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How does the Golgi apparatus change carbohydrate chains on glycoproteins?

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Diagram of the Golgi apparatus showing its stacked, curved membranes with labeled parts including cisternae, cis face, trans face, lumen, and various vesicles involved in cellular transport and secretion. — Golgi apparatus A membrane-bound organelle of eukaryotic cells, the Golgi apparatus is made up of a series of flattened, stacked pouches called cisternae that function as a series of compartments that progressively refine and direct cellular products to their proper destinations. .

The Golgi apparatus modifies carbohydrate chains on glycoproteins through a series of enzyme-driven reactions as molecules progress from the cis face to the trans face of the organelle.1

These modifications are facilitated by specific enzymes, including glycosidases and glycosyltransferases, which are compartmentalized within the Golgi stacks. The process typically involves:12

Cleavage: The removal of specific sugar residues, such as mannose, which occurs primarily in the cis and medial cisternae.12
Addition: The attachment of different sugar moieties, such as galactose and sialic acid, which occurs primarily in the trans cisternae.12

These chemical alterations are essential for the maturation of glycoproteins. Proper modification of these carbohydrate groups is often required for the stability, biological activity, or correct intracellular and extracellular targeting of the protein. If these chains are modified incorrectly or fail to form, the transport of certain secretory proteins may be halted.12