University:
Kentucky State University
Course:
CHE 315 | Biochemistry
Academic year:

2020
Views:

450

Pages:

92
Author:

Adowj7b

What Are Carbohydrates? Carbohydrates are members of a large class of naturally occurring polyhydroxy ketones and aldehydes Aldose A monosaccharide that contains an aldehyde carbonyl group. Ketose A monosaccharide that contains a ketone carbonyl group Have many hydroxyl groups on adjacent carbons together with either an aldehyde or ketone group 2 Roles of Carbohydrates Major energy sources Oligosaccharides play a key role in cell–cell interactions and immune recognition. Polysaccharides are essential structural components of several classes of organisms such as cellulose (a major component of grass and trees) 3 Carbohydrates – natural forms • Most carbohydrates are found naturally in bound form rather than as simple sugars – Polysaccharides (starch, cellulose, inulin, gums) – Glycoproteins and proteoglycans (hormones, blood group substances, antibodies) – Glycolipids (cerebrosides, gangliosides) – Glycosides – Mucopolysaccharides (hyaluronic acid) – Nucleic acids (DNA, RNA) Classification-Aldoses and ketoses Nomenclature: aldo- or keto- + latin number + ose Most common sugars are aldoses rather than ketoses 5 Classification • By the number of carbon atoms they contain – – – – – – Triose Tetrose Pentose Hexose Heptose … Isomerism Isomers that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. Isomers Stereoisomers Enantiomers L-isomers D-isomers An isomer is a molecule with the same molecular formula as another molecule, but with a different chemical structure. Constitutional isomers Diastereomers Epimers Molecules with the same molecular formula, but have different bonding patterns and atomic organization. Optical isomers (stereoisomers) Enantiomers More Cs, more stereoisomers D sugars, rather than L sugars, predominate in nature 8 How to determine chiral carbons? 9 Stereoisomers of an aldotetrose Number of stereoisomers=2n, n is the number of chiral carbons 10 Stereoisomers of an aldotetrose Diastereomers: non-superimposable, non-mirror image stereoisomers Diastereomers Diastereomers Epimers are diastereomers that differ in the configuration at only ONE chiral carbon D-erythrose and D-threose are epimers. 11 Stereoisomers 12 Isomers of glucose 2n (n is the number of chiral carbons in a sugar molecule) Search for: Glucose, Galactose Mannose Acetal/ketal vs. hemiacetal/hemiketal Hemiacetal and hemiketal: ether and alcohol on same carbon Acetal and ketal: two ethers on same carbon What is the difference between hemiacetal and hemiketal and the difference between acetal and ketal? Structure of Glucose 15 Structure of Glucose 16 Glucose anomers-alpha and beta Fischer projection formulas of three forms of glucose In some molecules, any anomer can be used In other sugars, only one anomer is used, e.g. only β-D-ribose and β-D-deoxyribose are found in RNA and DNA, respectively. 17 Structural representation of sugars 18 Carbohydrates • Mono- (one), Oligo- (a few) and (many) Polysaccharides • Monosaccharides (simple sugar) a carbohydrate with three to seven carbon atoms. General formula Cn(H2O)n. • A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose) 19 Glucose • The most important simple carbohydrate in human metabolism. • It is the final product of carbohydrate digestion and provides acetyl groups for entry into the citric acid cycle as acetyl-SCoA 20 Galactose • Plant gums and pectins • A component of the disaccharide lactose (milk sugar) • Is produced from lactose during digestion. • Is converted to glucose to provide energy • Is synthesized from glucose to produce lactose for milk and compounds needed in brain tissue. Jam with galactose in the pectin that stiffens it 21 Galactose An aldohexose It differs from glucose only in the orientation of the OH group at C4. 22 Galactose vs Glucose Galactose Glucose An aldohexose It differs from glucose only in the orientation of the OH group at C4. 23 Fructose • D-Fructose, often called levulose or fruit sugar • honey and many fruits. • one of the two monosaccharides combined in the disaccharide sucrose. • Fructose is produced commercially in large quantities by hydrolysis of cornstarch to make high fructose corn syrup (HFCS). 24 Fructose • Like glucose and galactose, fructose is a 6-carbon sugar. • A ketohexose • It forms a five-membered rings in solution 25 Ribose and 2-Deoxyribose • Both are 5-carbon aldehyde sugars. • Important as parts of larger biomolecules, such as coenzyme A , ATP, cyclic AMP and in oxidizing and reducing agent coenzymes. • 2-deoxyribose differs from ribose by the absence of one oxygen atom, that in the OH group at C2. • Both exist as mixtures of open-chain and cyclic hemiacetal forms. 26 Reactions of Monosaccharides Oxidation-Reduction Reactions Oxidation Aldehyde Cyclic aldose Ketose Carboxylic acid Lactone Aldose 27 Oxidation of a sugar to a lactone • Carbohydrates that react with oxidizing agents are classified as reducing sugars (they reduce the oxidizing agent). • Glucose can be detected specifically by the enzyme glucose oxidase. 