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A fully active analog of human parathyroid hormone (hPTH) has been produced by recombinant expression in Escherichia coli. Initially, a nucleotide sequence encoding hPTH(1-34)-Asp-Pro was ligated to a proinsulin gene in the plasmid pUC8, for the eventual expression of a fusion protein of 137 amino acids. Unexpectedly, the proinsulin gene and 340 bp downstream were deleted by an unknown mechanism during transformation of the E. coli. This resulted in a new plasmid encoding a small (72-amino acid) fusion product of hPTH(1-34)-Asp35-Pro36-X, where X is a 36-residue "arbitrary" downstream sequence of pUC8. The fusion product was efficiently expressed and the hPTH analog, [Asp35]hPTH-(1-35), was readily released by acid cleavage, with a yield of 100 mg/L. This analog had an effective concentration for half-maximal adenylyl cyclase stimulation (EC50) in rat osteosarcoma cells of 14 nM, which was identical to that for hPTH-(1-34). In the ovariectomized rat model of osteoporosis, [Asp35]hPTH-(1-35) was fully active as a bone anabolic agent.

The β-1,4-endoglucanase (EC 3.2.1.4) from the hyperthermophilic archaeon Pyrococcus horikoshii (EGPh) has strong hydrolyzing activity toward crystalline cellulose. When EGPh is used in combination with β-glucosidase (EC 3.2.1.21), cellulose is completely hydrolyzed to glucose at high temperature, suggesting great potential for EGPh in bioethanol industrial applications. The crystal structure of EGPh shows a triosephosphate isomerase (TIM) (β/α) 8 -barrel fold with an N-terminal antiparallel β-sheet at the opposite side of the active site and a very short C-terminal sequence outside of the barrel structure. We describe here the function of the peripheral sequences outside of the TIM barrel core structure. Sequential deletions were performed from both N and C termini. The activity, thermostability, and pH stability of the expressed mutants were assessed and compared to the wild-type EGPh enzyme. Our results demonstrate that the TIM barrel core is essential for enzyme activity and that the N-terminal β-sheet is critical for enzyme thermostability. Bioinformatics analyses identified potential key residues which may contribute to enzyme hyperthermostability.

Using site-directed mutagenesis we have investigated the catalytic residues in a xylanase from Bacillus circulans. Analysis of the mutants E78D and E172D indicated that mutations in these conserved residues do not grossly alter the structure of the enzyme and that these residues participate in the catalytic mechanism. We have now determined the crystal structure of an enzyme-substrate complex to 108 A resolution using a catalytically incompetent mutant (E172C). In addition to the catalytic residues, Glu 78 and Glu 172, we have identified 2 tyrosine residues, Tyr 69 and Tyr 80, which likely function in substrate binding, and an arginine residue, Arg 112, which plays an important role in the active site of this enzyme. On the basis of our work we would propose that Glu 78 is the nucleophile and that Glu 172 is the acid-base catalyst in the reaction.

Specific degenerate codons in the amino-terminal region of a synthetic human parathyroid hormone (PTH) gene exerted dramatic effects on both products and yield of expression of this 84-amino acid polypeptide in Escherichia coli. With adenine-rich degenerate codons constituting the PTH-(1-5) region, intact PTH has been expressed as the only PTH product at 6.5 mg/liter. In contrast, with guanine-rich degenerate codons, the predominent product was analogue PTH-(8-84). Use of cytosine- or thymine-rich degenerate codons generated only a small amount of immunoreactive product (0.2 mg/l). With the amino terminal region reconstituted with adenine-rich degenerate codons, the mid and carboxyl regions of the synthetic gene were also reconstructed to imitate the E. coli-favored codon degeneracy. Expression yielded the intact PTH at 20 mg/liter. Gel electrophoresis and Western blots, with antibodies specific to the amino or carboxyl terminus of PTH, indicated only a single PTH-related polypeptide, with the same mobility as a synthetic intact PTH sample. Amino acid sequencing, composition analysis, mass spectrometry, and the adenylate cyclase bioassays confirmed the purified product as the processed intact PTH.

