Question

A \(6\) month old infant presented with hypoglycemia, vomiting, diarrhea, protein-loss enteropathy, and hepatic fibrosis. Measurement of the glycosylation state of endogenous serum transferrin revealed Type Ib CDG (congenital disorders of glycosylation), which is a defect in phosphomannose isomerase activity. In Type Ic CDG, a defect in an enzyme transferring a glucosyl residue to a high-mannose dolichol pyrophosphate precursor, the carbohydrate structure would be part of a(n) A. \(N\) -linked glycoprotein. B. O-linked glycoprotein. C. proteoglycan. D. glycosaminoglycan. E. complex lipid.

Short answer

Answer: A. N-linked glycoprotein.

Step-by-step solution

Recalling the basics of glycoproteins

Glycoproteins are proteins that have carbohydrates attached to them. The carbohydrates may be in different forms, such as oligosaccharides or polysaccharides. Glycoproteins play vital roles in various biological processes, such as cell signaling, immune response, and cellular adhesion, among others. There are two main types of glycoproteins: N-linked and O-linked. The key difference between them is how the carbohydrate portion is attached to the protein. In N-linked glycoproteins, the carbohydrate is attached to the amide nitrogen of an asparagine residue. In O-linked glycoproteins, the carbohydrate is attached to the hydroxyl group of a serine or threonine residue. Proteoglycans and glycosaminoglycans are also closely related to glycoproteins. Proteoglycans are proteins that have glycosaminoglycans attached to them. Glycosaminoglycans are linear polysaccharides that consist of repeating disaccharide units. They play essential roles in the extracellular matrix, cell growth, and adherence.

Understanding Type Ic CDG

Type Ic CDG (congenital disorder of glycosylation) is a rare genetic disorder affecting the glycosylation process - the attachment of carbohydrates (specifically, a glucosyl residue) to a high-mannose dolichol pyrophosphate precursor. This process is critical in synthesizing N-linked glycoproteins.

Identifying the glycoprotein

Given the information from Step 2, we can deduce that the defective carbohydrate structure in Type Ic CDG is part of an N-linked glycoprotein. This is because the issue is with the enzyme transferring a glucosyl residue to a high-mannose dolichol pyrophosphate precursor, which is an essential step in N-linked glycoprotein synthesis. Therefore, the correct answer is: A. N-linked glycoprotein.