Question

Based on your knowledge the mechanism of glycoprotein synthesis, explain why all of the glycosylated portions of cell membrane proteins are on the extracellular domains of the proteins. Include a diagram that illustrates your explanation.

Answer 1

The fluid mosaic model given by singer and nicolson explains the structure of plasma membrane. Plasma membrane of the cells contains lipid bilayer with intrinsic and extrinsic proteins. Transmembrane proteins Or intrinsic proteins can function on both the sides of the membrane and transports molecules across it.

The majority of the transmembrane proteins are glycosylated. As in the glycolipids the sugar molecule is added into the lumen of the ER and Golgi apparatus and hence the oligosaccharide chains are always present on the non cystolic side of the membrane.

Another difference is the formation of disulphide bonds between cysteines on the cytosolic side of the membranes and non cytosolic sides of the membranes.

The reducing environment of the cytosol favours the formation of disulphide bonds towards the nonsystolic side of the membrane where it helps in stabilising the folded structure of the polypeptide chain Or it's association with the other polypeptide chain.

The presence of these oligosaccharide chains on the cell surface helps in the specific cell recognition processes.