KARBOHIDRATSTRUKTUR DAN FUNGSI By: Puji Lestari Prodi Kimia FST
Carbohydrates are polyhydroxy aldehydes orketones, or substances that yield suchcompounds on hydrolysis.Many, but not all, carbohydrates have theempirical formula (CH2O)n; some also containnitrogen, phosphorus, or sulfur.
There are three major size classes of carbohydrates 1. Monosaccharides simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D- glucose Monosaccharides of more than four carbons tend to have cyclic structures.
2. Oligosaccharides Consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. The most abundant are the disaccharides, with two monosaccharide units. Typical is sucrose (cane sugar), which consists of the six-carbon sugars D-glucose and D-fructose. In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to nonsugar molecules (lipids or proteins) in glycoconjugates.
3. PolysaccharidesPolysaccharides are sugar polymers containingmore than 20 or so monosaccharide units, and somehave hundreds or thousands of units. Some polysaccharides, such as cellulose, are linearchains; others, such as glycogen, are branched. Both glycogen and cellulose consist of recurringunits of D-glucose, but they differ in the type ofglycosidic linkage and consequently have strikinglydifferent properties and biological roles.
MONOSACCHARIDESAldoses (e.g., glucose) have Ketoses (e.g., fructose) havean aldehyde group at one a keto group, usually at C2.end. H O C CH2OH H C OH C O HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-glucose D-fructose
D vs L Designation CHO CHOD & L designations arebased on the H C OH HO C Hconfiguration about CH2OH CH2OHthe single asymmetric D-glyceraldehyde L-glyceraldehydeC in glyceraldehyde. CHO CHOThe lower H C OH HO C Hrepresentations areFischer Projections. CH2OH CH2OH D-glyceraldehyde L-glyceraldehyde
Sugar NomenclatureFor sugars with more O H O Hthan one chiral C Ccenter, D or L refers to H – C – OH HO – C – Hthe asymmetric C HO – C – H H – C – OHfarthest from the H – C – OH HO – C – Haldehyde or keto H – C – OH HO – C – Hgroup. CH2OH CH2OHMost naturally D-glucose L-glucoseoccurring sugars are Disomers.
Hemiacetal & hemiketal formation H HAn aldehyde canreact with an C O + R OH R O C OHalcohol to form R Ra hemiacetal. aldehyde alcohol hemiacetalA ketone can R Rreact with an C O + "R OH "R O C OHalcohol to form R Ra hemiketal. ketone alcohol hemiketal
Pentoses andhexoses can cyclizeas the ketone oraldehyde reactswith a distal OH.Glucose forms anintra-molecularhemiacetal, as theC1 aldehyde & C5OH react, to forma 6-memberpyranosering, named afterpyran. representations of the cyclic sugars are calledTheseHaworth projections.
1 CH2OH 2C O HO C H 1 CH2OH 3 HOH2C 6 O H C OH 4 5 H HO 2 H C OH H 4 3 OH 5 OH H 6 CH2OH D-fructose (linear) -D-fructofuranoseFructose forms either a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.
6 CH 2OH 6 CH 2OH 5 O 5 O H H H OH H H 4 H 1 4 H 1 OH OH OH OH OH H 3 2 3 2 H OH H OH -D-glucose -D-glucoseCyclization of glucose produces a new asymmetric centerat C1. The 2 stereoisomers are called anomers, & .Haworth projections represent the cyclic sugars as havingessentially planar rings, with the OH at the anomeric C1: (OH below the ring) (OH above the ring).
