RCT Chapter 7
Aldoses and Ketoses; Representative monosaccharides. (a)two trioses, an aldose and a ketose. The carbonyl group in each is shaded. An aldose contains an aldehyde functionality A ketose contains a ketone functionality
Formation of maltose. A disaccharide is formed from two monosaccharides (here, two molecules of D-glucose) when an OH (alcohol) of one glucose molecule (right) condenses with the intramolecular hemiacetal of the other glucose molecule (left), with elimination of H 2 O and formation of a glycosidic bond. The reversal of this reaction is hydrolysis attack by H 2 O on the glycosidic bond. The maltose molecule, shown here as an illustration, retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond. Because mutarotation interconverts the α and β forms of the hemiacetal, the bonds at this position are sometimes depicted with wavy lines, as shown here, to indicate that the structure may be either α or β.
Two common disaccharides. Like maltose in these are shown as Haworth perspectives. The common name, full systematic name, and abbreviation are given for each disaccharide. Formal nomenclature for sucrose names glucose as the parent glycoside, although it is typically depicted as shown, with glucose on the left.
Polysaccharides Natural carbohydrates are usually found as polymers These polysaccharides can be homopolysaccharides heteropolysaccharides linear branched Polysaccharides do not have a defined molecular weight. This is in contrast to proteins because unlike proteins, no template is used to make polysaccharides
Glycogen Glycogen is a branched homopolysaccharide of glucose Glucose monomers form ( 1 4) linked chains Branch-points with ( 1 6) linkers every 8 12 residues Molecular weight reaches several millions Functions as the main storage polysaccharide in animals
Starch Starch is a mixture of two homopolysaccharides of glucose Amylose is an unbranched polymer of ( 1 4) linked residues Amylopectin is branched like glycogen but the branch-points with ( 1 6) linkers occur every 24 30 residues Molecular weight of amylopectin is up to 200 million Starch is the main storage polysaccharide in plants
Glycosaminoglycans Linear polymers of repeating disaccharide units One monomer is either N-acetyl-glucosamine or N-acetyl-galactosamine Negatively charged Uronic acids (C6 oxidation) Sulfate esters Extended hydrated molecule Minimizes charge repulsion Forms meshwork with fibrous proteins to form extracellular matrix Connective tissue Lubrication of joints
Repeating units of some common glycosaminoglycans of extracellular matrix.
Heparin and Heparan Sulfate Heparin is linear polymer, 3 40 kda Heparan sulfate is heparin-like polysaccharide but attached to proteins Highest negative charge density biomolecules Prevent blood clotting by activating protease inhibitor antithrombin Binding to various cells regulates development and formation of blood vessels Can also bind to viruses and bacteria and decrease their virulence
Glycoconjugates: Glycoprotein A protein with small oligosaccharides attached Carbohydrate attached via its anomeric carbon About half of mammalian proteins are glycoproteins Carbohydrates play role in protein-protein recognition Only some bacteria glycosylate few of their proteins Viral proteins heavily glycosylated; helps evade the immune system
Glycoconjugates: Proteoglycans Sulfated glucoseaminoglycans attached to a large rod-shaped protein in cell membrane Syndecans: protein has a single transmembrane domain Glypicans: protein is anchored to a lipid membrane Interact with a variety of receptors from neighboring cells and regulate cell growth
This is heamaglutinin. The lectins are carbohydrate-binding proteins (not to be confused with glycoproteins, which are proteins containing sugar chains or residues) that are highly specific for sugar moieties, particularly, the high specificity of plant lectins for foreign glycoconjugates (e.g. those of fungi, invertebrates and animals). Lectins serve many different biological functions in animals, from the regulation of cell adhesion to glycoprotein synthesis and the control of protein levels in the blood. They may also bind soluble extracellular and intercellular glycoproteins. Some lectins are found on the surface of mammalian liver cells that specifically recognize galactose residues
Two families of membrane proteoglycans. (a) Schematic diagrams of a syndecan and a glypican in the plasma membrane. Syndecans are held in the membrane by hydrophobic interactions between a sequence of nonpolar amino acid residues and plasma membrane lipids; they can be released by a single proteolytic cut near the membrane surface. In a typical syndecan, the extracellular amino-terminal domain is covalently attached to three heparan sulfate chains and two chondroitin sulfate chains. Glypicans are held in the membrane by a covalently attached membrane lipid (GPI anchor), but are shed if the bond between the lipid portion of the GPI anchor and the oligosaccharide linked to the protein is cleaved by a phospholipase. All glypicans have 14 conserved Cys residues, which form disulfide bonds to stabilize the protein moiety, and either two or three glycosaminoglycan chains attached near the carboxyl terminus, close to the membrane surface.
Proteoglycan structure, showing the tetrasaccharide bridge. A typical tetrasaccharide linker (blue) connects a glycosaminoglycan in this case chondroitin 4-sulfate (orange) to a Ser residue in the core protein. The xylose residue at the reducing end of the linker is joined by its anomeric carbon to the hydroxyl of the Ser residue.
Proteoglycan aggregate of the extracellular matrix. Schematic drawing of a proteoglycan with many aggrecan molecules. One very long molecule of hyaluronan is associated noncovalently with about 100 molecules of the core protein aggrecan. Each aggrecan molecule contains many covalently bound chondroitin sulfate and keratan sulfate chains. Link proteins at the junction between each core protein and the hyaluronan backbone mediate the core protein hyaluronan interaction. The micrograph shows a single molecule of aggrecan, viewed with the atomic force microscope.
Interactions between cells and the extracellular matrix. The association between cells and the proteoglycan of the extracellular matrix is mediated by a membrane protein (integrin) and by an extracellular protein (fibronectin in this example) with binding sites for both integrin and the proteoglycan. Note the close association of collagen fibers with the fibronectin and proteoglycan.
Bacterial LPS. Schematic diagram of the LPS of the outer membrane of Salmonella typhimurium. Kdo is 3-deoxy-D-mannooctulosonic acid (previously called ketodeoxyoctonic acid); Hep is L-glycero-Dmanno-heptose; AbeOAc is abequose (a 3,6- dideoxyhexose) acetylated on one of its hydroxyls. There are six fatty acid residues in the lipid A portion of the molecule. Different bacterial species have subtly different LPS structures, but they have in common a lipid region (lipid A), a core oligosaccharide also known as endotoxin, and an O-specific chain, which is the principal determinant of the serotype (immunological reactivity) of the bacterium. The outer membranes of the gram-negative bacteria S. typhimurium and E. coli contain so many LPS molecules that the cell surface is virtually covered with O-specific chains.