins catalyze disulfide bond reduction and oxidation using the active site C-X-X-C sequence. In hyperthermophilic organisms, cysteine side chains were expected in low abundance since they were not believed to endure the high temperatures under which they grow. Recently it has been found that disulfide bonds in hyperthermophiles are more frequent, the higher the growth temperature of the organism. This is perhaps used as an adaptation to high temperature in order to stabilize proteins under harsh conditions. A protein with sequence and structural similarities to mesophilic members of the thioredoxin superfamily, called protein disulfide oxidoreductases (PDO), has been found in the genomes of recently sequenced hyperthermophilic genomes. In this study PDOs from the hyperthermophiles Aquifex aeolicus (AaPDO) and Pyrococcus furiosus (PfPDO) have been investigated. The molecular weight is about 26 kDa and their structures are comprised of two homologous thioredoxin folds, referred to as the N-unit and the C-unit, each containing a C-X-X-C motif. The sequence identity between the two units and the two proteins is low, but they are still structurally very similar. The function of these proteins in vivo is unknown. As a first step in characterizing the activity of these proteins, the redox characteristics of these domains will be investigated. During this project, the genes for AaPDO and PfPDO have been cloned into overexpression vectors, expressed in E. coli and purified to homogeneity. To allow for individual study of the activities of two units, mutated proteins were prepared in which the cysteine residues of the N-unit (AaPDOnm and PfPDOnm) and of the C-unit (AaPDOcm and PfPDOcm) and purified. Circular dichroism spectra recorded of the wild type and mutants indicate that all purified proteins are folded and that the N- and C-unit active site mutants are structurally similar to the corresponding wild type proteins