生物化學：Chapter 7 碳水化合物與糖生物學

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1、Chapter 7 Carbohydrates & Glycobiology碳水化合物與糖生物學碳水化合物與糖生物學2012-10-25Carbohydrate, (C-H2O)n = “Carbon + Water” Source of energy Structure (cotton fibers: cellulose) Building blocks Cellular recognition糖類主要是多羥基醛或酮類物質糖類主要是多羥基醛或酮類物質Carbohydrates in food are important sources of energy Upland cotton prod

2、uces the most widely used natural fibers, cellulose (a polysaccharide)Cellulose: the most abundant organic compounds in the biosphere 1015 kg （1000億噸） of cellulose is synthesized and degraded on Earth each year Contents Monosaccharides （單糖）& disaccharides （二糖） Polysaccharides （多糖） Glycoconjugates （復

3、合糖）: proteoglycans （蛋白聚糖）, glycoproteins （糖蛋白）, glycolipids （糖脂） Working with carbohydratesRequirements for Your Understanding Carbohydrate Chemistry Biological functions (Glycobiology): glycosaminoglycans & proteoglycans; glycoproteins; glycolipids Methods for carrying out research in glycobiology

4、fieldMonosaccharides and disaccharides1. Monosaccharides contain either:l aldehyde group (aldoses)l ketone group (ketoses) 醛糖醛糖 酮糖酮糖 典型單糖典型單糖丙糖丙糖己糖己糖戊糖戊糖甘油醛（丙醛糖）二羥丙酮（丙酮糖）Two common hexoses: Pentoses: components of nucleic acids(present in DNA)(present in RNA)2. Monosaccharides can be classified acco

5、rding to the number of carbons:單糖和二糖名字末尾綴以“-ose”l triose 3 carbonsl tetrose 4 carbonsl pentose5 carbonsl hexose6 carbonsl heptose7 carbons4. Monosaccharide stereoisomersHow many stereoisomers does an aldohexose contain ?3. Naming stereoisomers （立體異構體的命名）3 ways to represent the two steroisomers of gl

6、yceraldehydeenantiomer球棍模型球棍模型費希爾投影式費希爾投影式 透視式透視式D-醛糖醛糖 D- D-核糖核糖 D- D-阿拉伯糖阿拉伯糖 D- D-木糖木糖 D- D-來蘇糖來蘇糖 紅色標記的紅色標記的 C C 為為 手性碳；手性碳； 距離羰基最遠手性距離羰基最遠手性 C C的構型與的構型與D-D-甘油甘油 醛相同，稱醛相同，稱D D型型 異構體異構體； 方框中的糖是自然方框中的糖是自然 界中最普通的糖。界中最普通的糖。D-D-阿洛糖阿洛糖D-D-阿卓糖阿卓糖D-D-古洛糖古洛糖D-D-艾杜糖艾杜糖D-D-塔羅糖塔羅糖D-D-甘露糖甘露糖D-D-半乳糖半乳糖D-D-葡萄糖

7、葡萄糖Ketoses have one fewer asymmetric center than aldoses with the same number of carbonsNote: D-酮糖酮糖 二羥丙酮二羥丙酮D-赤蘚酮糖赤蘚酮糖 紅色標記的紅色標記的 C C 為為 手性碳；手性碳； 距離羰基最遠手性距離羰基最遠手性 C C的構型與的構型與D-D-甘油甘油 醛相同，稱醛相同，稱 D D 型型 異構體異構體； 方框中的糖是自然方框中的糖是自然 界中最普通的糖界中最普通的糖。D-核酮糖D-木酮糖D-山梨糖D-塔格糖D-阿洛酮糖D-果糖Epimers （差向異構體）: two sugars

8、that differ only in the configuration around one carbon atom D-glucose and two of its epimers5. Formation of hemiacetal（半縮醛） or hemiketal （半縮酮） can occur within a monosaccharide An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, creating a new chiral c

9、enter at the carbonyl carbon. 呋喃糖呋喃糖吡喃糖吡喃糖Only aldoses havingfive or more carbon atoms can form pyranose ringsmutarotationAnomers: differ in their configuration about either hemiacetal or hemiketal carbon atom Anomers(two cyclic forms of D-glucose)Create an asymmetric Center（異頭物碳原子）（異頭物碳原子）C-1: anom