We initially aligned 28 different cellulase sequences in pairwise fashion and found half of them have the sequence -Asn-Glu-Pro- located in a region flanked by hydrophobic-rich amino acids. Based on lysozyme as a model, the glutamate residue could be essential for enzyme function. We tested this possibility by site-directed mutagenesis of the genes coding Bacillus polymyxa and Bacillus subtilis endo-beta-1,4-glucanases. The genes and amino acid sequences of these two enzymes show very little similarity. Change of Glu-194 and Glu-169 to the isosteric glutamine form in these respective enzymes resulted in a dramatic loss of CMCase activity which could be restored by reverse mutation. Similar mutations to less-conserved residues, Glu-72 and Glu-147, of the B. subtilis enzyme did not cause any loss of activity.

HPLC6 is the major component of liver-type antifreeze polypeptides (AFPs) from the winter flounder, Pleuronectes americanus. To facilitate mutagenesis studies of this protein, a gene encoding the 37-amino acid mature polypeptide was chemically synthesized and cloned into the Tac cassette immediately after the bacterial ompA leader sequence for direct excretion of the AFP into the culture medium. Escherichia coli transformant with the construct placIQpar8AF was cultured in M9 medium. The recombinant AFP (rAFP) was detected by a competitive enzyme-linked immunosorbent assay (ELISA). After IPTG induction, a biologically active rAFP was expressed. The majority of the rAFP was excreted into the culture medium with only trace amounts trapped in the periplasmic space and cytoplasm. After 18 h of induction, the accumulated rAFP in the culture medium amounted to about 16 mg/L. The excreted AFP was purified from the culture medium by a single-step reverse-phase HPLC. Mass spectrometric and amino acid composition analyses confirmed the identity of the purified product. The rAFP, which lacked amidation at the C-terminal, was about 70% active when compared to the amidated wild-type protein, thus confirming the importance of C-terminal cap structure in protein stability and function.

Analogues of human parathyroid hormone (hPTH) truncated at the C-terminal end have been studied for adenylyl cyclase (AC) activity and for solution conformation by circular dichroism (CD) spectroscopy. Analogues of hPTH-(1-34)-NH2, containing the first 28-31 residues, had only a slightly diminished ability to stimulate AC in rat osteosarcoma (ROS) cells as compared to that of the parent analogue. CD data on hPTH-(16-34)-NH2 and C-terminal deletion mutants of hPTH-(1-34)-NH2 supported the presence of a partially stable alpha-helix over residues 17-28. A carboxyl-terminal mutant, hPTH-(1-30)-OH, showed both reduced helix and greatly reduced AC-stimulating activity as compared to the corresponding amide analogue. In contrast, both of these analogues, in the presence of palmitoyloleoylphosphatidylserine (POPS) vesicles, showed an equal stabilization of alpha-helix. All other analogues showed at least some enhancement of alpha-helix in the presence of POPS. However, both in neutral, aqueous buffer and in POPS, the relative amount of alpha-helix decreased greatly as the peptide was shortened below the 1-28 sequence. These data provide additional support for an amphiphilic alpha-helix over residues 21-28 being the conformation for receptor binding of hPTH for stimulation of AC activity. Modeling human parathyroid hormone-related peptide as an alpha-helix over this same region, and comparison to hPTH, suggests that both may bind via the hydrophobic face to the receptor.

Synthetic genes encoding the 190 amino acid Trichoderma reesei xylanase II (TrX) and the closely related Trichoderma viride xylanases have been synthesized in a two-step procedure. Initially, a partial gene encoding amino acids 92–190 was constructed in fusion with the N-terminal half of the Bacillus circulons xylanase (BcX). The remaining BcX gene sequence was replaced during the assembly of the coding sequence for amino acids 1–91. Expression of the synthetic genes in Escherichia coli yielded recombinant xylanases with specific activity generally identical with the natural TrX. However, the recombinant TrX showed thermostability and temperature optimum lower than those of the natural TrX, thus indicating that the posttranslational modifications of the latter in its fungal host are essential to its greater stability. A mutation N19K further decreased the thermostability of the recombinant TrX.Key words: xylanase, thermostability.

