北大生物化学完整chapter9

1、Qin, Yongmei, PhDPhD (秦咏梅)Chapters included: May 26 Chapter 7 Carbohydrates & GlycobiologyJune 2 Chapter 10 Lipids June 9 Chapter 11 Biological Membranes and TransportJune 16 Chapter 12 Biosignaling Contact information: email: Tel: 6275 8885 Address: New building of our college: Room 214Chapter 7 Ca

2、rbohydrates & GlycobiologyLectured by Dr. Qin Yongmei (秦咏梅): May 26, 2007Reference: Carbohydrates: the Sweet Molecules of Life by Robert V Stick What do we eat ?high protein, low-carbohydrates, no-fat ? Science is evolutionary, and new research points in different directions are coming up every day.

3、 But, there isnt enough research on nutrition and popular diets to prove anyone right or wrong. Carbohydrates in food are important sources of energy Upland cotton produces the most widely used natural fibers, cellulose (a polysaccharide)Cellulose: the most abundant organic compounds in the biospher

4、e 1015 kg of cellulose is synthesized and degraded on Earth each year Carbohydrates & GlycobiologyCarbohydrate, (C-H2O)n = “Carbon + Water” Source of energy Structure (cotton fibers: cellulose) Building blocks Cellular recognitionContents Monosaccharides and disaccharides Polysaccharides Glycoconjug

5、ates: proteoglycans, glycoproteins, glycolipidsWorking with carbohydratesMonosaccharides and disaccharides1. Monosaccharides contain either:l aldehyde group (aldoses)l ketone group (ketoses)Two common hexoses: Pentoses: components of nucleic acids(present in RNA)(present in DNA)2. Monosaccharides ca

6、n be classified according to the number of carbons:l triose 3 carbonsl tetrose 4 carbonsl pentose5 carbonsl hexose6 carbonsl heptose7 carbons4. Monosaccharide stereoisomersHow many stereoisomers does an aldohexose contain ?3. Naming stereoisomers3 ways to represent the two steroisomers of glyceralde

7、hydeenantiomerD-Aldoses(triose)(tetrose)(pentose)(hexose)D-ketosesKetoses have one fewer asymmetric center than do aldoses with the same number of carbonsNote:Epimers: two sugars that differ only in the configuration around one carbon atom D-glucose and two of its epimers5. Formation of hemiacetal o

8、r hemiketal can occur within a monosaccharide Two monosaccharides are connected through glycosidic linkage to form disaccharides.O-glycosidic bondreducingendhemiacetalacetalaldehydeOnly aldoses havingfive or more carbon atoms can form pyranose ringsmutarotationAnomers: differ in their configuration

9、about either hemiacetal or hemiketal carbon atom Anomers(two cyclic forms of D-glucose)Create an asymmetric centerC-1: anomeric carbon atomHaworth perspective formulas : commonly used to show the stereochemistry of ring forms of monosaccharides Fructose can form both five-membered furanose and six-m

10、embered pyranose ringspredominate in many fructose derivativespredominate in fructose free in solutionConformation of pyranose ringsThe chair form is more stable because of less steric hindrance as the axial positions are occupied by hydrogen atoms. b-D-glucopyranose steric hindrance(predominated fo

11、rm)6. Reactions of monosaccharides The aldehyde and primary alcohol groups can be oxidized to acids (aldonic and uronic acids) reducing sugaroxidationApplication to diagnosis of diabetes mellitus (determination of glucose concentration)Fehlings reactionMore sensitive method: using glucose oxidase7.

12、Monosaccharide derivativesaldonic aciduronic acid8. Formation of disaccharides Two monosaccharides are connected through glycosidic linkage to form disaccharides.O-glycosidic bondreducingendSome common disaccharides Nonreducing sugar ! Contains no free anomeric carbon atom nonreducing disaccharides

13、are named as glycosidesSome rules to name oligosaccharides Give the configuration at the anomeric carbon joining the first monosaccharide unit to the second. Name the nonreducing reside; to distinguish five- and six-membered ring structures, insert “furano” or “pyrano” into the name. Indicate the pa

14、rentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers. Name the second reside. If there is a third residue, describe the second glycosidic bond by the same conventions. Example: maltoseconventional name: -D-glucopyranosyl-(14)-D-glucopyranosesimplifie

15、d name: Glc ( 14) GlcPolysaccharides (glycans) As high as 2 x 107 Daltons Storage forms of glucose starch (plant) amylose (un-branched) amylopection (branched) glycogen (animals) Structural roles Cellulose (eg. cotton) Chitin (Arthropods) Glycosaminoglycans (animals)1. Differences between homopolysa

