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Chapter 7

Chapter 7. Carbohydrates and the Glycoconjugates of Cell Surfaces Biochemistry by Reginald Garrett and Charles Grisham. Essential Question. What is the structure, chemistry, and biological function of carbohydrates? (CH 2 O) n or (C · H 2 O) n Breakdown of carbohydrates provides energy.

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Chapter 7

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  1. Chapter 7 Carbohydrates and the Glycoconjugates of Cell Surfaces Biochemistry by Reginald Garrett and Charles Grisham

  2. Essential Question • What is the structure, chemistry, and biological function of carbohydrates? • (CH2O)n or (C ·H2O)n • Breakdown of carbohydrates provides energy. • Glycolipids and glycoproteins are glycoconjugates involved in recognition between cell types or recognition of cellular structures by other molecules.

  3. Outlines • How Are Carbohydrates Named? • What Is the Structure and Chemistry of Monosaccharides? • What is the Structure and Chemistry of Oligosaccharides? • What is the Structure and Chemistry of Polysaccharides? • What Are Glycoproteins, and How Do They Function in Cells? • How Do Proteoglycans Modulate Processes in Cells and Organisms?

  4. 7.1 – How Are Carbohydrates Named? Carbohydrates are hydrates of carbon. • Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions. • Oligo = "a few" - usually 2 to 10 • Polysaccharides are polymers of the simple sugars.

  5. 7.2 – What Is the Structure and Chemistry of Monsaccharides? An organic chemistry review • Aldoses and ketoses contain aldehyde and ketone functions, respectively. • Triose, tetrose, etc. denotes number of carbons. • Aldoses with 3C or more and ketoses with 4C or more are chiral. • Review Fischer projections and D,L system.

  6. Stereochemistry Review Read text on p. 204-207 carefully! • D,L designation refers to the configuration of the highest-numbered asymmetric center. • D,L only refers the stereocenter of interest back to D- and L-glyceraldehyde! • D,L do not specify the sign of rotation of plane-polarized light! • All structures in Figures 7.2 and 7.3 are D. • D-sugars predominate in nature.

  7. More Stereochemistry • Know these definitions • Stereoisomers that are mirror images of each other are enantiomers. • Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers. • Any 2 sugars in a row in Figures 7.2 and 7.3 are diastereomers. • Two sugars that differ in configuration at only one chiral center are epimers.

  8. Cyclic monsaccharide structures and anomeric forms • Glucose (an aldose) can cyclize to form a cyclic hemiacetal. • Fructose (a ketose) can cyclize to form a cyclic hemiketal. • Cyclic form of glucose is mainly a pyranose. • Cyclic form of fructose is mainly a furanose.

  9. Cyclic monsaccharide structures and anomeric forms • Cyclic forms possess anomeric carbons. • For D-sugars,  has OH down,  has OH up. • For L-sugars, the reverse is true. • Mutarotation: The optical rotation of glucose solution could change with time. It involves interconversion of - and -D-glucose. • []D20 = 112.2 for -D-glucose • []D20 = 18.7 for -D-glucose

  10. Monosaccharide Derivatives • Reducing sugars: sugars with free anomeric carbons - they will reduce oxidizing agents, such as peroxide, ferricyanide and certain metals (Cu2+ and Ag+). • Fehling’s reagent: CuSO4 (blue) + RC(=O)H  Cu2O (red) + RCO2- • Tollen’s reagent: Ag+  Ag0 • These redox reactions convert the sugar to a sugar acid. • Glucose is a reducing sugar --- so these reactions are the basis for diagnostic tests for blood sugar.

  11. More Monosaccharide Derivatives • Sugar alcohols (alditols): sweet-tasting,from mild reduction of sugars • Deoxy sugars: constituents of DNA, etc. • Sugar esters: phosphate esters like ATP are important. • Amino sugars contain an amino group in place of a hydroxyl group. • Acetals, ketals and glycosides: basis for oligo- and poly-saccharides.

  12. 7.3 – What is the Structure and Chemistry of Oligosaccharides? It’s not important to memorize structures, but you should know the important features. • Be able to identify anomeric carbons and reducing and nonreducing ends. • Sucrose is NOT a reducing sugar. • Browse the structures in Figure 7.19 and Figure 7.20. • Note carefully the nomenclature of links! Be able to recognize (1,4), (1,4), etc.

  13. 7.4 – What is the Structure and Chemistry of Polysaccharides? Functions: storage, structure, recognition • Nomenclature: homopolysaccharide vs. heteropolysaccharide. • Lower the osmotic pressure. • Starch and glycogen are energy storage molecules. • Chitin and cellulose are structural molecules. • Cell surface polysaccharides are recognition molecules.

  14. Starch A plant storage polysaccharide • Two forms: amylose and amylopectin • Most starch is 10-30% amylose and 70-90% amylopectin. • Amylose has (1,4) links and one reducing end. • Amylopectinhas (1,6) branches in every 12-30 residues.

  15. Starch • Amylose and amylopectin are poorly soluble in water, but form micellar suspensions. • In these suspensions, amylose is helical and iodine fits into the helices to produce a blue color. Amylopectin produces a red-violet color with I2. • Salivary -amylase, an endoamylase, is (14)-glucan 4-glucanhydrolase. • -amylase is an exoamylase, cleaving maltose units. • (16)-glucosidase is required for complete hydrolysis of amylopepctin.