The thermostability of the 20 396 Da Bacillus circulans xylanase was increased by the introduction of both intra- and intermolecular disulfide bridges by site-directed mutagenesis. Based on the 3-D structure of the enzyme, sites were chosen where favourable geometry for a bridge existed; in one case, to obtain favourable geometry additional mutations around the cysteine sites were designed by computer modelling. The disulfide bonds introduced into the xylanase were mostly buried and, in the absence of protein denaturants, relatively insensitive to reduction by dithiothreitol. The mutant proteins were examined for residual enzymatic activity after various thermal treatments, and were assayed for enzymatic activity at elevated temperatures to assess their productivity. We have examined one of these mutants by X-ray crystallography. All of the disulfide bond designs tested increased the thermostability of the B. circulans xylanase, but not all enhanced the activity of the enzyme at elevated temperatures.

An efficient expression system for a low-molecular mass xylanase in Escherichia coli has been developed. A gene encoding the mature Bacillus circulans ( Bc ) xylanase was designed to imitate the frequency of degenerate codons used in E. coli . Appropriate degenerate codons were used to create multiple unique restriction sites for future mutagenesis studies. The synthetic gene was constructed in two stages, both involving ligation of overlapping oligonucleotides. The synthetic Bc gene was then converted to a Bacillus subtilis ( Bs ) xylanase gene via a single codon substitution (Thrl47Ser). The plasmids containing both synthetic genes were further modified for the direct expression in E. coli . Under the control of the lac promoter, recombinant xylanase has been produced at levels as high as 300 mg/liter in soluble form in the cytoplasm. This efficiency represented a dramatic improvement over all previous attempts involving the expression of the natural genes, with the xylanase being secreted in those cases. Characterization of our gene products indicated that the purified recombinant product was correctly processed and enzymatically active.

Human parathyroid hormone (hPTH) and several deletion analogues were examined for the presence of secondary structure using circular dichroism spectroscopy. The spectra of hPTH and the deletion analogues 8-84, 34-53, 53-84, 1-34, 13-34, 1-19, and 20-34, in neutral, aqueous buffer, gave no evidence for extensive secondary structure. An alpha-helical-like spectral contribution was found to arise from a region within peptide 13-34. This spectral contribution was speculated to arise from partial stability of a helix consisting of residues 17-29. Molecular dynamics simulations of peptide 1-34 suggested that this peptide tends to fold with a bend defined by residues 10-14, with the amino-terminal and carboxyl-terminal residues tending to be in more extended forms and the other residues in helical-like conformations. The addition of trifluoroethanol promoted the formation of alpha-helix, mainly in the 1-34 region. The putative helix comprised of residues 17-29 was stabilized by the addition of 10-20% TFE, while a second putative helix proximal to the amino terminus, and comprised of residues 3-11, was stabilized by slightly higher concentrations of TFE. An amphiphilic sequence was identified within the 20-34 fragment. The development of alpha-helix on binding this fragment, and other analogues containing this sequence, to palmitoyloleoylphosphatidylserine vesicles provided experimental evidence for the potential role of this amphiphilic sequence in binding to membranes or to a membrane receptor. The relationships between these alpha-helical regions in 1-34, either potentiated by trifluoroethanol or lipid vesicles, are discussed in terms of different receptor-binding regions within hPTH.

Expression of the human parathyroid hormone (PTH) gene in E. coli yielded intact PTH and PTH-(8–84). To determine if PTH-(8–84) is the result of a competing translation initiated from methionine codon-8 or degradation of the intact PTH, twelve new gene constructs with or without an internal ribosome-binding site (iRBS) in the PTH-(1–5) region were prepared via substitution with degenerate codons. Expression of constructs without iRBS produced only intact PTH. Constructs with weak iRBS, including one that resembles the cDNA sequence, yielded PTH-(8–84) as a minor product. In contrast, constructs with strong iRBS produced predominantly or exclusively this shorter analogue.