16、ccharides and heteropolysaccharides2. Homopolysaccharides Starch (amylose, amylopectin) unbranched polymer of D-glucose-(1,4)-D-glucose branched polymer of D- glucose-(1,4)-D-glucose with (1,6) branch point A cluster of amylose and amylopectin (in starch granules) strands of amylopectin form double-

17、helical structures with each other or with amylose strands Glucose residues at the nonreducing ends are removed enzymatically during mobilization of starch for energy production.3. Some homopolysaccharides serve structural roles Cellulose unbranched polymer of D-glucose-b(1,4)-D-glucose the b config

18、uration allows cellulose to form very long and straight chain. scale drawing of segments of two parallel cellulose chain 3. Some homopolysaccharides serve structural roles Chitin unbranched polymer of D-N- acetylglucosamine-b(1,4)-D-N- acetylglucosamine4. Steric factors and hydrogen bonding influenc

19、e homopolysaccharide foldingThe folding of polysaccharides in three dimensions follows the same principles as those governing polypeptide structure: hydrogen boding hydrophobic interactions van der Waals interactions electrostatic interactionsThe three-dimensional structures are described in terms o

20、f dihedral angles, analogous to angles made by the peptide bond. gal(b13)galRelative energyA map of favored conformations for oligosaachrides and polysaccharidesThe structure of starch (amylose)Coiled helical structureMost stable conformationis curved, rather than linearFrom the structures of cellul

21、ose and starch (glycogen), we learn: The different consequences of the and b linkages have biological significance. The straight chain formed by b linkages is optimal for the construction of fibers having a high tensile strength. The open helix formed by linkages is well suited to form an accessible

22、 store of sugars. Mammals lack cellulases that therefore cannot digest wood and vegetable fibers. Why not store glucose in its monomeric form ? It has been calculated that hepatocytes store glycogen equivalent to glucose = 0.4M; the actual glycogen = 0.01 mM. If in cytosol glucose = 0.4 M, leading t

23、o osmotic entry of water that might rupture the cell. If intracellular glucose = 0.4 M, compared with extracellular (in blood of a mammal) glucose = 5 mM, the free-energy change for glucose uptake into cells would be prohibited.4. Bacteria and algal cell walls contain structural heteropolysaccharide

24、s alternating (b1,4)-linked N- acetylglucosamine and N- acetylmuramic acid residuesPeptidoglycan of the cell wallof Staphlococcus aureus5. Glycosaminoglycans are heteropolysaccharides of the extracellular matrixAt least one of sugars in the repeating unit has a negatively charged carboxylate or sulf

25、ate groupb-D-Glucuronate透明质酸软骨素硫酸角质素肝素1. 透明质酸:存在于眼球玻璃体、关节液和皮肤等组织中作为润滑剂和撞击缓冲剂，并有助于阻滞入侵的微生物及毒性物质的扩散。2. 硫酸软骨素:动物组织的基础物质，用以保持组织的水分和弹性。包括软骨素、等数种，软骨素是软骨的主成分。和肝素相似，可用以降血脂，改善动脉样硬化症状。此外，硫酸软骨素还有消除皱纹、使皮肤保持及富弹性的作用。硫酸软骨素主要存在于鸡皮、鱼翅、鲑鱼头和鸡等软骨内。3. 肝素:高度硫酸酯化的右旋多糖，与蛋白质结合大量存在于肝脏之中，其他器官和血液中也有。肝素有强的抗凝血作用，临床上用肝素钠盐预防或治疗血栓的

26、形成，肝素也有消除血液脂肪的作用。proteins: fibroblast growth factor (FGF1) FGFR (R: receptor)ligand: heparin Their interaction: electrostatic interactionGlycoconjugates: proteoglycans, glycoproteins & glycolipidsProteoglycans: macromolecules of the cell surface or extracellular matrix in which one or more glycosami

27、noglycan chains are joined covalently to a membrane protein or a secreted proteinGlycoproteins: proteins covalently attached by one or several oligosaccharidesGlycolipids: membrane lipids in which the hydrophobic head groups are oligosaccharides. 1. Proteoglycans: glycosaminoglycan-containing macrom

28、olecules(glycosaminoglycan)(linker)Function as lubricants and structural components in connective tissue, mediate adhesion of cells to the extracellular matrix and bind factors that stimulate cell proliferation NA domain: N-acetylated domainS domain: Sulfated domain Proteoglycans structure of an int

29、egral membrane protein Proteoglycans form aggregates at the extracellular matrix Proteoglycan aggregatescan interact with collagen in the extracellular matrix of cartilage, contributingto the development and tensile strength of theconnective tissue. Proteoglycans resemble polysaccharides more than p

