Improvement of oxygen-carrying properties of human hemoglobin by chemical modification with a benzene hexacarboxylate-monosubstituted polyoxyethylene

Molecular insight into the steric shielding effect of PEG on the conjugated staphylokinase: biochemical characterization and molecular dynamics simulation

PEGylation is a successful approach to improve potency of a therapeutic protein. The improved therapeutic potency is mainly due to the steric shielding effect of PEG. However, the underlying mechanism of this effect on the protein is not well understood, especially on the protein interaction with its high molecular weight substrate or receptor. Here, experimental study and molecular dynamics simulation were used to provide molecular insight into the interaction between the PEGylated protein and its receptor. Staphylokinase (Sak), a therapeutic protein for coronary thrombolysis, was used as a model protein. Four PEGylated Saks were prepared by site-specific conjugation of 5 kDa/20 kDa PEG to N-terminus and C-terminus of Sak, respectively. Experimental study suggests that the native conformation of Sak is essentially not altered by PEGylation. In contrast, the bioactivity, the hydrodynamic volume and the molecular symmetric shape of the PEGylated Sak are altered and dependent on the PEG chain length and the PEGylation site. Molecular modeling of the PEGylated Saks suggests that the PEG chain remains highly flexible and can form a distinctive hydrated layer, thereby resulting in the steric shielding effect of PEG. Docking analyses indicate that the binding affinity of Sak to its receptor is dependent on the PEG chain length and the PEGylation site. Computational simulation results explain experimental data well. Our present study clarifies molecular details of PEG chain on protein surface and may be essential to the rational design, fabrication and clinical application of PEGylated proteins.

Mono-N-terminal poly(ethylene glycol)-protein conjugates

A site-directed method of joining proteins to poly(ethylene glycol) is presented which allows for the preparation of essentially homogeneous PEG-protein derivatives with a single PEG chain conjugated to the amine terminus of the protein. This selectivity is achieved by conducting the reductive alkylation of proteins with PEG-aldehydes at lower pH. Working examples demonstrating the application of this method to improve the delivery characteristics and therapeutic value of several proteins are provided.

Hemoglobin linked to polyanionic polymers as potential red blood cell substitutes

A variety of chemical modifications of hemoglobin (Hb) have been proposed in order to transform it into cell-free oxygen carriers. These modifications are intended to increase the plasmatic half-life of the protein and to lower its affinity for oxygen. We have designed and prepared derivatives of dextran and polyoxyethylene functionalized so that, after their covalent fixation onto Hb, they can act as permanent effectors and thus lower the oxygen affinity of the protein while decreasing its renal excretion. The main characteristic of these functionalized polymers is that they possess a relatively high density of anionic groups. Benzene hexacarboxylate (BHC) and tetracarboxylate (BTC) linked to water-soluble polymers such as polyoxyethylene or dextran, yielded polymeric derivatives which, even after reaction with oxyHb, gave rise to conjugates with a lower oxygen affinity than the native protein. We showed on a conjugate obtained by the fixation of a BHC-monosubstituted polyoxyethylene onto oxyHb, that the low oxygen affinity was due to the preferential binding of the polymer-linked BHC to the beta-terminal valine residues. In the reaction of dextran-linked benzene tetracarboxylate (dex-BTC) with oxyHb, a lot of parameters had to be optimized in order to obtain conjugates well fitted to the purpose of blood substitute. Thus the polymer content in BTC and the amounts of reagents used for the reaction determined the oxygen-binding properties, the molecular weights and the biochemical characteristics of the conjugates, as well as the viscosity and oncotic pressure of the solutions. This optimization resulted in products which are now studied in-vivo.