Question

Glycan groups of membrane glycoproteins and glycolipids serve as high-specificity binding sites for cell recognition proteins. Which has higher structural complexity: a mixture of tripeptides generated from Thr, Glu, and Ala or a mixture of trisaccharides generated from D-xylose, D-glucose, and D-galactose? Explain your answer using numerical examples to illustrate the extent of structural variety in the mixtures.

Answer 1

Glycoproteins

There are three glycoprotein hormone receptors (GpHRs) functioning, respectively, as the receptors for the pituitary hormones

thyrotropin (TSH receptor)

follitropin (FSH receptor)

and lutropin (LH receptor).

The hormones, TSH, FSH, and LH/CG are dimeric proteins of about 30kDa made of a common alpha subunit and specific beta subunits. The beta subunits of TSH, FSH, and LH are encoded by paralogous genes displaying substantial sequence similarity (about 40% in terms of encoded amino acids). Their corresponding receptors, FSHr, LH/CGr and TSHr, are members of the rhodopsin-like G protein-coupled receptor family. Their serpentine portion contains seven transmembrane helices with sequence signatures typical of this receptor family. In addition, they display a large (350-400 residues) amino terminal ectodomain responsible for the high affinity and selective binding of the corresponding hormones .

Glycolipids.

Glycolipids are glycoconjugates of lipids that are generally found on the extracellular face of eukaryotic cellular membranes, and function to maintain stability of the membrane and to facilitate cell–cell interactions. Glycolipids can also act as receptors for viruses and other pathogens to enter cells.

The main function of glycolipids in the body is to serve as recognition sites for cell–cell interactions. The saccharide of the glycolipid will bind to a specific complementary carbohydrate or to a lectin (carbohydrate-binding protein), of a neighboring cell. The interaction of these cell surface markers is the basis of cell recognitions, and initiates cellular responses that contribute to activities such as regulation, growth, and apoptosis.