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本课件为英文教材第四章，从糖类的基本分子式与命名来源讲起，交代其作为能源分子、前体与结构组分的功能及特征化学性质；随后讲单糖的分类与结构、寡糖与二糖的连接方式，最后讲多糖的结构与命名规则。
Chapter 4Carbohydrates"The discovery of honey"—Piero de Cosimo (1462). (Courtesy of the Worcester Art Museum)
Carbohydrates are the single most abundant class of organic molecules found in nature. The name carbohydratearises from the basic molecular formula (CH2O)n,which can be rewritten (C•H2O)nto show that these substances are hydrates of carbon, where n=3 or more.
The functions ofcarbohydratesEnergy molecules;Precursors of many other molecules;Structures (e.g. glycolipids); Recognition(e.g. glycoproteins)
The characteristic chemicalfeatures of carbohydrates(1)the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glycosidicbonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment.
Carbohydrate NomenclatureCarbohydrates are generally classified into three groups: monosaccharides(and their derivatives), oligosaccharides, polysaccharides.
The monosaccharides are also called simple sugars and have the formula (CH2O)n. Monosaccharides cannot be broken down into smaller sugars under mild conditions.
Oligo-saccharides derive their name from the Greek word oligo, meaning “few,” and consist of from two to ten simple sugar molecules. Disaccharides are common in nature, and trisaccharides also occur frequently.
Four-to six-sugar-unit oligosaccharides are usually bound covalently to other molecules, including glycoproteins .
polysaccharides are polymers of the simple sugars and their derivatives. They may be either linear or branched polymers and may contain hundreds or even thousands of monosaccharide units. Their molecular weights range up to 1 million or more.
4.1•MonosaccharidesClassificationMonosaccharides consist typically of three to seven carbon atoms and are described either as aldosesor ketoses(suffix –ose means saccharide), depending on whether the molecule contains an aldehyde function or a ketone group.
Monosaccharides,either aldoses or ketoses, are often given more detailed generic names to describe both the important functional groups and the total number of carbon atoms. Thus, one can refer to aldotetrosesand ketotetroses, aldopentosesand ketopentoses, aldohexosesand ketohexoses, and so on. Sometimes the aldoses do not need to use prefix ald-, e.g. tetroses, pentoses, hexoses, and heptoses, but the ketosesmust use prefix keto-.
Sometimes the ketone-containing monosaccharides are named simply by inserting the letters-ul-into the simple generic terms, such as tetruloses, pentuloses, hexuloses, heptuloses, and so on.
Monosaccharides can be categorized according to their value of 'n,'n Category3 Triose4 Tetrose5 Pentose6 Hexose7 Heptose8 OctoseMonosaccharides can exist as aldehydesor ketonesand are called aldosesor ketoses, respectively.Monosaccharides
Stereochemistry Chiral centerMirror image
ConfigurationSolid wedge-shaped bonds point toward the readers, dasted wedges point away.
prefix D or L refers to the configuration of the highest numbered asymmetric carbon (the asymmetric carbonfarthestfrom the carbonyl carbon).
The D-AldosesMost distant chiral center (shaded red)
The D-ketosesMost distant chiral center (shaded red)
Stereoisomers that are mirror images of each other are called enantiomers(对映体）, or sometimes enantiomericpairs.D-Fructose and L-fructose, an enantiomeric pair. Note that changing the configuration only at C5would change D-fructose to L-sorbose
Pairs of isomers that haveopposite configurations at oneor more of the chiral centersbut that are not mirror imagesof each other are calleddiastereomers(非对映体）ordiastereomericpairs.
Two sugars that differ in configuration at only one chiral center are described as epimers（差向异构体）. For example, D-mannose and D-talose are epimers and D-glucose and D-mannose are epimers, whereas D-glucose and D-talose are notepimers but merely diastereomers .
D-glucose and two of its epimers are shown as projection formulas. Each epimer differs from D-glucose in the configuration at one chiral center (shaded red)
Optical rotation is to be specified in the name, the Fischer convention of D or L designations may be used along with a + (plus) or -(minus) sign. Thus, D-glucose may also be called D(+)-glucose because it is dextrorotatory, whereas D-fructose, which is levorotatory, can also be named D(-)-fructose.
