Site-selective protein glycation and PEGylation

A two step protocol has been set up to selectively conjugate PEG to buried amino acids of proteins. The process involves site-specific glycation followed by PEGylation of the oxidized glycosides. Aimed at glycating the cysteine groups of proteins, two maleimide-glycosylic linkers have been synthesised: galactosyl-glucono-CO–NH–(CH2)12–NH–CO–(CH2)2-maleimide and maltosyl-glucono-CO–NH–(CH2)12–NH–CO–(CH2)2-maleimide. The linkers were extensively characterized by 1H NMR, FT-IR, ESI–TOF mass spectrometry and colorimetric assays. Complete conjugation of the activated linkers to Cys34 of human serum albumin was obtained in about 2 h. The selective oxidation of the galactosyl and maltosyl moieties by periodate treatment yielded two and three available aldehyde groups, respectively. The PEG-hydrazide conjugation to the aldehyde groups was found to be 100% in about 40 h, whereas less than 30% protein modification was obtained by direct conjugation of commercial PEG-maleimide to Cys34. The pH dependent PEG-glycosyl hydrazone bond hydrolysis at various pH values was verified. PEG release was faster under mild acidic and basic conditions than at neutral pH. Furthermore, the maltosyl derivatives, by virtue of the higher number of coupled PEG chains, showed a slower protein release as compared to the galactosyl counterpart, indicating that the choice of the glycosylic linker allows for control of protein release kinetics.