UBC Theses and Dissertations
Expression and characterization of two recombinant mammalian metalloproteins : |b bovine microsomal cytochrome b₅ and human serum transferrin (N lobe) Funk, Walter David
Two separate systems were developed for the expression of recombinant metalloproteins. A synthetic gene encoding the lipase-solubilized form of bovine liver microsomal cytochrome b₅ was designed and assembled for expression in E. coli. Analysis of the initial recombinant cytochrome revealed differences in several physical characteristics of the molecule compared to the authentic bovine liver species, including a reduction potential that was lower by 17 mV. Further studies showed the primary sequence of the initial recombinant differed from the authentic protein in the amidation status of three residues which, when corrected yielded a recombinant protein identical in behaviour to the authentic protein. The participation of Ser64 in the stabilization of the oxidized form of cytochrome b₅ was investigated using site-directed mutagenesis to alter this residue to Ala, which was predicted to ablate a hydrogen bond formed between the protein and heme-propionate 7. Spectroelectrochemical analysis of this variant showed that the reduction potential had been shifted downwards by 7 mV, in contrast to predictions from a structural model describing the red/ox behaviour of cytochrome b₅ (Argos and Mathews, 1975). The role of heme carboxylates in determining the reduction potential was confirmed for both the wild-type and Ala64 variants by heme replacement studies using the esterified derivative of protoporphyrin IX, suggesting that the presence of free carboxylates contributes to the stabilization of the oxidized species. In addition, constructions for the expression of the trypsin-solubilized form of bovine liver microsomal cytochrome b₅ and the erythrocytic form of human cytochrome b₅ are described. A tissue culture cell system was developed for the expression of the N-terminal half molecule of human serum transferrin. The recombinant molecule (hTF/2N) was secreted at high levels from selected eukaryotic cells, and displayed high identity with the proteolytically-derived molecule from authentic human serum transferrin as judged by sequence analysis, electrophoretic mobility and iron binding capacity. A construction for the expression of the C-terminal half molecule was assembled but failed to express recombinant protein when introduced into tissue culture cells. The production of these two heterologous expression systems allows for high-level recovery of recombinant protein and provides a convenient approach to structure-function studies employing site-directed mutagenesis techniques.
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