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The gene encoding the human erythrocyte form of cytochrome b5 (97 residues in length) has been prepared by mutagenesis of an expression vector encoding lipase-solubilized bovine liver microsomal cytochrome b5 (93 residues in length) (Funk et al., 1990). Efficient expression of this gene in Escherichia coli has provided the first opportunity to obtain this protein in quantities sufficient for physical and functional characterization. Comparison of the erythrocytic cytochrome with the trypsin-solubilized bovine liver cytochrome b5 by potentiometric titration indicates that the principal electrostatic difference between the two proteins results from two additional His residues present in the human erythrocytic protein. The midpoint reduction potential of this protein determined by direct electrochemistry is -9 +/- 2 mV vs SHE at pH 7.0 (mu = 0.10 M, 25.0 degrees C), and this value varies with pH in a fashion that is consistent with the presence of a single ionizable group that changes pKa from 6.0 +/- 0.1 in the ferricytochrome to 6.3 +/- 0.1 in the ferrocytochrome with delta H degrees = -3.2 +/- 0.1 kcal/mol and delta S degrees = -11.5 +/- 0.3 eu (pH 7.0, mu = 0.10). The 1D 1H NMR spectrum of the erythrocytic ferricytochrome indicates that 90% of the protein binds heme in the "major" orientation and 10% of the protein binds heme in the "minor" orientation (pH 7.0, 25 degrees C) with delta H degrees = -2.9 +/- 0.3 kcal/mol and delta S degrees = -5.4 +/- 0.9 eu for this equilibrium.

The objective of this work is to determine the molecular mechanism and regulation of short-chain acyl-CoA dehydrogenase (SCAD) from Megasphaera elsdenii. To achieve this, the gene coding for SCAD from M. elsdenii was cloned and sequenced. Site-directed mutagenesis was then used to identify an amino acid residue that is required for the proposed mechanism. To clone the gene, the amino acid sequence of the 50 N-terminal residues of SCAD from M. elsdenii was determined. This sequence information was utilized to synthesize two sets of mixed oligonucleotide primers which were then used to generate a 120-bp specific probe from M. elsdenii DNA by the polymerase chain reaction (PCR) method. The 120-bp probe was used to screen a M. elsdenii genomic DNA library cloned into Escherichia coli. The gene encoding M. elsdenii SCAD was identified from this library, sequenced, and expressed. The cloned SCAD gene contained an open reading frame which revealed a high degree of sequence identity with an open reading frame protein sequence of the human SCAD and the rat medium-chain acyl-CoA dehydrogenase (MCAD) (44% and 36% identical residues in paired comparisons for human SCAD and rat MCAD, respectively). Recombinant SCAD expressed from a pUC119 vector accounted for 35% of the cytosolic protein in the Escherichia coli crude extract. The expressed protein had similar activity, redox potential properties, and nearly identical amino acid composition to native M. elsdenii SCAD. In addition, a site-directed Glu367 Gln mutant of SCAD expressed from a pUC119 vector was shown to have minimal reductive and oxidative pathway activity with butyryl-CoA and crotonyl-CoA, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The coding sequence for human serum transferrin was assembled from restriction fragments derived from a full-length cDNA clone isolated from a human liver cDNA library. The assembled clone was inserted into the expression vector pNUT and stably transfected into transformed baby hamster kidney (BHK) cells, leading to secretion of up to 125 mg/L recombinant protein into the tissue culture medium. As judged by mobility on NaDodSO4-PAGE, immunoreactivity, spectral properties (indicative of correct folding and iron binding), and the ability to bind to receptors on a human cell line, initial studies showed that the recombinant transferrin, is identical to three commercial human serum transferrin samples. Electrospray mass spectrometry (ESMS), anion-exchange chromatography, and urea gel analysis showed that the recombinant protein has an extremely complex carbohydrate pattern with 16 separate masses ranging from 78,833 to 80,802 daltons. Mutation of the two asparagine carbohydrate linkage sites to aspartic acid residues led to the expression and secretion of up to 25 mg/L nonglycosylated transferrin. ESMS, anion-exchange chromatography, and urea gel analysis showed a single molecular species that was consistent with the expected theoretical mass of 75,143 daltons. In equilibrium binding experiments, the nonglycosylated mutant bound to HeLa S3 cells with the same avidity and to the same extent as the glycosylated protein and the three commercial samples. These studies demonstrate conclusively that carbohydrate has no role in this function.

