Both the N-terminal and C-terminal nonmembrane domains have many hydrophilic amino acids that are not found in the trans-membrane α-helix segment. This segment is totally devoid of sugars. Finally, glycophorin a has a 29 amino acid C-terminal segment that extends into the aqueous space in the erythrocyte interior. Glycophorin a is held in the membrane via a 22-amino acid single span trans-membrane α-helix. O-linked serine and threonine and N-linked asparagine are the usual way sugars are linked to membrane proteins ( Chapter 7). The attached sugar tetrasaccharides are O-linked to serine or threonine while the larger oligosaccharides are N-linked to asparagines. As with cytochrome b 5, this extracellular segment can be cleaved by trypsin. These carbohydrates are attached to only a few of the first 50 amino acids of glycophorin's 80 amino acid exterior sequence. All of glycophorin's carbohydrates are attached to amino acids on the N-terminal, exterior segment. Glycophorin a is a single span integral protein of 131 amino acids whose cartoon structure is depicted in Fig. 6.7. With age, the loss of sugars triggers destruction of old erythrocytes. The negative charges on glycophorin cause the erythrocytes to repel one another, preventing them from clumping in the blood. Abundant sialic acids ( Chapter 7) impart a negative charge to the erythrocyte outer surface. By weight, 60% of glycophorin is carbohydrate that is entirely attached to the protein's outer leaflet surface. Glycophorins a and b are the major glycoproteins in the erythrocyte plasma membrane and bear the antigenic determinants for the MN and Ss blood groups. However, despite the fact that glycophorin is one of the most studied of all membrane proteins, its biological function remains uncertain. Many of the classic membrane biochemical and biophysical studies discussed in later chapters were first worked out on erythrocytes, and many of the membrane protein studies were first done on resident glycophorin.
![hydrophobic amino acids in membran hydrophobic amino acids in membran](http://www.russelllab.org/aas/bbk_images2/Trp.gif)
Since erythrocytes, unlike other human cells, do not have any complicating internal membranes, and are easily obtained in pure form from blood, they have served for decades as the “laboratory for membrane studies”. Single Trans-Membrane α-Helix: Glycophorin William Stillwell, in An Introduction to Biological Membranes (Second Edition), 2016 4 Type I. Membrane proteins can be posttranslationally modified with lipids and carbohydrates, among other modifications. Some proteins are kinetically stabilized in the membrane, with a finite lifetime before denaturation to biologically inactive forms. Membrane proteins can diffuse in the plane of the membrane, though that can be restricted. Other membrane proteins form β-barrels, with hydrophobic residues pointing to the outside of the barrel. This produces a signature by which integral membrane proteins can often be identified by their linear sequence. The primary structure of many transmembrane proteins is organized to include linear sequences of 19–23 hydrophobic amino acids to span the hydrophobic interior of a membrane in a helix. Peripheral membrane proteins bind to integral membrane proteins through compatible binding sites and decorate the surfaces of membranes to support membrane functions.
![hydrophobic amino acids in membran hydrophobic amino acids in membran](https://www.78stepshealth.us/plasma-membrane/images/3342_121_160-integral-membrane-proteins.jpg)
Integral membrane proteins may be transmembrane (exposed on both sides of the membrane) or anchored (and exposed on only one side of the membrane).
![hydrophobic amino acids in membran hydrophobic amino acids in membran](https://img.homeworklib.com/questions/29562810-20ad-11eb-81e0-6952c7408fe8.png)
Integral membrane proteins have hydrophobic surfaces that allow and demand that they are incorporated into the hydrophobic portion of the lipid bilayer. Membrane proteins expose surfaces that are ideally suited for incorporation into, or binding to, membranes. Yeagle, in The Membranes of Cells (Third Edition), 2016 Abstract