Monosaccharides Are Reducing Agents Monosaccharides can be oxidized by relatively mild oxidizingagents such as ferric (Fe3+) or cupric (Cu2+) ion The carbonyl carbon is oxidized to a carboxyl group Glucose and other sugars capable of reducing ferric or cupricion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitativetest for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by asolution of a sugar, it is also possible to estimate the concentrationof that sugar For many years this test was used to detect and measureelevated glucose levels in blood and urine in the diagnosis ofdiabetes mellitus
Sugar derivatives COOH CHO CH2OH H C OH H C OH H C OH HO C H HO C H H C OH H C OH H C OH H C OH H C OH H C OH CH2OH CH2OH COOH D-ribitol D-gluconic acid D-glucuronic acid sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol. sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
CH2OH CH2OH H O H H O H H H OH H OH H OH OH OH O OH H NH2 H N C CH3 H -D-glucosamine -D-N-acetylglucosamineamino sugar - an amino group substitutes for a hydroxyl.An example is glucosamine.The amino group may be acetylated, as in N-acetylglucosamine.
O H H3C C NH O COO R HC OH H H R= HC OH H OH CH2OH OH H N-acetylneuraminate (sialic acid)N-acetylneuraminate (N-acetylneuraminic acid, alsocalled sialic acid) is often found as a terminal residueof oligosaccharide chains of glycoproteins.Sialic acid imparts negative charge toglycoproteins, because its carboxyl group tends todissociate a proton at physiological pH, as shown here.
Disaccharides (such as maltose, lactose, and sucrose) consist oftwo monosaccharides joined covalently by an O-glycosidic bond, whichis formed when a hydroxyl group of one sugar reacts with theanomeric carbon of the other Glycosidic bonds are readily hydrolyzed by acid but resist cleavageby base,they can be hydrolyzed to yield their free monosaccharidecomponents by boiling with dilute acid.The oxidation of a sugar’s anomeric carbon by cupric or ferric ion(the reaction that defines a reducing sugar) occurs only with the linearform, which exists in equilibrium with the cyclic form When the anomeric carbon is involved in a glycosidic bond, thatsugar residue cannot take the linear form and therefore becomes anonreducing sugar The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.
POLYSACCHARIDES (GLYCANS) Most carbohydrates found in nature occur as polysaccharides, polymers of medium to high molecular weight. Differ from each other in: their monosaccharide units the length of their chains the types of bonds linking the units the degree of branching. Homopolysaccharides contain only a single type of monomer Heteropolysaccharides contain two or more different kinds
Homopolysaccharides serve as: storage forms of monosaccharides that are used as fuels (starch and glycogen) structural elements in plant cell walls and animal exoskeletons (cellulose and chitin,) Heteropolysaccharides provide extracellular supportfor organisms of all kingdoms. the rigid layer of the bacterial cell envelope (peptidoglycan) is composed in part of a heteropolysaccharide built from two alternating monosaccharide units. In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides, which form a matrix that holds individual cells together and provides protection, shape, and support to cells, tissues, and organs.
Homopolysaccharides1. Some Homopolysaccharides Are Stored Forms of Fuel The most important storage polysaccharidesare: starch in plant cells glycogen in animal cells Both polysaccharides occur intracellularly as large clusters or granules Glycogen and starch ingested in the diet are hydrolyzed by α-amylases, enzymes in saliva and intestinal secretions that break (α1→4) glycosidic bonds between glucose units
StarchContains two types of glucose polymer, amylose and amylopectin Amylose consists of long, unbranched chains of D-glucose residues connected by (α1→4) linkages, vary in molecular weight from a few thousand to more than a million Amylopectin also has a high molecular weight (up to 100 million) but is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (α1→4), the branch points (occurring every 24 to 30 residues) are (α1→6) linkages.