Recombinant analogues of human parathyroid hormone [hPTH-(1-84)] were expressed in Escherichia coli harboring plasmids containing synthetic genes under the control of the lac promoter. The level of expression of the gene encoding the truncated analogue, hPTH-(3-84), was greater than that of the gene encoding full-length hPTH-(1-84) but less than that of the gene encoding proparathyroid hormone (hProPTH). This may be due in part to the relative efficiency of translation of the mRNA as suggested by secondary structure analysis and in part because of enhanced stability of the extended peptide. Formylmethionyl derivatives of hProPTH and of hPTH-(3-84) and underivatized hPTH-(3-84) were purified by HPLC, and their identity was confirmed by NH2-terminal sequencing and amino acid analysis. The bioactivity of these recombinant peptides was then tested in skeletal and renal adenylate cyclase assays in vitro and in assays examining effects on plasma and urine calcium and phosphate levels and on urine cyclic AMP levels in vivo. The NH2-terminally extended analogue fMet-hProPTH displayed 10% of the in vitro activity of hPTH-(1-84) and was a partial agonist in vivo. The peptides hPTH-(3-84) and fMet-hPTH-(3-84) were inert in vitro and were very weak in vitro antagonists when compared to the NH2-terminal analogue bovine [Nle8,18Tyr34]PTH-(3-34)-NH2. In vivo, hPTH-(3-84) and the bPTH-(3-34) analogue, when assayed at a 10:1 molar ratio relative to bPTH-(1-84), were each inert, and neither demonstrated antagonist activity at these concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for phosphorylating a protected deoxyribooligonucleotide containing phosphotriester linkages is described. The modified phosphotriester method of chemical synthesis is further refined in terms of (i) better final deblocking conditions and (ii) new chromatography solvent systems containing acetone-water-ethyl acetate to yield pure oligomers. The effectiveness of these improvements has been demonstrated in the rapid and efficient synthesis of seventeen fragments constituting the sequence of human insulin C-chain DNA.

Enhanced accumulation of human proinsulin synthesized in Escherichia coli has been achieved by inserting a short leader of homooligopeptide at the amino end of proinsulin. Out of 20 amino acid oligomers studied, (Ala)6, (Asn)6, (Cys)7, (Gln)7, (His)6, (Ser)6, and (Thr)6 leaders were the most effective, with the yield of proinsulin ranging between 6% and 26% of the total bacterial protein. These constructions were made by inserting a synthetic oligodeoxyribonucleotide duplex, coding for a small homooligopeptide, between a synthetic proinsulin gene and an eight-codon beta-galactosidase gene residue in vector pUC8. Cyanogen bromide cleavage of the 102 amino acid fused polypeptide yielded a species identical to authentic proinsulin, as judged by NaDodSO4/PAGE and radioimmunoassay.

5'-0-Dimethoxytritylthymidine (2) was phosphorylated and base-modified simultaneously to yield the 4-triazolopyrimidinone nucleotide (3). Coupling between (3) and other common deoxyribonucleotides gave a fully protected nonamer (4). Deblocking under different conditions yielded the nonamer as phosphodiester with concomitant conversion of 4-triazolopyrimidinone to 5-methylcytosine (aqueous ammonia) or thymine (N1,N1,N3,N3-tetramethyl-guanidinium syn-4-nitrobenzaldoximate solution).

The purification of protected deoxyribooligonucleotides containing phosphotriester internucleotidic linkages has been improved by developing a deactivated silica gel chromatographic technique. The efficiency of this technique as applied in the modified phosphotriester approach has been demonstrated in the rapid synthesis of seventeen pure fragments constituting the sequence of human insulin B and mini-C DNA. The sequence of each oligomer was confirmed by the two-dimensional mobility shift method of fingerprinting.

