Glycosylation and Protein Processing Quiz

Study Notes

Glycosylation is a process where carbohydrate molecules are added to proteins and lipids.
This process occurs in various locations, depending on the organism and the target molecule.
Prokaryotes, fungi, and some eukaryotic organisms use glycosylation to maintain cell wall structure, resist osmotic stress, and provide structural integrity to their cellular components.
In eukaryotes, glycosylation is crucial for protein folding, solubility, and preventing unwanted interactions with other molecules.
Furthermore, glycosylation plays a crucial role in determining the function and stability of membrane proteins, such as receptors, adhesion molecules, and channels.

Protein processing begins when ribosomes assemble proteins from peptides in the cytoplasm, which subsequently enter the rough endoplasmic reticulum (ER).
The ER modifies the proteins.
Vesicles transport proteins from the ER to the Golgi apparatus where they are further processed.
These vesicles contain finished proteins and travel to the cell membrane to release the proteins outside the cell.

Secreted proteins are often N-glycosylated, O-glycosylated, modified with glycosylphosphatidylinositol (GPI) anchors, or modified with glycosaminoglycans.
N-linked glycans and GPI anchors are pre-assembled before being added to proteins and further modified in the ER-Golgi pathway.
O-linked glycans and glycosaminoglycans, as well as the glycosylation of lipids, occur in both the ER and Golgi.
Various enzymes are involved in all these glycosylation processes.

Glycans are added to growing chains inside the ER or Golgi, facing the inside of the lumen, then face the outside cell.
Some glycan chains are made on the cytoplasmic side and then flipped to the other side.
Some proteins never enter the ER lumen, but instead are directly transferred through the plasma membrane (e.g., lectins).
Different glycotransferases have distinct roles in adding glycans to the protein and the types of glycans differ between the two sides of the membrane.

Glycosyltransferases use activated monosaccharides to add glycans to proteins and other molecules.
Nucleotide sugars and lipid-phosphate-linked sugars (e.g. dolicholphosphate mannose) are used by the glycosyltransferases.
Glycan modifications such as sulfation, acetylation, and methylation, often are carried out by other types of enzymes.
Almost all glycosylation donors are made in the cytosolic compartment and then transported to the ER and Golgi.

Type II membrane proteins have an amino terminal cytoplasmic tail, a transmembrane region, and a carboxy terminal catalytic domain.
Golgi-lumen enzymes synthesize glycan chains on proteins and lipids.
Some enzymes are synthesized in the Golgi (soluble) or derived from integrated enzymes in the Golgi compartments.
Secretory glycosyltransferases can add sugars to the cell surface.

Enzymes, their glycan substrates, and nucleotide sugar donors must be in the same compartment.
Enzymatic reactions in the biosynthesis pathway are separated into distinct overlapping compartments in the Golgi.
Different enzyme types segregate into specific areas (cis, medial, trans) of the Golgi for different phases of biosynthesis.
Mechanisms ensure enzymes remain in appropriate regions of the Golgi.

Enzyme complexes catalyze sequential reactions within the same Golgi cistern.
Different types of glycoconjugates involve sequential enzymes in different compartments of the Golgi.