ABSTRACT
Many proteins require post-translational modifications to insure the full biological activity that is essential for their use as efficacious biopharmaceuticals. The most widely recognized form of post-translational modification is glycosylation that requires an extensive sequence of processing and trimming events in the endoplasmic reticulum (ER) and Golgi apparatus. Because eukaryotic cells have the metabolic capability for these types of modifications they have become the host cells used in biopharmaceutical processes. More specifically, mammalian cells (particularly the hamster BHK and CHO cells) have become the host cells of choice because the glycosylation patterns generated from these cells are most similar to equivalent human patterns. Most therapeutic proteins are glycoproteins that have an extensive heterogeneity of glycoforms arising from incomplete glycosylation processes. Although glycosylation is the most extensive and widely studied form
of post-translational modification, there are other modifications that are equally important for the activity of specific proteins. The blood clotting factor, factor X, is a good example to illustrate a range of post-translational modifications on a single complex protein. The recognized modifications necessary to ensure bioactivity of this factor include: removal of signal sequences, formation of disulfide bonds, gamma-carboxylation of glutamic acid residues, modification of aspartate to beta-hydroxyaspartate, N-and O-glycosylation, removal of an internal tripeptide, and removal of a propeptide (Camire et al., 2000). In order to express this protein as a recombinant form in a cell culture bioprocess, it is necessary to insure that the host cell has the appropriate metabolic capabilities for efficient posttranslational modification and also to insure that the culture conditions are suitable to drive the metabolism to high productivity of the active protein. This chapter describes some of the most important and well-studied
forms of post-translational modifications of proteins, which have been associated with the production of glycoproteins as biopharmaceuticals from mammalian cell culture systems. Most attention is given to glycosylation because of its general importance for the activity of all these
proteins. However, other modifications are also described and related to the production of specific proteins.