  16. Why branching in Starch? Consider the phosphorylase reaction... • Phosphorylase releases glucose-1-P, products from the amylose or amylopectin chains. • The more branches, the more sites for phosphorylase attack. • Branches provide a mechanism for quickly releasing (or storing) glucose units for (or from) metabolism.

  17. Glycogen --- the glucose storage device in animals • Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass. • Glycogen is stored energy for the organism. • Only difference from amylopectin: number of branches. • (1,6) branches every 8-12 residues . • Like amylopectin, glycogen gives a red-violet color with iodine. • Hydrolyzed by -, -amylase, and glycogen phosphorylase.

  18. Dextrans A small but significant difference from starch and glycogen. • If you change the main linkages between glucose from (1,4) to (1,6), you get a new family of polysaccharides – dextrans. • Branches can be (1,2), (1,3), or (1,4). • Dextrans formed by bacteria are components of dental plaque. • Cross-linked dextrans are used as "Sephadex" gels in columnchromatography. • These gels are up to 98% water!

  19. Structural Polysaccharides Composition similar to storage polysaccharides, but small structural differences greatly influence properties. • Cellulose is the most abundant natural polymer on earth. • Cellulose is the principal strength and support of trees and plants . • Cellulose can also be soft and fuzzy - in cotton.

  20. Other Structural Polysaccharides • Chitin - exoskeletons of crustaceans, insects and spiders, and cell walls of fungi. • similar to cellulose, but C-2s are N-acetyl • cellulose strands are parallel, chitins can be parallel or antiparallel. • Alginates – Ca2+-binding polymers in algae. • Agarose and agaropectin - galactose polymers • Glycosaminoglycans - repeating disaccharides with amino sugars and negative charges.

  21. Bacterial Cell Walls Composed of 1 or 2 bilayers and peptidoglycan shell • To resist high internal osmotic pressure, to maintain cell shape and size of bacteria. • Gram-positive: One bilayer and thick peptidoglycan outer shell. • Gram-negative: Two bilayers with thin peptidoglycan shell in between . • Gram-positive: pentaglycine bridge connects tetrapeptides. • Gram-negative: direct amide bond between tetrapeptides.

  22. More Notes on Cell Walls • Note the -carboxy linkage of isoglutamate in the tetrapeptide • Peptidoglycan is called murein - from Latin "murus", for wall • Gram-negative cells are hairy! Note the lipopolysaccharide "hair" in Figures 7.35 and 7.36.

  23. Cell Surface Polysaccharides A host of important functions! • Animal cell surfaces contain an incredible diversity of glycoproteins (on the dell surface) and proteoglycans (in the extracellular matrix). • In glass dishes, heart myocytes “beat” and liver cells avoid contact with kidney cells. Cancer cells grow without contact inhibition. • These polysaccharide structures regulate cell-cell recognition and interaction.They contain several points for linkage (-OH) and are more informative than linear proteins and nucleic acids. • The uniqueness of the "information" in these structures is determined by the enzymes that synthesize these polysaccharides.

  24. 7.5 – What Are Glycoproteins, and How Do They Function in Cells? Many structures and functions! • May be N-linked or O-linked. • N-linked saccharides are attached via the amide nitrogens of asparagine residues. • O-linked saccharides are attached to hydroxyl groups of serine, threonine or hydroxylysine. • See structures in Figure 7.39

  25. O-linked Saccharides of Glycoproteins • Function in many cases is to adopt an extended and relatively rigid conformation. • These extended conformations resemble "bristle brushes“. • Bristle brush structure extends functional domains up out of the glycocalyx. • See Figure 7.40

  26. N-linked Oligosaccharides Many functions known or suspected • N-glycosylation of proteins can alter the chemical and physical properties of proteins, altering solubility, mass, and electrical charges. • N-linked oligosaccharide moieties can (1) stabilize protein conformations,(2) protect against proteolysis and (3) promote correct folding of certain globular proteins (p. 239). • Cleavage of monosaccharide units from N-linked glycoproteins in blood targets them for degradation in the liver. - see pages 238, 239

  27. 7.6 - Proteoglycans --- Glycoproteins whose carbohydrates are mostly glycosaminoglycans. • Components of the cell membrane and glycocalyx. • Consist of proteins with one or two types of glycosaminoglycan. • See structures, Figure 7.44

  28. 7.6 – How Do Proteoglycans Modulate Processes in Cells and Organisms? • Proteoglycans are glycoproteins whose carbohydrate moieties are predominantly glycosaminoglycans. • Example: syndecan - transmembrane protein - inside domain interacts with cytoskeleton, outside domain interacts with fibronectin. • Highly sulfated glycosaminoglycans bind specific proteins (e.g. fibronectin) at sites containing basic amino acid residues. (charge interactions) • A particular pentasaccharide sequence in heparin finds to antithrombin III. (sequence-specific)

  29. Proteoglycan Functions • Modulation of cell growth processes • Binding of growth factor proteins by proteoglycans in the glycocalyx provides a reservoir of growth factors at the cell surface. • Cushioning in joints • Cartilage matrix proteoglycans absorb large amounts of water. During joint movement, cartilage is compressed, expelling water!

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