PLEASE ANSWER ALL 4 QUESTIONS Compare the types of protein glycosylation in the

Question

PLEASE ANSWER ALL 4 QUESTIONS

Compare the types of protein glycosylation in the ER and Golgi in terms of function, diversity of carbohydrate chains produced, and site to which carbohydrate chains are added.

Discuss the path taken by secreted proteins as well as by soluble proteins that are not destined for secretion and explain the general characteristics that allow proteins to be targeted to a specific organelle.

Explain where you would expect to find a particular domain of a plasma membrane transmembrane protein based on where this domain was found when the protein was still present at the ER membrane

Discuss how proteins and lipids synthesized at the ER get sorted to the correct destinations (e.g. plasma membrane, extracellular space, lysosomes, ER, Golgi)

Explanation / Answer

N-linked glycosylation of proteins in Rough endoplasmic reticulum:

biosynthesis of N-linked oligosaccharides begins in RER with the addition of a large preformed oligosaccharide precursor. this precursor oligosaccharide is linked by a pyrophosphoryl residue to dolichol, a long chain (75-95 carbon atoms) polyisoprenoid lipid that is firmly embedded in the ER membrane and acts as a carrier for the oligosaccharide. the structure of the N-linked oligosaccharide is same in plants, animals, and single-celled eukaryotes- a branched oligosaccharide, containing three glucose, nine mannose, and two N-acetylglucosamine molecules which is written as (Glc3 Man9 GlcNAc2).

biosynthesis of oligosaccharide begins on the cytoplasmic face of the ER membrane with the transfer of N-acetyl glucosamine. the entire Glc3 Man9 GlcNAc9 oligosaccharide is transferred en bloc from the dolichol carrier to an asparagine residue on a nascent polypeptide, a reaction catalyzed by oligosaccharide-protein transferase. only asparagine residues in the tripeptide sequences Asn-X-Ser and Asn-X-Thr (where X is any amino acid except proline) are substrates for this transferase.

immediately after the oligosaccharide is transferred to a nascent polypeptide, all three glucose residues and one particular mannose residue are removed by different enzymes. the glucose residues act as a signal that the oligosaccharide is complete and ready to be transferred to a protein.

O-linked glycosylation of polypeptides in Golgi

N-linked oligosaccharide chains are altered as the proteins pass through the Golgi cisternae en route from ER. some proteins undergo O-linked glycosylation in Golgi. O-linked oligosaccharides are linked to the hydroxyl group of serine or threonine via N-acetylgalactosamine (GalNac) or (in collagens) to the hydroxyl group of hydroxylysine via galactose. O-linked oligosaccharides are generally short, often containing only one to four sugar residues. O-linked sugars are added one at a time, and each sugar transfer is catalyzed by a different glycotransferase enzyme.

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