Application of the reaction of dithioesters with epsilon-amino groups in lysine to the chemical modification of proteins

Protein thioacylation: 2. Reagent stability in aqueous media and thioacylation kinetics

Several thioacylating reagents have been tested toward hydrolysis under conditions suitable for protein modifications: 20-35 degrees C and buffered solutions at pH 7.5-8.5. Aliphatic dithioesters are sufficiently stable in aqueous media at room temperature (or below) if protein modification reaction time does not exceed 24 h, whereas at 35 degrees C reaction times must be limited to a few hours. Kinetic data obtained in gelatin thioacylation at room temperature using aliphatic dithioesters and dithio acid are consistent with a second-order reaction rate with respect to amine concentration. The pH dependence of the second-order reaction rate constants indicate that dithioester reacts exclusively with the free amine form of lysine residue, whereas dithiocarboxylate ion reacts with both amine and ammonium ion, probably through a more complex mechanism. Interestingly thioacylation using dithio acids may be obtained in pH near neutrality or in slightly acidic media, thus offering protein modification possibilities at pH 5-9. Thioacylation reaction rates may be expressed as R = -(dAt/dt) = k[H3O+](-b)At2[thioacylating agent] in which At is the amine concentration at time t, constants k and b depending on the reagent nature.

Protein thioacylation. 1. Reagents design and synthesis

Thioacylation is a new way for protein chemical modification. Carboxylic dithioesters and -acids react selectively and rapidly at room temperature with aliphatic amines such as lysine epsilon-amino groups leading to thioamide formation, without any other reagent or catalyst. Various thioacylating reagents were synthesized: monofunctional dithioesters bearing on the acylating end various chemical groups such as: aliphatic chains, phenyl group, mono- and dicarboxylic acids, dialkylphosphonic ester, phosphonic acid, thiol, phenol, or quaternary ammonium group. Bifunctional dithioesters containing either a polymethylene chain or an ethylene oxide oligomer as spacer group as well as some mono- and bis(dithio acids) are described. Applications of thioacylation may be involved either in enzyme chemical modifications or in the obtention of new materials from proteins. Bifunctional reagents might be used as cross-linking or coupling reagents.

Thermostabilized chemical derivatives of horseradish peroxidase

Horseradish peroxidase finds a variety of uses in analysis, immunology, organic synthesis, and biosensors. Although moderately stable, its applicability to biosensors and other fields would be greatly enhanced if it could be made yet more stable. Appropriate chemical modification can substantially stabilize enzymes. Here we describe the use of bis-imidates and of bis-succinimides to modify free amino groups of commercial horseradish peroxidase under mild conditions of pH and temperature. Imidates yielded a marginal stabilization. Some of the succinimide derivatives, however, are much more thermostable than the native enzyme. Apparent half-lives indicate stabilizations of 6- to 23-fold, depending on the bis-succinimide used. These modifications preserve the carbohydrate side chains for subsequent reaction or immobilization.

Effect of linking allyl and aromatic chains to histidine 170 in horseradish peroxidase

Histidine residues in horseradish peroxidase (HRP) were modified chemically with diethyl pyrocarbonate, 4,omega-dibromoacetophenone or diallylpyrocarbonate. Histidines were chosen as His-170, the fifth ligand of the heme iron atom, forms part of the active site of this enzyme. Good yields of hemoprotein were obtained in all cases. Analysis by HPLC of peptides obtained after tryptic digestion showed that His-170 of HRP was in fact modified. The specific activity remained satisfactory after chemical modification of the histidine residues, and so the active site of HRP can thus be altered without a dramatic loss of hemoprotein or peroxidase activity. This may open routes to the preparation of novel biocatalysts.