Immobilized anhydrochymotrypsin as a biospecific affinity adsorbent for the peptides produced by chymotryptic hydrolysis

Inhibitory specificity change of the ovomucoid third domain of the silver pheasant upon introduction of an engineered Cys14-Cys39 bond

The ovomucoid third domain from silver pheasant (OMSVP3), a typical Kazal-type inhibitor, strongly inhibits different serine proteases of various specificities, i.e., chymotrypsin, Streptomyces griseus protease, subtilisin, and elastase. Structural studies have suggested that conformational flexibility in the reactive site loop of the free inhibitor may be related to broad specificity of the ovomucoid. On the basis of the structural homology between OMSVP3 and ascidian trypsin inhibitor (ATI), which has a cystine-stabilized alpha-helical (CSH) motif in the sequence, we prepared the disulfide variant of OMSVP3, introducing an engineered disulfide bond between positions 14 and 39 near the reactive site (Met18-Glu19) by site-directed mutagenesis. The disulfide variant P14C/N39C retained potent inhibitory activities toward alpha-chymotrypsin (CHT) and S. griseus proteases A and B (SGPA and SGPB), while this variant lost most of its inhibitory activity toward porcine pancreatic elastase (PPE). We determined the solution structure of P14C/N39C, as well as that of wild-type OMSVP3, by two-dimensional nuclear magnetic resonance (2D NMR) methods and compared their structures to elucidate the structural basis of the inhibitory specificity change. For the molecular core consisting of a central alpha-helix and a three-stranded antiparallel beta-sheet, essentially no structural difference was detected between the two (pairwise rmsd value = 0.41 A). In contrast to this, a significant difference was detected in the loop from Cys8 to Thr17, where in P14C/N39C it has drawn approximately 4 A nearer the central helix to form the engineered Cys14-Cys39 bond. Concomitantly, the Tyr11-Pro12 cis-peptide linkage, which is highly conserved in ovomucoid third domains, was isomerized to the trans configuration. Such structural change in the loop near the reactive site may possibly affect the inhibitory specificity of P14C/N39C for the corresponding proteases.

The interaction of thrombomodulin with Ca2+

Thrombomodulin (TM) is a cofactor for protein C activation by thrombin and each residue of a consensus Ca2+ site in the sixth epidermal growth factor domain (EGF6) is essential for this cofactor activity [Nagashima, M., Lundh, E., Leonard, J.C., Morser, J. & Parkinson, J.F. (1993) J. Biol. Chem. 268, 2888-2892]. Three soluble analogs of the extracellular domain of TM, solulin (Glu4-Pro490), TME1-6 (Cys227-Cys462) and TMEi4-6 (Val345-Cys462) were prepared for equilibrium dialysis experiments by exhaustive dialysis against Ca2+-depleted buffer. However, all three analogs still contained one tightly bound Ca2+ (Kd approximately 2 microm), which could only be removed by EDTA. Epitope mapping with Ca2+-dependent monoclonal antibodies to EGF6 provided further localization of this tight Ca2+ site. Equilibrium dialysis of the soluble TM analogs in [45Ca2+] between 10 and 200 microm revealed a second Ca2+ site (Kd = 30 +/- 10 microm) in both solulin and TME1-6, but not in TMEi4-6. Ca2+ binding to this second site was unaffected by bound thrombin and we attribute it to the consensus Ca2+ site in EGF3. A 75-fold decrease in the binding affinity of thrombin to TM was observed with immobilized solulin treated with EDTA to remove the high affinity Ca2+ by measuring kassoc and kdiss rates in a BIAcoretrade mark instrument. Ca2+-dependent conformational transitions detected by CD spectroscopy in the far UV indicate a more ordered structure upon Ca2+ binding. Bound Ca2+ stabilized soluble TM against protease digestion at a trypsin-like protease-sensitive site between Arg456 and His457 in EGF6 compared with protease treatment in EDTA. Finally, TM containing EGF domains 4-6, but lacking the interdomain loop between EGF3 and 4 (TME4-6), has an identical Ca2+ dependence for the activation of protein C as found for TMEi4-6, indicating this interdomain loop is not involved in Ca2+ binding.