10、eric carbon atom變旋光變旋光aD25C = + 112aD25C = - 19旋光性 Optical rotation 異頭物異頭物六元環(huán)的吡喃醛糖比五元環(huán)的呋喃酮糖穩(wěn)定變旋現(xiàn)象變旋現(xiàn)象（mutarotation)異頭物異頭物（anomer）的形成的形成D-葡萄糖C-1的醛基與C-5的羰基反應形成半縮醛，具有兩種立體異構體，a 和b異頭物。a 和b異頭物之間的相互轉化的過程稱為變旋現(xiàn)象。Definition of specific rotation aD25CaD25C = observed optical rotation ()Optical path length (dm)

11、 x concentration (g/mL)The optical activity （光活性）is expressed quantitatively by its optical rotation（旋光性）（旋光性）: Levorotatory isomer (-): rotates plane-polarized light to the left （左旋異構體） (counterclockwise);Dextrorotatory isomer(+): rotates plane-polarized light to the right （右旋異構體） (clockwise)當平面偏振光

12、通過手性化合物溶液后，偏振面的方向就被旋轉了一個角度。這種能使偏振面旋轉的性能稱為旋光性。手性化合物都具有旋光性。 hemiacetalacetalaldehyde醛醛半縮醛半縮醛 縮醛縮醛麥芽糖 Two monosaccharides are connected through glycosidic linkage to form disaccharides. O-glycosidic bond Substitution of a second alcohol molecule produces an acetal or ketal. Reducing endEn example of D-

13、glucose mutarotation(Please refer: problem No.4 of Chapter 7)The equilibrium mixture consists of 64% -D-glucopyranose and 36% -D-glucopyranose. It contains trace amount of the open-chain form.Mutarotation 變旋現(xiàn)象Mutarotation 變旋現(xiàn)象Mutarotation is the change in specific rotation that accompanies the inter

14、-conversion of a and b-anomers in aqueous solution. Mutarotation is common to all carbohydrates that exist in hemiacetal forms, and it was one of the phenomena that suggested that aldoses might exist as cyclic hemiacetals. Mutarotation occurs when pure anomers are dissolved in aqueous solution. An a

15、ldehyde cannot undergo mutarotation.Haworth perspective formulas （Haworth 透視式透視式） : commonly used to show the stereochemistry of ring forms of monosaccharides (Sir Water N. Haworth won 1937 Nobel Prize in Chemistry)呋喃呋喃吡喃吡喃 吡喃葡萄糖吡喃葡萄糖 呋喃果糖呋喃果糖Conformation of pyranose ringsThe chair form is more stab

16、le because of less steric hindrance as the axial positions are occupied by hydrogen atoms. b-D-glucopyranose steric hindrance(predominated form)AxisEquatorialposition吡喃糖的構象形式空間障礙軸赤道椅式船式6. Reactions of monosaccharides 單糖具有還原性a mixture of products （reducing 3 mol of Cu2+ per mole of glucose）Applicatio

17、n to diagnosis of diabetes mellitus (determination of glucose concentration)Fehlings reaction 費林反應 (not specific !)More sensitive method: using glucose oxidase (only specific for glucose)Nonenzymatic reaction of glucose with a primary amino group in hemoglobinAmadori rearrangementcyclizationglycatio

18、n end products The nonenzymatic reaction of glucose with a primary amino group in hemoglobin:(1) formation of a Schiff base;(2) undergoes the Amadori rearrangement to generate a stable product; (3) this ketoamine can further cyclize to yield GHB;(4) Subsequent reactions generate advanced glycation e

19、nd products (AGEs), such as -N-carboxymethyllysine and methylglyoxal, compounds (5) AGEs damage other proteins by cross-linking them, causing pathological changes.7. Monosaccharide derivativesaldonic aciduronic acid糖醛酸醛糖酸葡萄糖醛酸葡萄糖酸葡糖胺胞壁酸半乳糖胺甘露糖胺脫氧糖類鼠李糖墨角藻糖N-乙酰神經(jīng)氨酸 （唾液酸）8. Formation of disaccharides T

20、wo monosaccharides are connected through glycosidic linkage （糖苷鍵）to form disaccharides.O-glycosidic bondreducingend二糖的形成二糖的形成Some common disaccharides Nonreducing sugar ! Contains no free anomeric carbon atom nonreducing disaccharides are named as glycosides乳糖乳糖蔗糖蔗糖海藻糖海藻糖Some rules to name oligosacc

21、harides 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 using the parentheses the two carbon atoms joined by the

22、glycosidic bond, with an arrow connecting the two numbers. Name the second residue. If there is a third residue, describe the second glycosidic bond by the same conventions. A double-headed arrow is used for nonreducing disaccharide (glycosides). Sucrose: Glc (a1 2b) FruExample: maltoseconventional