Glycogen The main storage polysaccharide of animal cells A polymer of (α1→4)-linked subunits of glucose, with (α1→6)-linked branches, but glycogen is more extensively branched on average, (every 8 to 12 residues) and more compact than starch. especially abundant in the liver (7% of the wet weight) also present in skeletal muscle
2. Some Homopolysaccharides Serve Structural Roles Cellulose Cellulose, a fibrous, tough, water-insoluble substance, is found in thecell walls of plants Cellulose constitutes much of themass of wood, and cotton is almostpure cellulose cellulose molecule is alinear, unbranchedhomopolysaccharide,consisting of 10,000 to 15,000 D-glucose units. The glucose residues in cellulose arelinked by (β1→4) glycosidic bonds
Most animals cannot use celluloseas a fuel source, because they lackan enzyme to hydrolyzethe (β1→4) linkages. Termites readily digest cellulose (andtherefore wood), but only because theirintestinal tract harbors a symbioticmicroorganism, Trichonympha, thatsecretes cellulase, which hydrolyzesthe (β1→4) linkages Wood-rot fungi and bacteria alsoproduce cellulase
Chitin a linear homopolysaccharide composed ofN-acetylglucosamine residues in β- linkage the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs A spotted June beetle (Pellidnota punetatia), showing its surface armor probably the second most abundant (exoskeleton) of chitin. polysaccharide, next to cellulose, in nature.
Heteropolysaccharides1. Bacterial Cell Walls Contain Structural Heteropolysaccharides The rigid component of bacterial cellwalls is a heteropolymer of alternating(β1→4)-linked N-acetylglucosamine andN-acetylmuramic acid residues The linear polymers lie side by side in thecell wall, crosslinked by short peptides The enzyme lysozyme kills bacteria byhydrolyzing the (β1→4)glycosidic bondbetween N-acetylglucosamine and N-acetylmuramic acid Penicillin and related antibiotics killbacteria by preventing synthesis of thecross-links, leaving the cell wall too weakto resist osmotic lysis
2. Algal Cell Walls Contain Structural Heteropolysaccharides Certain marine red algae, including someof the seaweeds, have cell walls thatcontain agar, a mixture of sulfatedheteropolysaccharides made up of D-galactose and an L-galactose derivativeether-linked between C-3 and C-6 The two major components of agarare the unbranched polymer agarose(Mr ~120,000) and a branchedcomponent, agaropectin The remarkable gel-forming property of agarose makes it useful in the biochemistrylaboratory Agarose gels are used as inert supports for the electrophoretic separation of nucleic acids, an essential part of the DNA sequencing process Agar is also used to form a surface for the growth of bacterial colonies Agar is also used for the capsules in which some vitamins and drugs are packaged
3. Glycosaminoglycans Are Heteropolysaccharides of the Extracellular Matrix The extracellular space in the tissues ofmulticellular animals is filled with a gel-like material (ground substance), whichholds the cells together and provides aporous pathway for the diffusion ofnutrients and oxygen to individual cells The extracellular matrix is composed ofan interlocking meshwork ofheteropolysaccharides and fibrousproteins such ascollagen, elastin, fibronectin, andlaminin These heteropolysaccharides, the glycosaminoglycans, are a family of linear polymers composed of repeating disaccharide units Glycosaminoglycans are attached toextracellular proteins to formproteoglycans
Hyaluronic Acid Serve as lubricants in the synovial fluid of joints and give the vitreous humor ofthe vertebrate eye its jellylike consistency An essential component of the extracellular matrix of cartilage andtendons, to which it contributes tensile strength and elasticity as a result of itsstrong interactions with other components of the matrix Hyaluronidase, an enzyme secreted by some pathogenic bacteria, canhydrolyze the glycosidic linkages of hyaluronate, rendering tissues moresusceptible to bacterial invasion In many organisms, a similar enzyme in sperm hydrolyzes an outer glycosaminoglycan coat around the ovum, allowing sperm penetration
Chondroitin sulfate contributes to the tensile strength of cartilage, tendons, ligaments, and thewalls of the aortaDermatan sulfate (Greek derma, “skin”) contributes to the pliability of skin and is also present in blood vessels and heart valves. Keratan sulfates present in cornea, cartilage, bone, and a variety of horny structures formed of dead cells: horn, hair, hoofs, nails, and claws Heparin a natural anticoagulant made in mast cells (a type of leukocyte) and released into the blood, where it inhibits blood coagulation by binding to the protein antithrombin