Site-selective enzymatic chemistry for polymer conjugation to protein lysine residues: PEGylation of G-CSF at lysine-41
Microbial transglutaminase (mTGase) is an enzyme that catalyzes site-specific protein derivatization at specific glutamines. mTGase-mediated conjugation with PEG-NH2 to granulocyte colony stimulating factor (G-CSF) yields a site selective mono-derivative conjugate involving Gln135. The same enzymatic reaction of mTGase, i.e. the transfer of the Gln acyl group to an amino donor, was investigated by reversing the substrates. A specific acyl donor PEG derivative was synthesized by coupling the Z-QG mTGase substrate to PEG. The mTGase-mediated conjugation of this PEG-ZQG in the presence of G-CSF generated a high-yield PEG-G-CSF conjugate in which the polymer was selectively coupled to Lys41 of the protein. The PEG-K41-G-CSF conjugate was compared with the PEG-Q135-G-CSF one obtained through mTGase conjugation of PEG-NH2 to Gln135. Biophysical characterization showed that the two positional isomers have similar behaviors, and pharmacokinetic studies in rats demonstrated that they have comparable half-life extensions. Overall, the study demonstrates that mTGase protein derivatization is linked to inherent advantages because it carries with it the possibility of targeting lysines or glutamines, in both cases with a high site-selective specificity.