C-terminal site-specific PEGylation of a truncated thrombomodulin mutant with retention of full bioactivity

Addition of poly(ethylene glycol) to bioactive proteins (PEGylation) improves their plasma half-life, enhances stability against proteolytic cleavage, and may also decrease protein immunogenicity. Characteristically, PEGylation usually involves a reaction to available lysine amino groups, some of which may be within or near a bioactive site. Thus, most protocols are nonspecific and result in a loss of protein activity. We report herein a strategy for site-specific PEGylation of a thrombomodulin (TM) derivative at the C terminus. A truncated TM mutant consisting of epidermal growth factor (EGF)like domains 4-6 was expressed in Escherichia coli with a C-terminal azido-methionine. The TM mutant was site-specifically conjugated to a methyl-PEG-triarylphosphine compound via the Staudinger reaction. Enzymatic activity of the TM construct before and after PEGylation was unchanged, which confirms the utility of this site-specific PEGylation scheme.