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Protein Labeling and Conjugation

The most common functional groups targeted for bioconjugation on a protein are hydroxyl (present on Thr, Ser, and the phenolic group in Tyr), carboxylic acid (present at the C-terminus and on Asp and Glu), sulfhydryl (present on Cys), and amine (present at the N-terminus and Lys). Histidine's imidazolyl nitrogen and arginine's guanidinyl groups can also be targeted, but are less commonly used. Most proteins have multiple copies of the same amino acids, and conjugation chemistry targeting the functional groups of natural amino acids can result in heterogeneous bioconjugates.

To specifically label a protein site, an orthogonal functional group can be introduced into the system. So far, the most popular approach uses Cys incorporation by mutagenesis. At CellMosaic, we are experts in dealing with Cys mutant proteins and labeling them with different molecules. Alternatively, if the protein is glycosylated, it can be site-specifically labeled at its glycosylation site. In general, the hydroxyl groups on adjacent carbon atoms of carbohydrates are oxidized to generate formyl groups. At CellMosaic, we have different linkers and molecules that can be synthesized to specifically react with the formyl group under very mild conditions. The less controllable approach for obtaining homogeneous protein conjugates is through manipulation of the reaction conditions in combination with the purification method(s).