Glycosylation and Protein Processing Quiz

Glycosylation: Location and Activity

• 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 and Secretion

• 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.

Types of Glycosylation

• 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.

Initiation and Maturation of Eukaryotic Glycoconjugates

• 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

• 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.

Role of Glycotransferases in the Golgi Apparatus

• 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.

Golgi Glycosylation Enzyme Localization

• 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.

Mechanisms in Golgi Compartments

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