Site-directed enzymatic PEGylation of the human granulocyte colony-stimulating factor

Poly(ethylene glycol) (PEG) is a widely used polymer employed to increase the circulating half-life of proteins in blood and to decrease their immunogenicity and antigenicity. PEG attaches to free amines, typically at lysine residues or at the N-terminal amino acid. This lack of selectivity can present problems when a PEGylated protein therapeutic is being developed, because predictability of activity and manufacturing reproducibility are needed for regulatory approval. Enzymatic modification of proteins is one route to overcome this limitation. Bacterial transglutaminases are enzyme candidates for site-specific modification, but they also have rather broad specificity. The need arises to be able to predict a priori potential PEGylation sites on the protein of interest and, especially, to be able to design mutants where unique PEGylation sites can be introduced when needed. We investigated the feasibility of a computational approach to the problem, using human granulocyte colony-stimulating factor as a test case. The selected protein is therapeutically relevant and represents a challenging problem, as it contains 17 potential PEGylation sites. Our results show that a combination of computational methods allows the identification of the specific glutamines that are substrates for enzymatic PEGylation by a microbial transglutaminase, and that it is possible to rationally modify the protein and introduce PEG moieties at desired sites, thus allowing the selection of regions that are unlikely to interfere with the biological activity of a therapeutic protein.