Although Fischer projections are useful for presenting the structures of particular monosaccharides and their stereoisomers , they ignore one of the most interesting facets（方面）of sugar structure—the ability to form cyclic structures with formation of an additional asymmetric center. Alcohols react readily with aldehydes to form hemiacetalsCyclic Structures andAnomeric（异头的）Forms
Cyclization sugars from formation of hemiacetal or hemiketal
Anomeric Forms
Cyclization of Glucose -Anomers
Cyclization to form furanose rings
Conformations of pyranose and furanose rings
HaworthProjections Another of Haworth’s lasting contributions to the field of carbohydrate chemistry was his proposal to represent pyranose and furanose structures as hexagonal（六角型）and pentagonal（五角型）rings lying perpendicular（垂线）to the plane of the paper, with thickened lines indicating the side of the ring closest to the reader.
Summary of terminologyof the sugars1 Enantiomersand diastereomers2 Epimers(D, L)3 Rings (pyranose form and furanose form) 4 Anomeric Forms (αβ )5Optical rotation (+ dextrorotatory,-levorotatory) 6Conformations(boat form, chair from)e.g. α-D(+)-Glucosefα-D(+)-Glcf
Derivatives of Monosaccharides A variety of chemical and enzymatic reactions produce derivatives of the simple sugars. These modifications produce a diverse array of saccharide derivatives. Some of the most common derivations are discussed here.
Sugar AcidsSugar AlcoholsDeoxy SugarsSugar EstersAmino SugarsAcetals(缩醛）, Ketals（缩酮）, and Glycosides
Sugar Acids-aric acid(二酸）uronic acid -uronate ( 醛酸）-nate( 醛酸）
Sugar Alcohols
Deoxy Sugars
Sugar Esters
Examples of phspho-esthers of monosaccharides (M table 9.3)
Amino Sugars
Amino sugars (M p292)
Examples ofamino-sugars(M p292)
Acetals, Ketals , and Glycosides
Glycosides
Glycosides (M9.15)苦杏苷乌本(箭毒)苷
Abbreviations ofcommon sugars (Mtable 9.4)
4.2 • Oligosaccharides Disaccharides The simplest oligosaccharides are the disaccharides, which consist of two monosaccharide units linked by a glycosidic bond. As in proteins and nucleic acids, each individual unit in an oligosaccharide is termed a residue.
Formation of a disaccharide (M p297)
Biosynthesis oflactose (M9.17)
Disaccharides withβconnection (M9.16b)
Honey—An AncestralCarbohydrate TreatMost of the sugar in the honay is glucose and fructose. Sucrose accounts for only about 1% of the sugar in the final product, with fructose at about 38% and glucose at 31% by weight.
Honey
Higher Oligosaccharides
Glycosaminoglycans(粘多糖)
Natural occurrence of commonoligosaccharides (M table 9.5)
4.3• Polysaccharides Structure and NomenclatureBy far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures.
Polysaccharides, also called glycans,consist of monosaccharides and their derivatives. If a polysaccharide contains only one kind of monosaccharide molecule, it is a homopolysaccharide,or homoglycan,whereas those containing more than one kind of monosaccharide are heteropolysaccharides.
-anssuffix Glucans glucose homopolysaccharides,mannans mannose homopolysaccharides, Other homopolysaccharide names are just as obvious: galacturonans, arabinans,and so on. Homopolysaccharides of uniform linkage type are often named by including notation to denote ring size and linkage type. Thus, cellulose is a (1-4)-b-D-glucopyranan.
storage materials starch, glycogen and other storage polysaccharidesstructural components Chitinand celluloseprovide strong support for the skeletons of arthropods（节肢动物）and green plants, respectivelyprotective substances such as the hyaluronicacids（透明质酸）,form protective coats on animal cells. cellular recognition，intercellular communication.Polysaccharide Functions
Storage polysaacharidesStorage polysaccharidesare an important carbohydrate form in plants and animals. It seems likely that organisms store carbohydrates in the form of polysaccharides rather than as monosaccharides to lower the osmotic pressure of the sugar reserves. Because osmotic pressures depend only on numbers of molecules,the osmotic pressure is greatly reduced by formation of a few polysaccharide molecules out of thousands (or even millions) of monosaccharide units.
Polysaccharides (amylopectin) (M9.19)
Starch
Amylose form aa-helix secondarystructure (M9.20)
Suspensions（悬浮液）of amylose in water adopt a helical conformation. Iodine (I2) can insert into the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch.
Salivary（唾液的）a-amylase(a(1-4)-glucan 4-glucanohydrolase), Salivary a-amylase is an endoamylasethat splits a(1-4)glycosidic linkages only within the chain. b-Amylase,an enzyme absent in animals but prevalent in plants and microorganisms, cleaves disaccharide (maltose) units from the termini of starch chains and is an exoamylase.
