Size of hirudin sequence required to fold into an active core domain

Novel method of expression and purification of hirudin based on pBAD TOPO, pTYB12 vectors and gene synthesis

To express recombinant hirudins in Escherichia coli cells, a fragment of chemically synthesized DNA was used, containing codons for the individual amino acids preferred by the host cells. Gene synthesis was based on the design of two DNA fragments, so-called mega primers H1 and H2 with a complementary fragment, and their incubation with Taq polymerase. The gene obtained in this fashion was multiplied using the PCR, and then expressed in E. coli cells with the use of TOPO vectors pBAD and pTYB12. Using this method, hirudins were obtained in the amount of 17 mg/l E. coli strain, with the activity of 17 antithrombin units (ATU)/mg protein. The method can be considered as an easy and inexpensive route to small protein synthesis.

Chemical synthesis of the RGD-protein decorsin: Pro→Ala replacement reduces protein thermostability

Decorsin is a 39-residue polypeptide chain, crosslinked by three disulfide bridges, that strongly inhibits platelet aggregation. We report the chemical synthesis and characterization of analogs of decorsin with the aim of investigating the role of proline residues in protein structure, stability and biological activity. Decorsin analogs have been synthesized in which one (P23A and P24A decorsin) or two (P23,24A decorsin) proline residues have been substituted by alanine. The crude synthetic polypeptides were purified by reversed-phase HPLC in their reduced form and allowed to refold oxidatively to their disulfide-crosslinked species. The homogeneity of the synthetic mini-proteins, and also the correct pairing of the three disulfide bridges, were established by a number of analytical criteria, including fingerprinting analysis of the refolded synthetic analogs by using thermolysin and proteinase K as proteolytic enzymes. Replacement of proline by alanine results in a significant and cumulative decrease of the high thermal stability (Tm 74 degrees C) of native decorsin. The mono-substituted analogs display a Tm of 66-67 degrees C, while the double-substituted analog a Tm of 50 degrees C. On the other hand, the overall secondary and tertiary structures were not affected by the Pro-->Ala exchanges, as judged from circular dichroism measurements. Platelet aggregation assays established that the proline substitutions do not impair significantly the biological activity of decorsin. The results of this study clearly indicate that proline residues contribute significantly to the protein thermal stability. Our results are in line with the 'proline rule', previously advanced for explaining the unusual thermal stability of thermophilic enzymes, which usually show an enhanced content of proline residues with respect to their mesophilic counterparts.

Minithioredoxin: a folded and functional peptide fragment of thioredoxin

A peptide fragment comprising the first 83 residues from the N-terminus of E. coli thioredoxin is purified by hydroxylamine cleavage of the intact protein. At physiological pH, the secondary and tertiary structure contents of the peptide are 70 and 35%, respectively, compared to the intact protein. Peptide 83 is able to display dual biological functions of thioredoxin, namely, a substrate for the enzyme E. coli thioredoxin-reductase and a processivity factor of T7 DNA polymerase. At present, peptide 83 represents the minimum functional and folding unit of thioredoxin. The highly conserved residue Phe 81 appears to play an important role in the folding of peptide 83, as judged from the packing analysis. Peptide 83 also mimics a particular kinetic folding intermediate of thioredoxin in terms of spectral properties and may serve as an equilibrium peptide model for the former.

Chemical synthesis and structural characterization of the RGD-protein decorsin: a potent inhibitor of platelet aggregation

Decorsin is a 39-residue RGD-protein crosslinked by three disulfide bridges isolated from the leech Macrobdella decora belonging to the family of GPIIb-IIIa antagonists and acting as a potent inhibitor of platelet aggregation. Here we report the solid-phase synthesis of decorsin using the Fmoc strategy. The crude polypeptide was purified by reverse-phase HPLC in its reduced form and allowed to refold in the presence of glutathione. The homogeneity of the synthetic oxidized decorsin was established by reverse-phase HPLC and capillary zone electrophoresis. The results of amino acid analysis after acid hydrolysis of the synthetic protein, NH2-terminal sequencing and mass determination (4,377 Da) by electrospray mass spectrometry were in full agreement with this theory. The correct pairing of the three disulfide bridges in synthetic decorsin was determined by a combined approach of both peptide mapping using proteolytic enzymes and analysis of the disulfide chirality by CD spectroscopy in the near-UV region. Synthetic decorsin inhibited human platelet aggregation with an IC50 of approximately 0.1 microM, a figure quite similar to that determined utilizing decorsin from natural source. In particular, the synthetic protein was 2,000-fold more potent than a model RGD-peptide (e.g., Arg-Gly-Asp-Ser) in inhibiting platelet aggregation. Thermal denaturation experiments of synthetic decorsin, monitored by CD spectroscopy, revealed its high thermal stability (Tm approximately 74 degrees C). The features of the oxidative refolding process of reduced decorsin, as well as the thermal stability of the oxidized species, were compared with those previously determined for the NH2-terminal core domain fragment 1-41 or 1-43 from hirudin. This fragment shows similarity in size, pairing of the three disulfides and three-dimensional structure with those of decorsin, even if very low sequence similarity. It is suggested that the less efficient oxidative folding and the enhanced thermal stability of decorsin in respect to those of hirudin core domain likely can be ascribed to the presence of the six Pro residues in the decorsin chain, whereas none is present in the hirudin domain. The results of this study indicate that decorsin can be obtained by solid-phase methodology in purity and quantities suitable for structural and functional studies and thus open the way to prepare by chemical methods novel decorsin derivatives containing unusual amino acids or even non-peptidic moieties.