2.12:_Glycoproteins

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<p class="lt-bio-3790" style="background-color: unset;">Glycoproteins have carbohydrate attached to them &mdash; a process called <strong>glycosylation</strong>. The attachment is a covalent linkage to:</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">the hydroxyl (-OH) group of the R group of serine or threonine - called &quot;<strong>O-linked</strong>&quot; in both cases or to</li> <li class="lt-bio-3790" style="background-color: unset;">the amino group (-NH<sub><font size="2">2</font></sub>) in the R group of asparagine - called &quot;<strong>N-linked</strong>&quot;</li>
</ul> <p class="lt-bio-3790" style="background-color: unset;">The carbohydrate consists of short, usually branched, chains of</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">plain sugars (e.g., glucose, galactose)</li> <li class="lt-bio-3790" style="background-color: unset;">amino sugars (sugars with an amino group, e.g., N-acetylglucosamine), and</li> <li class="lt-bio-3790" style="background-color: unset;">acidic sugars (sugars with a carboxyl group, e.g., sialic acid)</li>
</ul> <p class="lt-bio-3790" style="background-color: unset;">Sugars are very hydrophilic thanks to their many -OH groups. Their presence</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">makes glycoproteins far more hydrophilic than they would be otherwise and</li> <li class="lt-bio-3790" style="background-color: unset;">are often essential for the proper folding of the protein into its tertiary structure</li>
</ul> <p class="lt-bio-3790" style="background-color: unset;">Most of the proteins exposed to the watery surroundings at the surface of cells are glycoproteins.</p> <p class="lt-bio-3790" style="background-color: unset;"><img class="internal right" style="width: 325px; height: 350px; float: right;" alt="Glycophorin spans a lipid bilayer, with carbohydrate chains attached to its extracellular region." loading="lazy" width="325px" height="350px" src="https://bio.libretexts.org/@api/deki/files/5158/glycophorin.gif?revision=1&amp;size=bestfit&amp;width=325&amp;height=350" aria-describedby="remedy-description-30dae56b3695ea0a" /><span id="remedy-description-30dae56b3695ea0a" data-remedy-image-description="30dae56b3695ea0a" style="display: block; white-space: pre-line;">A chain of labeled amino-acid circles runs from the N-terminal end outside the cell, through the lipid bilayer, to the C-terminal end inside. Residue numbers mark positions along the chain. The key identifies blue O-linked carbohydrate symbols attached to orange Ser or Thr residues, and green N-linked carbohydrate attached to a dark blue Asn residue. These carbohydrate attachments occur on the outside portion; the membrane-spanning and inside portions have none shown. The bilayer is drawn as two rows of heads with tails facing inward.</span>This image shows the primary structure of <strong>glycophorin A</strong>, a glycoprotein that spans the plasma membrane (&quot;Lipid bilayer&quot;) of human red blood cells. Each RBC has some 500,000 copies of the molecule embedded in its plasma membrane.</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">Fifteen carbohydrate chains are &quot;O-linked&quot; to serine (Ser) and threonine (Thr) residues</li> <li class="lt-bio-3790" style="background-color: unset;">One carbohydrate chain is &quot;N-linked&quot; to the asparagine (Asn) at position 26</li>
</ul> <p class="lt-bio-3790" style="background-color: unset;">Two polymorphic versions of glycophorin A, which differ only at residues 1 and 5, occur in humans. These give rise to the MN blood groups</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">The M allele encodes Ser at position 1 (Ser-1) and Gly at position 5 (Gly-5)</li> <li class="lt-bio-3790" style="background-color: unset;">The N allele encodes Leu-1 and Glu-5</li>
</ul> <span id="Genotype_to_Phenotype"></span><span id="Genotype_to_Phenotype-2"></span><h2 style="background-color: unset;" class="lt-bio-3790">Genotype to Phenotype</h2> <p class="lt-bio-3790" style="background-color: unset;">Individuals who inherit two N alleles (are homozygous) have blood group N.</p> <ul> <li class="lt-bio-3790" style="background-color: unset;">Individuals who are homozygous for the M allele have blood group M.</li> <li class="lt-bio-3790" style="background-color: unset;">Heterozygous individuals produce both proteins and have blood group <strong>MN</strong>.</li>
</ul> <p class="lt-bio-3790" style="background-color: unset;">Glycophorin A is the most important attachment site by which the parasite <em>Plasmodium falciparum</em> invades human red blood cells.</p>

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