30、roteins as much as the carbohydrate makes up as much as 95% of the biomolecule by weight the association between cellsand the proteoglycan of extra-cellular matrix is mediated by a membrane protein (integrin)and by an extracellular protein (fibronectin) with binding sitesfor both integrin and thepro

31、teoglycan.Cell matrix interactions anchor cells to extracellular matrix direct the migration of cells in developing tissue convey information in both directions across plasma membrane (via integrins)2. Glycoproteins are information-rich containing oligosaccharidesglycosidic bondN-glycosyl bondHow ar

32、e glycoproteins sorted in the cell ?A pentasaccharide core is common to all N-linked oligosaccharides and serves as the foundation for a wide variety of N-linked oligosaccharides What are the biological advantages of adding oligosaccharides to proteins ? alter the polarity and solubility of their co

33、njugated proteins influence protein-folding events protect some proteins from attack by proteolytic enzymes complex of oligosaccharides is enormously rich in structural information some proteins present tissue glycoforms (may act as a tissue-specific mark) Calnexin(chaperone) Calnexin(chaperone) An

34、unfolded or misfolded protein will receive a glucose residue; Such glycosylated protein bind to calnexin that allow multiple attempts to attain correct folding; Properly folded proteins are not re- glucosylated3. Glycolipids and lipopolysaccharides are membrane componentsGangliosides: membrane glyco

35、lipids of eukaryotic cells in which the polar head group forms the outer surface of the membraneExample: the oligosaccharide moieties of the gangliosides determine human blood groupsLipopolysaccharides: dominant surface feature of the outer membrane of gram-negative bacteria. - target of antibody -

36、toxic to humans The principal determinant of the serotype(immunological reactivity) of the bacterium4. Oligosaccharide-lectin interactions mediate many biological processesLectin: proteins that bind carbohydrates with high affinity (hydrogen-bonding) and specificity (lecitins can be easily distingui

37、shed by their binding with various similar sugars). involved in a wide variety of cell-cell recognition and adhesion processesSelectins: a family of lectins, found in plasma membranes, that mediate cell-cellrecognition and adhesion.Role of lectin-ligand interactions in lymphocyte movement to the sit

38、e of an infection or injuryHelicobacter pylori cellsadhering to the gastric surfacecauses ulcers by interactions between a bacterial surface lectinand the Leb oligosaccharide (a bloodgroup antigen) of the gastric epitheliumLectin-Carbohydrate Interaction- very strong & highly specific Neu5Ac (N-acet

39、ylneuraminic acid) binding sitemannose 6PCa 2+Sialic acid-specific lectinMannose 6P receptorRoles of oligosaccharides in recognition and adhesion at the cell surfaceWorking with carbohydrates Analyze composition of monosaccharides (high-performance liquid chromatography, HPLC or gas-liquid chromatog

40、raphy ) Determination of position(s) of glycosidic bonds Methylate free hydroxyl group (-OH) by CH3I, then hydrolyze the methylated polysaccharide in acid. The only free -OH(s) present in the monosaccharide are those involved in glycosidic bonds Determination of sequence of monosaccharides Mass spec

41、trometry and NMR spectroscopyMALDI-TOF spectrometryEnzymatic hydrolysis with specific glycosidasepeptide N-glycosidase F & neuraminidasepeptide N-glycosidase F & neuraminidase &b-1,4-galactosidaseSummary Sugars are compounds containing an aldehyde or ketone group and two or more hydroxyl groups; Mon

42、osaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms; Monosaccharides commonly form internal hemiacetals or hemiketals; A hydroxy group of one monosaccharide can add to the anomeric carbon of a second monosaccharide to form an acetal. In this

43、 disaccharide, the glycosidic bond protects the anomeric carbon from oxidation; Oligosaccharides are short polymers of several monosaccharides joined by glycosidic bonds; The common nomenclature for di- or oligosaccharides specifies the order of monosaccharide units, the configuration at each anomer

44、ic carbon, and the carbon atoms involved in the glycosidic linkage (s); Polysaccharides (glycans) serve as stored fuel and as structural components of cell walls and extracellular matrix; The homopolysaccharides starch and glycogen are stored fuels in plant, animal, and bacterial cells. The homopoly

45、saccharide cellulose, chitin, and dextran serve structural roles. Homopolysaccharides fold in three dimensions; Bacterial and algal cell walls are strengthened by heteropolysaccharides-peptidoglycan in bacteria, agar in red algae; Glycosaminoglycans are extracellular heteropolysaccharides in which one of the two monosaccharide units is a uronic acids and the other an N-acetylated amino sugars. Proteoglycans and glycoconjugates in which a core protein is attached covalently to on