GlycogenThe major form of storage polysaccharide in animals is glycogen. Liver glycogen consists of granules containing highly branched molecules, with a(1-6) branches occurring every 8 to 12 glucose units. Like amylopectin, glycogen yields a red-violet color with iodine. Glycogen can be hydrolyzed by both a-and b-amylases, yielding glucose and maltose, respectively, as products and can also be hydrolyzed by glycogen phosphorylase.
Glycogen StructureActual molecule has more layersGlycogenin (G) is a proteinthat starts the molecule
glycogenamylopectin
DextranAnother important family of storage polysaccharides are the dextrans,which are a(1-6)-linked polysaccharides of D-glucose with branched chains found in yeast and bacteria . Dextran is a branched polymer of D-glucose units. The main chain linkage is a(1-6), but 1-2, 1-3, or 1-4 branches can occur.
Sephadex gels are formed from dextran chains cross-linked with epichlorohydrin（表氯醇）. The degree of cross-linking determines the chromatographic properties of Sephadex gels. Sephacryl gels are formed by cross-linking of dextran polymers with N,N9-methylene bisacrylamide.
Structural PolysaccharidesCellulose
CelluloseCharacteristics/FunctionServes as structural component in bark, wood,stems, etc. of plantsInsoluble in water, tough, fibrous material Many H bonds create high tensile strength（抗张强度）of cellulose Nondigestable by mammals; termites（白蚁）, wood-rot fungi and bacteria can.
Giraffes, cattle, deer, and camels are ruminant（反刍）animals that are able to metabolize cellulose, thanks to bacterial cellulase in the rumen（瘤胃）, a large first compartment in the stomach of a ruminant.
ChitinA polysaccharide that is similar to cellulose, both in its biological function and its primary, secondary, and tertiary structure, is chitin. Chitin is present in the cell walls of fungi and is the fundamental material in the exoskeletons of crustaceans(甲壳类动物), insects, and spiders.
Structure of chitin (M p303)Repeating units are N-acetyl-D-glucosamines in b(1-4) linkage
The application of chitinChitin-based coatings can extend the shelf life of fruits, and a chitin derivative that binds to iron atoms in meat has been found to slow the reactions that cause rancidity(恶臭）and flavor loss. Without such a coating, the iron in meats activates oxygen from the air, forming reactive free radicals that attack and oxidize polyunsaturated lipids, causing most of the flavor loss associated with rancidity. Chitin-based coatings coordinate(调整) the iron atoms, preventing their interaction with oxygen.
Chitinase gene can be usedin plant genetic engineering toproduce anti-disease plants.
Alginates(盐藻糖)A family of novel extended ribbon structures thatbind metal ions, particularly calcium, in theirstructure are thealginatepolysaccharides ofmarine brown algae (Phaeophyceae). Theseincludepoly(b-D-mannuronate )andpoly(a-L-guluronate ),
Poly(a-L-guluronate) strands dimerize in the presence of Ca2+, forming a structure known as an “egg carton.”
AgaroseAn important polysaccharide mixture isolatedfrom marine red algae (Rhodophyceae) isagar,which consists of two components,agaroseandagaropectin. Agarose is a chain of alternatingD-galactose and 3,6-anhydro-L-galactose, with
D-galactose and3,6-anhydro-L-galactose
The favored conformation of agarose in water is a double helix with a threefold screw axis.
The ability of agarose to assemble in complex bundles to form gels in aqueous solution makes it useful in numerous chromatographic procedures, including gel exclusion chromatography and electrophoresis. Cells grown in culture can be embedded in stable agarose gel “threads” so that their metabolic and physiological properties can be studied. 。
Sepharose is a trade name which is used commonly in lab as a separated material.
Peptidoglycanlayer of Grampositive bacteria(M9.26)
Peptidoglycans
GlycosaminoglycansA class of polysaccharides known as glycosaminoglycansis involved in a variety of extracellular (and sometimes intracellular) functions.软骨素-4-硫酸透明质酸盐硫酸角质素
Glycoconjugates ProteoglycansGlycoproteinsglycolipid
Proteoglycan structure, showing the trisaccharide bridge
Proteoglycan structure of anintegral membrane protein
Interactions between cells and extracellularmatrix
Glycoprotein are information-rich conjugates containing oligosaachrides
Examples of glycoproteins (M table9.6)
Glycolipids and lipopolysacharides are membrane componants
Bacterial lipopolysacharides
Bacterial lipopolysacharides
Lectins are protein that bindcarbohydrate with high affinityand specificity
Glycoproteins are the materials involved in cellular recognition.
The ABO bloodgroups (M9.29)
A, B, O Blood GroupsAntigens on surface of cells lead todifferent blood groupsAntigens are carbohydrates
Methods of carbohydrate analysis
The end
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