23、name: a-D-glucopyranosyl-(14)-D-glucopyranosesimplified name: Glc (a 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,節(jié)肢動物) G

24、lycosaminoglycans (animals)多多 糖糖1. Differences between homopolysaccharides and heteropolysaccharides 同多糖同多糖 雜多糖雜多糖2. Homopolysaccharides Starch (amylose 直鏈淀粉, amylopectin 支鏈淀粉 ) unbranched polymer of D-glucose-a(1,4)-D-glucose branched polymer of D-glucose-a(1,4)-D-glucose with a (1,6) branch point

25、A cluster of amylose and amylopectin (in starch granules) strands of amylopectin form double-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 s

26、erve structural roles Cellulose unbranched polymer of D-glucose-b(1,4)-D-glucose the b configuration 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 幾丁質幾丁質 unbranched polymer of

27、 D-N- acetylglucosamine-b(1,4)-D-N- acetylglucosamine由由b(1,4）糖苷鍵連接）糖苷鍵連接N-N-乙酰乙酰-D-D-葡糖胺而形成的同多糖葡糖胺而形成的同多糖4. Steric factors and hydrogen bonding influence homopolysaccharide foldingThe folding of polysaccharides in three dimensions follows the same principles as those governing polypeptide structure:

28、 hydrogen bond hydrophobic interactions van der Waals interactions electrostatic interactionsThe three-dimensional structures are described in terms of dihedral angles, analogous to angles made by the peptide bond. A map of favored conformations for oligosaachrides and polysaccharidesRelative energy

29、Disaccharide Gal(13)GalSimilar to : Ramachandran plot (to visualize dihedral angles against of amino acid residues in protein structure. The structure of starch (amylose)Coiled helical structureMost stable conformationis curved, rather than linearThe hydroxyl groups have been omitted.Cellulose break

30、down by wood fungiA wood fungus growing on an oak log（橡樹）. All wood fungi have the enzyme cellulase, which breaks the (14) glycosidic bonds in cellulose, so that wood is a source of metabolizable sugar (glucose) for the fungus. The only vertebrates able to use cellulose as food are cattle and other

31、ruminants (sheep, goats, camels, giraffes). The extra stomach compartment (rumen) of a ruminant teems with bacteria and protists that secrete cellulase.From the structures of cellulose and starch (glycogen), we learn: The different consequences of the a and b linkages have biological significance. T

32、he straight chain formed by b linkages is optimal for the construction of fibers having a high tensile strength. The open helix formed by a linkages is well suited to form an accessible store of sugars. Mammals lack cellulases that therefore cannot digest wood and vegetable fibers. Why not store glu

33、cose 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 to osmotic entry of water that might rupture the cell. If intracellular glucose = 0.4 M, compared with extracellular

34、 (in blood of a mammal) glucose = 5 mM, the free-energy change for glucose uptake into cells would be prohibited.Epimers （差向異構體）: two sugars that differ only in the configuration around one carbon atom D-glucose and two of its epimersEpimerizationThe carbon atom at which the initial deprotonation ta

35、kes place is a stereocenter. If, for example, D-Glucose (an Aldose) rearranges to D-Fructose, the ketose, the stereochemical configuration is lost in the enol form. In the chemical reaction the enol can be protonated from two faces, resulting in the backformation of glucose or the formation of the e

36、pimer D-mannose. Lobry-de Bruyn-van Ekenstein transformation It is the base or acid catalyzed transformation of an aldose into the ketose isomer or vice versa, with a tautomeric (互變異構的) enediol as reaction intermediate. On the origin of elementary hexoses(Hirabayashi J.) (1) Formal reaction, an auto

37、catalytic polymerization reaction, occurred on the primitive earth under weakly alkaline conditions to generate various small molecular compounds. (2) Among them, glyceraldehyde and dihydroxyacetone undergo aldol condensation to produce stable ketohexoses, fructose and sorbose （山梨糖）. (3) Subsequentl

38、y, Lobry de Bruyn rearrangement converts fructose into stable aldohexoses, glucose, and mannose. Q Rev Biol. 1996 Sep;71(3):365-80.(4) Thus, prebiotically synthesized “first triplet” hexoses would have been available for utilization by ancestral primitive microorganisms. (5) After the development of

39、 biochemical pathways, various saccharides, including galactose, were biosynthesized from glucose and mannose as “bricolage products”; the utilization of galactose as a key recognition molecule, based on its distinctive axial 4-OH and its outermost location in glycoconjugates, owing to its late arri