Five site-directed mutants of the N-terminal half-molecule of human serum transferrin have been expressed in baby hamster kidney cells and purified to homogeneity. Expression levels and overall yields varied considerably from the wild-type protein, depending on the mutant in question. The mutants are D63S, D63C, G65R, K206Q, and H207E and are based on mutations observed in a variety of transferrins of known sequence. Their molecular masses, determined by electrospray mass spectrometry, agree with theory, except for the D63C mutant, which appears to be cysteinylated. All mutants bind iron but with varying affinities; qualitatively, in increasing order D63S approximately D63C approximately G65R much less than wild type less than or equal to H207E much less than K206Q. In general, reduction of formal negative charge within the binding cleft shifts the visible spectral maximum of the iron complex toward the blue and reduces the affinity for iron, and increasing the formal negative charge shifts the visible maximum toward the red and increases the affinity for iron. The K206Q mutant is exceptional inasmuch as its visible maximum shows a blue shift, but its affinity for iron is the greatest of all of the mutants studied. All mutants reported, in addition to the wild-type protein, exhibit very similar visible molar extinction coefficients for the iron complex and very similar changes in extinction coefficients at 240 nm on binding Fe(III) or Ga(III). These results suggest that in all cases the bound metal ion is coordinated by two tyrosyl side chains.

A gene coding for lipase-solubilized bovine liver microsomal cytochrome b5 has been synthesized, expressed in Escherichia coli, and mutated at functionally critical residues. Characterization of the recombinant protein revealed that it has a reduction potential that is approximately 17 mV lower than that of authentic wild-type protein at pH 7 (25 degrees C). Structural studies determined that the recombinant protein differed in sequence from authentic wild-type cytochrome b5 owing to three errors in amidation status in the published sequence for the protein on which the gene synthesis was based. The structural origin of the lower reduction potential exhibited by the triple mutant has been investigated through X-ray crystallographic determination of the three-dimensional structure of this protein and is attributed to the presence of Asp-57 within 3.3 A of heme vinyl-4 in the mutant. In addition, the model developed by Argos and Mathews [Argos, P., & Mathews, F.S. (1975) J. Biol. Chem. 250, 747] for the change in cytochrome b5 oxidation state has been studied through mutation of Ser-64 to Ala. In this model, Ser-64 is postulated to stabilize the oxidized protein through H-bonding interactions with heme propionate-7 that orients this propionate group 6.2 A from the heme iron. Spectroelectrochemical studies of a mutant in which Ser-64 has been changed to an alanyl residue demonstrate that this protein has a reduction potential that is 7 mV lower than that of the wild-type protein; moreover, conversion of the heme propionate groups to the corresponding methyl esters increases the potential by 67 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

A human liver cDNA library was screened with a synthetic oligonucleotide, complementary to the 5' region of human transferrin mRNA, as a hybridization probe. The full-length human cDNA clone isolated from this screen contained part of the 5' untranslated region, the complete coding region for the signal peptide and the two lobes of transferrin, the 3' untranslated region, and a poly(A) tail. By use of oligonucleotide-directed mutagenesis in vitro, two translational stop codons and a HindIII site were introduced after the codon for Asp-337. This fragment was inserted into two different expression vectors that were then introduced into Escherichia coli. As judged by NaDodSO4-polyacrylamide gel electrophoresis and Western blot analysis, however, recombinant hTF/2N was undetectable in bacteria transformed by these plasmids. Concurrently, we developed a plasmid vector for the expression of recombinant hTF/2N in eukaryotic cells. In this case, a DNA fragment coding for the natural signal sequence, the hTF/2N lobe, and the two stop codons was cloned into the expression vector pNUT, such that the expression of hTF/2N was controlled by the mouse metallothionein promoter and the human growth hormone termination sequences. Baby hamster kidney cells containing this hTF/2N-pNUT plasmid secreted up to 20 mg of recombinant hTF/2N per liter of tissue culture medium. Recombinant hTF/2N was purified from the medium by successive chromatography steps on DEAE-Sephacel, Sephadex G-75, and FPLC on Polyanion SI. The purified protein was characterized by NaDodSO4-PAGE, urea-PAGE, amino-terminal sequence analysis, UV-visible spectroscopy, iron-binding titration, and proton NMR.(ABSTRACT TRUNCATED AT 250 WORDS)