Publisher Summary This chapter presents an experiment by designing a small protective cap for the efficient expression of the desired polypeptide. In the experiment, a short duplex oligonucleotide, which encodes a homooligopeptide of hydrophilic and protease-resistant amino acid residues, was fused to the proinsulin gene at the amino terminus. Small foreign proteins, which can be produced in bacteria via recombinant DNA, are often degraded rapidly by the proteolytic enzyme system of the host. A widely used approach is to produce the target protein as fusion product with a host protein, which often constitutes an undesirably large portion of the fused polypeptide. This lowers the yield and complicates the purification of the desired protein. The fused gene was found to be expressed efficiently in Escherichia coli . Structure of the cell wall was distorted because of accumulation of the new polypeptide, which resisted degradation. After removal of the homopeptide leader sequence by cyanogens bromide, the fused polypeptide was converted to proinsulin. The chapter discusses the materials and methods required for the expression of the fused protein-containing proinsulin and cyanogen bromide cleavage of the fused protein to give intact proinsulin.

Bypassing any intermediate steps of purification and gene assembly, several synthetic oligonucleotides constituting a DNA duplex with a small base-mismatching region were phosphorylated, annealed, and ligated directly into a linearized plasmid vector. After transformation in bacteria, the two plasmid strands individualy yielded two different plasmids bearing altered versions of the same gene. Via this approach, DNA coding sequences of the human parathyroid hormone and analogues were synthesized and cloned in Escherichia coli.

The "duplex crossover linker" technique was simplified and used to delete the beta-galactosidase (beta-Gal)-coding sequence upstream from the multiple restriction sites in pUC plasmids. A single-stranded crossover linker, with a homology-searching sequence as short as 5 bases, was initially ligated to a linearized plasmid. Inside Escherichia coli, the plasmid was circularized by intramolecular, homologous recombination between the (5'-or 3'-) protruding homology-searching sequence and a targeted region in the opposite terminus. As a consequence, sequences beyond the point of integration were deleted. Specific deletion of sequences up to 1472 bp was demonstrated. The single-stranded linkers apparently avoided generation of undesirable mutants associated with the usage of duplex linkers. A mechanism has been proposed for the intramolecular recombination directed by the crossover linkers. It principally involves either 3'- or 5'-exonucleolytic breakdown of the homologous terminus of the plasmid, circularization by spontaneous pairing of the exposed complementary strands, and subsequent degradation of any redundant sequence.

A 74-bp DNA sequence coding for the pre sequence of human preproinsulin and containing EcoRI termini was synthesized by the chemical enzymatic method, joined with previously synthesized proinsulin DNA, and cloned in the M13mp8 vector. A clone pNB82-121 was identified by DNA sequence which confirmed the correct orientation of the pre sequence to the proinsulin DNA. The EcoRI site at the junction of pre- and proinsulin DNA was eliminated by removing a triplet ATT using a synthetic 19-mer primer. To simplify preproinsulin isolation and to study its expression in the M13 system, a 25-bp affinity leader sequence coding for (glu)7 was inserted at the remaining EcoRI site; this put the preproinsulin DNA in a correct reading frame with the AUG initiation codon of β-galactosidase. Preproinsulin was expressed under lac promoter control as analyzed by a radioimmunoassay (RIA) against C-peptide.

Infrared spectra of E. coli strain JM103 and transformants which overproduced recombinant proinsulin have been measured as a function of pressure up to 38 kbar. It is the first time that high-pressure infrared spectra of live bacteria have been successfully measured. In ambient conditions, spectra of the host strain JM103 and the transformants are generally identical. However, under pressure, distinct shifting pattern can be observed in specific spectral parameters of transformants, presumably due to accumulation of proinsulin in form of cytoplasmic inclusion bodies. In particular, the pressure-induced frequency shift of the amide III band (1235 cm-1) in the proinsulin-producing transformants is much smaller than in the host JM103. This pressure effect can potentially be an efficient approach to monitor maximum gene expression in microorganisms. Contrary to predictions based on model system, the pressure-induced denaturation and the sharp transition from disordered liquid crystalline state to the ordered gel state commonly observed in the aqueous solution of protein and aqueous bilayer dispersion of lipids, respectively, do not occur in the bacterial proteins and cell membrane of E. coli.