40、val, may have evolved concomitantly with the evolution of multicellular organisms.(b b or a a )(b b or a a )Mutarotation also occurs on oligosaccharides and polysaccharides ! But, for polysaccharides, it is not detectable ! 5. Glycosaminoglycans are heteropolysaccharides of the extracellular matrixA

41、t least one of sugars in the repeating unit has a negatively charged carboxylate or sulfate group透明質酸硫酸軟骨素硫酸角質素肝素 糖胺聚糖糖胺聚糖糖醛酸糖醛酸N-N-乙酰葡糖胺乙酰葡糖胺艾杜糖醛酸艾杜糖醛酸 半乳糖半乳糖 細胞外介質：細胞外介質： 雜多糖雜多糖 + + 纖維蛋白纖維蛋白 （膠原蛋白、彈性蛋白）（膠原蛋白、彈性蛋白）成纖維細胞1. Hyaluronate ( (透明質酸透明質酸):):存在于眼球玻璃體、關節(jié)液和皮膚等組織中作為潤滑劑和撞擊緩沖劑，并有助于阻滯入侵的微生物及毒性物質的擴

42、散。2. Chondroitin 4-sulfate (硫酸軟骨素硫酸軟骨素):動物組織的基礎物質，用以保持組織的水分和彈性。包括軟骨素、等數(shù)種，軟骨素是軟骨的主成分。和肝素相似，可用以降血脂，改善動脈樣硬化癥狀。此外，硫酸軟骨素還有消除皺紋、使皮膚保持及富彈性的作用。硫酸軟骨素主要存在于雞皮、魚翅、鮭魚頭和雞等軟骨內。3. Haparin (肝素肝素):高度硫酸酯化的右旋多糖，與蛋白質結合大量存在于肝臟之中，其他器官和血液中也有。肝素有強的抗凝血作用，臨床上用肝素鈉鹽預防或治療血栓的形成，肝素也有消除血液脂肪的作用。 The molecules are copolymers of alter

43、nating uronic acid and amino sugar residues (keratan sulfate is the exception), with sulfate esters in any of several positions, except in hyaluronan. The ionized carboxylate and sulfate groups give these polymers their characteristic high negative charge. Therapeutic heparin contains primarily idur

44、onic acid (IdoA) and a smaller proportion of glucuronic acid (GlcA), and is generally highly sulfated and heterogeneous in length. 葡萄糖醛酸艾杜糖醛酸 Space-filling model shows a heparin segmentIdoAO OSproteins: fibroblast growth factor (FGF1) FGFR (R: receptor)ligand: heparin Their interaction: electrostati

45、c interactionHeparin is shown in a ball-and-stick representation, with the negative charges (SO3 and COO) attracted to the positive (blue) surface of the FGF1 protein. Heparin was used in this experiment, but the glycosaminoglycan that binds FGF1 in vivo is heparan sulfate on the cell surface. Inter

46、action between a glycosaminoglycan and its binding proteinGlycoconjugates: proteoglycans, 蛋白聚糖蛋白聚糖glycoproteins（糖蛋白糖蛋白） & glycolipids（糖脂糖脂）Proteoglycans: macromolecules of the cell surface or extracellular matrix in which one or more glycosaminoglycan chains are joined covalently to a membrane prote

47、in or a secreted proteinGlycoproteins: proteins covalently attached by one or several oligosaccharidesGlycolipids: membrane lipids in which the hydrophilic head groups are oligosaccharides. 復合糖1. Proteoglycans: glycosaminoglycan-containing macromolecules(glycosaminoglycan)(linker)Function as lubrica

48、nts and structural components in connective tissue, mediate adhesion of cells to the extracellular matrix and bind factors that stimulate cell proliferation 蛋白聚糖：蛋白聚糖：糖胺聚糖糖胺聚糖 + + 蛋白蛋白蛋白聚糖蛋白聚糖the association between cellsand the proteoglycan of extra-cellular matrix is mediated by a membrane protein

49、 (integrin)and by an extracellular protein (fibronectin) with binding sitesfor both integrin and theproteoglycan.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 integrin

50、s)纖連蛋白整合素肌動蛋白絲（微絲） 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 prote

51、ins as much as the carbohydrate makes up as much as 95% of the biomolecule by weight 透明質酸分子結構透明質酸分子結構 Electron micrograph of a proteoglycan（蛋白聚糖） aggregate purified from calf cartilageRoseman S (2001) Reflections on glycobiology J. Biol. Chem. 276, 41,527-542 One very long molecule of hyaluronan is

