Biochemistry Online: An Approach Based on Chemical Logic

CHAPTER 3 - CARBOHYDRATES/GLYCANS

B: More Complex Carbohydrates

03/15/16

Learning Goals/Objectives for Chapter 3B: After class and this reading, students will be able to

state the differences between the homopolysaccharides glycogen, starch cellulose, and chitin and those with dissacharide repeat units (glycosoaminoglycans)
draw cartoon models of complex oligosaccharides such as peptidoglycans of bacterial cell walls, N and O linked glycoproteins, and proteoglycans showing the linkage of protein and CHO
describe the role of protein and cell surface CHO in binding and biological function
given diagrams of leukocyte and endothelial cells interactions, describe the role of selectins, selectin ligands, integrins and cellular adhesion molecules in immune cell/blood vessel interactions.

This chapter on complex carbohydrates (glycans/glycoconjugates) will review those features that are deemed especially important for a one semester course dealing with structure and function of biomolecules.

B6. Eukaroytic Cell Membranes

We have studied lipids, proteins, and carbohydrates. Although phospholipid can spontaneously form biliayers, the actual structure of biological membranes is made much more complicated through addition of protein and carbohydrate substituents to the membrane. Soluble proteins can be made to insert into bilayers by addition of nonpolar attachments. Several examples of such attachments include:

N-myristoylation (attached myristic acid - 14:0 - through an amide link)
S-palmitoylation (attached palmitic acid - 16:0 - through a thioester link with a Cys
farnesyl or geranylgeranyl additon to a CAAX carboxy-terminal sequence in a target protein, where C is Cys, A is aliphatic, and X is any amno acid
addition of a protein to a glycosyl phophatidylinositol (GPI), through a complex which usually contains a conserved tetrasaccharide core of 3 Man and 1 GlcNAc residues linked to a protein. The GPI can be further modified with extra Gal's and Man, as well as additions to the PI group, which secures the protein in the membrane. GPIs are found in eukaryotic cells, and link many surface antigens, adhesion molecules, and hydrolases to the membrane. GPIs from Plasmoidium falciparum, the malarial parasite which kills about two million people each year, appears to act as a toxin and is the most common CHO modification of the parasite protein. Mice immunized against the GPI sequence, NH2-CH2-CH2-PO4-Man (a1-2) 6Man (a1-2) Man (a1-6) Man (a1-4) GlcNH2 (a1-6) myo-inositol-1,2-cyclic-phosphate, were substantially protected from malarial symptoms and death after they were exposed to the actual parasite.

Figure: Biological Membranes: Simple to Complex

Figure: A cool view of a membrane surface

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