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Glycosylation is affected by several factors, including: movement of the protein through the ER and Golgi apparatus, environmental factors, and sugar availability. The movement of protein through ER and Golgi apparatus can be affected by cholesterol and other free fatty acid availability that impact health of ER and Golgi membranes. Environmental factors include cell culture parameters such as pH, CO2, dissolved oxygen. Sugar availability is driven by the media constituents and any feeds that are used. Finally, lot-to-lot variation in commercially available supplements and feeds also introduce variation in glycosylation. It is important to select vendors and materials that have proven lot-to-lot consistency.
The wide variety of factors impacting glycosylation can make controlling the quality profile a complex challenge. Further, these same factors that drive post-translational modification also impact protein titer. In many cases scientists will focus optimization efforts on either titer or glycosylation first, but find that both titer and glycosylation are affected by any one change. In some cases, scientists will have to make a hard choice between quality and yield. It is possible to focus on the physiology of the cell and target the ER and Golgi to increase both titer and consistency.