Editing Cisterna (section)

== Golgi glycosylation ==
The Golgi apparatus plays a critical role in the modification of proteins through [[glycosylation]], particularly [[N-linked glycosylation|''N''-linked glycosylation]], which is a crucial process for the proper folding, stability, and function of many secretory and membrane-bound proteins.<ref name=":2">{{Cite journal |last1=Liu |first1=Jianyang |last2=Huang |first2=Yan |last3=Li |first3=Ting |last4=Jiang |first4=Zheng |last5=Zeng |first5=Liuwang |last6=Hu |first6=Zhiping |date=2021-04-01 |title=The role of the Golgi apparatus in disease (Review) |journal=International Journal of Molecular Medicine |volume=47 |issue=4 |pages=1 |doi=10.3892/ijmm.2021.4871 |pmid=33537825 |pmc=7891830 |issn=1107-3756}}</ref> ''N''-linked glycosylation involves the attachment of [[oligosaccharide]]s to the nitrogen atom of asparagine residues in proteins. These oligosaccharides are composed of various sugar units, including N-acetylglucosamine (GlcNAc), mannose (Man), galactose (Gal), and N-acetylneuraminate (NANA, also known as sialic acid). These glycosylated structures are integral for proper protein function, influencing cellular interactions, protein trafficking, and immune recognition.<ref name=":3">{{Cite book |last=Lodish |first=Harvey F. |title=Molecular cell biology (Eighth edition ed.) |date=2016 |publisher=New York: W. H. Freeman-Macmillan Learning. |isbn=978-1-4641-8339-3}}</ref>

''N''-linked glycosylation begins in the rough endoplasmic reticulum (ER), where a precursor oligosaccharide is synthesized on a lipid carrier called dolichol. The precursor consists of a core structure made up of two N-acetylglucosamine (GlcNAc) residues, nine mannose (Man) residues, and three glucose (Glc) residues.<ref name=":3" /> The precursor is then transferred to a protein's asparagine residue as soon as the protein enters the ER lumen. The attachment of the oligosaccharide to the asparagine is catalyzed by the enzyme oligosaccharyltransferase.<ref>{{Cite journal |last1=Mohanty |first1=Smita |last2=P Chaudhary |first2=Bharat |last3=Zoetewey |first3=David |date=2020 |title=Structural Insight into the Mechanism of N-Linked Glycosylation by Oligosaccharyltransferase |journal=Biomolecules |language=en |volume=10 |issue=4 |pages=624 |doi=10.3390/biom10040624 |doi-access=free |pmid=32316603 |pmc=7226087 |issn=2218-273X}}</ref>

Once the glycosylated protein enters the ER, further processing of the oligosaccharide occurs. Three specific enzymes play key roles in this early stage of glycosylation. First, [[Mannosyl-oligosaccharide glucosidase|glucosidase I]] removes one glucose residue from the oligosaccharide. Then, [[glucosidase II]] removes two more glucose residues, leaving behind a core oligosaccharide attached to the protein. Finally, a mannosidase enzyme removes one mannose residue.<ref name=":3" /> After this initial trimming, the oligosaccharide is ready to move from the ER to the Golgi apparatus for more elaborate modifications.

In the Golgi, further trimming and addition of sugar residues occur, particularly the removal of [[mannose]] and the addition of various sugars such as [[GlcNAc]], [[galactose]] (Gal), and [[sialic acid]] (NANA). Golgi [[mannosidase I]] and [[mannosidase II]] remove additional mannose residues from the oligosaccharide, further refining its structure. GlcNAc transferase then adds GlcNAc residues to the growing oligosaccharide chain by transferring GlcNAc from UDP-GlcNAc. In the medial-Golgi, the oligosaccharide undergoes more modifications, including the addition of two GlcNAc units, three Gal residues, and finally three sialic acid (NANA) residues in the trans-Golgi network.<ref name=":3" />

Each compartment of the Golgi plays a distinct role in glycosylation and protein processing. The cis-Golgi network is involved in the phosphorylation of oligosaccharides on lysosomal proteins, a modification that helps target proteins to the lysosomes. The medial-Golgi is the site of important reactions like the trimming of mannose and the addition of GlcNAc, which is essential for the formation of complex glycan structures. In the trans-Golgi, galactose is added to the oligosaccharide, further refining the glycan structure. The trans-Golgi network is responsible for adding sialic acid (NANA) and sorting proteins into [[Vesicle (biology and chemistry)|vesicle]]s destined for lysosomes or [[protein secretion|secretion]].<ref name=":3" /> These specialized modifications and sorting are crucial for protein functionality and their subsequent cellular destinations.

The organization of the Golgi compartments into cisternae—stacks of membrane-bound structures—ensures that enzymes are properly localized to each region, facilitating the sequential and highly regulated modification of oligosaccharides. The Golgi apparatus plays a pivotal role in ''N''-linked glycosylation, a process that begins in the ER and is elaborated within the Golgi. Through the sequential trimming and addition of sugars like GlcNAc, mannose, galactose, and sialic acid, the Golgi ensures that proteins are properly modified for their final functional roles. The distinct regions of the Golgi, from the cis-Golgi to the trans-Golgi network, work in concert to facilitate the precise modification and sorting of glycoproteins, which are essential for a wide range of cellular functions.