52、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 proteinhyaluronan

53、 interaction. 膜蛋白聚糖（membrane proteoglycans） Syndecans are held in the membrane by hydrophobic interactions between a sequence of nonpolar amino acid residues and plasma membrane lipids; Sydecans can be released by a single proteolytic cut near the membrane surface. In a typical syndecan, the extrace

54、llular aminoterminal domain is covalently attached (by tetrasaccharide linkers) to three heparan sulfate chains and two chondroitin sulfate chains. Membrane Proteoglycan: glypicanCurrent Opinion in Structural Biology 2012, 22: 583-590Syndecans cooperate with integrins to regulate cell adhension Glyp

55、icans are held in the membrane by a covalently attached membrane lipid (GPI anchor), and are shed if the lipid-protein bond 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 glycosaminoglyc

56、an chains attached near the carboxyl terminus, close to the membrane surface. Membrane Proteoglycan: GlypicanDeficiency in glypican-3(GCP3) gene, which encodes a heparan sulfate proteoglycan (HSPG，硫酸乙酰肝素類蛋白聚糖硫酸乙酰肝素類蛋白聚糖 ), resulted in Simpson-Golabi-Behmel syndrome;This syndrome is characterized by

57、a wide variety of clinical manifestations including pre-natal and post-natal overgrowth syndrome, phenotypically similar to Beckwith-Wiedemann syndrome (貝-威綜合征 ).貝貝- -威綜合征威綜合征( (常染色體顯性遺傳常染色體顯性遺傳):):以巨大舌，臍膨出和以巨大舌，臍膨出和生長過剩生長過剩為三大主要特征的先天性疾病，同為三大主要特征的先天性疾病，同時伴有內臟腫大（主要為肝、腎和脾的腫大），出生時低血糖，時伴有內臟腫大（主要為肝、腎和脾的腫

58、大），出生時低血糖，單側肥大（身體的一側生長過剩）等生長異常。單側肥大（身體的一側生長過剩）等生長異常。Reference: Simpson-Golabi-Behmel Syndrome associated with renal dysplasia and embryonal tumor: Localization of the gene to Xqcen-q21.Hughes-Benzie RM, Hunter AGW, Allanson JE, Mackenzie AE. Am. J. Med. Genet 1992; 43: 428-435Dr. David Reid of the D

59、epartment of Chemistry at the University of Cambridge, revealed that the study has shown that the characteristic toughness and stiffness of bone is predominantly due to the presence of specialized sugars, not proteins. They observed that sugars, particularly proteoglycans and glycosaminoglycans, app

60、eared to play a role which is as important as proteins in controlling bone mineralization, the process by which newly-formed bone is hardened with minerals such as calcium phosphate. New findings on treatments for osteoporosis (骨質疏松)One of the NS domains is shown in more detail, revealing a high den

61、sity of modified residues: GlcNS (N-sulfoglucosamine), with a sulfate ester at C-6; and both GlcA and IdoA, with a sulfate ester at C-2. The exact pattern of sulfation in the NS domain differs among proteoglycans.NA domain: N-acetylated domainNS domain: Sulfated domainFour types of protein interacti

62、ons with NS domains of heparan sulfate抗凝血酶：antithrombinFGF: 成纖維細胞生長因子成纖維細胞生長因子 2. Glycoproteins are information-rich containing oligosaccharidesglycosidic bondN-glycosyl bond 糖蛋白：富含信息糖蛋白：富含信息What are the biological advantages of adding oligosaccharides to proteins ? alter the polarity and solubility

63、 of their conjugated 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)3. Glycolipids（糖脂） and lipopolys

64、accharides （脂多糖） are membrane componentsGangliosides: 神經(jīng)節(jié)苷脂神經(jīng)節(jié)苷脂 membrane glycolipids 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(LPS, 脂多糖， endotoxin

65、, 細菌內毒素細菌內毒素): dominant surface feature of the outer membrane of gram-negative bacteria. - target of antibody - toxic to humans - inflammation stimulator (炎癥刺激物炎癥刺激物) The principal determinant of the serotype(immunological reactivity) of the bacterium4. Oligosaccharide-lectin interactions mediate ma

66、ny biological processesLectin（凝集素 ）: proteins that bind carbohydrates with high affinity (hydrogen-bonding) and specificity (lectins can be easily distinguished by their binding with various similar sugars). involved in a wide variety of cell-cell recognition and adhesion processesLectin microarray technology analysis of glycansCurrent opinion in Chemistry Biology 2009, 13: 427-432Selectins: 選擇蛋白a family of lectins, found in plasma membranes, that mediate cell-cellrecognition and adhesion.Role o