Selective on-line enrichment and separation of peptides having aromatic amino acids at their C-termini by column-switching high-performance liquid chromatography using an anhydrochymotrypsin-immobilized precolumn

A column switching high-performance liquid chromatographic system in which peptides retained on an anhydrochymotrypsin (AHC)-immobilized diol-silica precolumn were selectively transferred to and separated on a reversed-phase analytical column was developed. An investigation of the affinity characteristics of 40 peptides to the AHC precolumn showed that among eleven peptides having Tyr or Phe at their C-termini and more than five amino acid residues, ten were retained almost quantitatively on the precolumn, and two peptides having Trp at their C-termini were less retained. Two peptides having C-terminal PheNH2 were also retained, but the peptide having C-terminal D-PheNH2 was not retained. Among eighteen peptides having no aromatic amino acids at their C-termini, two were retained, one slightly and the other moderately. Calibration graphs for rat atrial natriuretic peptide constructed at various sample sizes were nearly identical, indicating that the peptide could be enriched by this system. The AHC precolumn showed no loss of analytical performance over 1 year, during which about 450 samples were analysed.

Trypsin and affinity chromatography

Affinity adsorbents for trypsin which were prepared by immobilizing product-type ligands, that is, peptides having C-terminal arginine, proved to be effective not only for preparative purposes but also for basic research on molecular recognition. The properties of the binding site of trypsin were revealed by chromatographic experiments. Quantitative analysis based on the theory of frontal affinity chromatography proved to be extremely effective. As an extension of the product-type ligands, peptide argininals were also used and information on the mechanism of action of these inhibitors was obtained. Anhydrotrypsin, which lost the hydroxyl group of Ser183, was found to gain increased binding ability for product-type compounds. This inactivated enzyme was also used as an immobilized ligand and the unique affinity adsorbent thus prepared proved to be extremely effective for the separation of peptides and recombinant proteins based on their C-terminal structures. High-performance affinity chromatography of trypsin and related enzymes using a polymer-based support was also developed.

Specific isolation by anhydrotrypsin-agarose chromatography of a recombinant protein tagged with an affinity tail arginine at the C-terminus

A characteristic property of anhydrotrypsin, i.e., its ability to strongly bind C-terminal arginine, proved to be useful as a tool for specific enrichment of a recombinant protein. An arginine tail was introduced at the C-terminus, for example, of a human beta-galactoside-binding lectin by site-directed mutagenesis. The resulting mutant recombinant lectin, which retained sugar-binding activity as high as the wild-type lectin, became recognizable by anhydrotrypsin. It was adsorbed on an anhydrotrypsin-agarose column and eluted with benzoylglycylarginine. The added arginine tail could be specifically removed by carboxypeptidase B. When E. coli lyzate containing the mutant lectin was applied to the column more than 10-fold enrichment of the mutant lectin was attained. This procedure should be generally applicable and may be advantageous because the addition of a single arginine residue may have minimal effect on the structure and function of the target protein.

Anhydroelastase: enhanced affinity toward product-type ligands revealed by affinity chromatography

Anhydroelastase was effectively isolated by a single operation of affinity chromatography from a complex mixture produced by phenylmethylsulfonylation and alkaline treatment of porcine pancreatic elastase. The adsorbent used for the chromatography was 6-aminohexanoyl-trialanine, which corresponds to a product of elastase action, immobilized on Sepharose 4B. Successful resolution by the operation indicated that this immobilized ligand possesses the highest affinity for anhydroelastase among various proteins including regenerated elastase in the mixture. Comparative affinity chromatography on immobilized anhydroelastase and on immobilized native elastase further confirmed the stronger interaction of anhydroelastase with the product-type peptides. Immobilized anhydroelastase was also found to be useful in the purification and search for naturally occurring proteinase inhibitors.