28 Biochemical application on sugar oxidation •Vitamin C (ascorbic acid) is an unsaturated lactone. •Air oxidation of ascorbic acid, followed by hydrolysis of the ester bond, leads to loss of activity as a vitamin. •A lack of fresh food can cause vitamin C deficiencies, which, in turn, can lead to scurvy 29 Benedict’s test Sugar acids (oxidation) • Where is it oxidized? What does it form? Example 1 Example 2 Example 3 Ketoses can become aldoses • In basic solution, ketoses are also reducing sugars. • Rearrangement. 35 Reduced sugars (deoxy sugars) L-fucose (L-6-deoxygalactose) is found in the carbohydrate portions of some glycoproteins including the ABO blood-group antigens. D-2-deoxyribose is found in DNA 36 Reduced sugars (polyhydroxy alcohols or alditols) Sorbitol and xylitol are used commercially as sweeteners in sugarless chewing gum and candy. 37 Reactions of Monosaccharides Esterification of sugars Phosphate esters formation (phosphorylated sugars) Important for the metabolism of sugars 38 O-Glycosides • What is the reacting functional group? Where does it react? What are the end products? Where are they used? N-glycosides • What is the reacting functional group? Where does it react? What are the end products? Where are they used? • Examples: nucleotides (DNA and RNA) Note • Glycosides derived from furanoses are called furanosides, and those derived from pyranoses are called pyranosides, regardless if they are Nor O-linkded. Sugar derivatives Amino sugars An amino group (—NH2) or one of its derivatives is substituted for the hydroxyl group of the parent sugar. Components of bacterial cell walls 42 Reactions of Monosaccharides Reaction with Alcohols: Glycoside Formation Hemiacetals react with alcohols with the loss of water to yield acetals • Monosaccharides are cyclic hemiacetals that react with alcohols to form acetals (glycosides). • O-glycosides not N-glycosides • Furanosides are glycosides derived from furanoses, and pyranosides are glycosides derived from pyranoses. 43 Hydrolysis of a disaccharide • Hydrolysis reaction takes place during digestion of all carbohydrates. 44 Reactions of Monosaccharides Glycosidic bond between monosaccharides 2 glucose molecules Chemical natures of oligo-and polysaccharides depend on: Monosaccharides linked together The type of glycosidic bond formed (anomer type and carbons linked) 45 Linear and branched-chain polymers of α-D-glucose 46 Polysaccharides and reducing sugars 47 Oligosaccharides Disaccharides are Oligosaccharides 48 Distinctions of disaccharides • The 2 specific sugar monomers involved and their stereoconfigurations (D- or L-) • The carbons involved in the linkage (C-1, C-2, C-4, or C-6) • The order of the two monomer units, if different (example: galactose followed by glucose) • The anomeric configuration of the OH group on carbon 1 of each residue (α or β) Sucrose • Common table sugar, cane sugar, beet sugar • Hydrolysis of sucrose yields one molecule of D-glucose and one molecule of D-fructose. • The 50:50 mixture of glucose and fructose (invert sugar) is sweeter than sucrose and is used as a food additive. • No hemiacetal group because a 1,2 link joins both anomeric carbon atoms. • Sucrose is not reducing because it does not have a hemiacetal group • Sucrose is the only common disaccharide that is not a reducing sugar. 50 Biochemical Application: Synthetic sugars and sweetness 51 Biochemical application: Sucralose 52 Lactose (milk sugar) • • • • Human milk is about 7% lactose. A disaccharide of β-D-galactose and either α or β-D-glucose. The two monosaccharides are connected by a β-1,4 link. A reducing sugar because the glucose ring (on the right) is a hemiacetal at C1. 53 Clinical hint-Lactose intolerance • Is prevalent in adults in all populations. • Due to lactase enzyme deficiency. • Because lactose remains in the intestines rather than being absorbed, it raises the osmolarity, which draws in excess water. • Intestinal bacteria ferment lactose to produce lactate, carbon dioxide, hydrogen gas, and methane resulting in bloating, cramps, and diarrhea. • Lactose free food, the use of commercial enzyme preparations and Lactaid, milk that has been treated with lactase to reduce its lactose content 54 Clinical application • Galactosemia: Missing a galactose-metabolizing enzyme can result in galactosemia where nonmetabolized galactose accumulates within cells and is converted to the hydroxy sugar galactitol, which cannot escape cells. Water is drawn into cells and the swelling causes cell damage, particularly in the brain, resulting in severe and irreversible retardation. It also causes cataract. 55 Maltose (Malt sugar) • • • • Present in fermenting grains Can be prepared by enzyme-catalyzed degradation of starch. Used in prepared foods as a sweetener. Produced during starch digestion by α-amylase in the small intestine • Hydrolyzed to glucose by maltase. 56 Maltose • Two -D-glucose molecules are joined in maltose by an α-1,4 link • It is both an acetal (at C1 in the left-side glucose) and a hemiacetal (at C1 in the right-side glucose). • The hemiacetal group on the right can react with an oxidizing agent, thus maltose can be a reducing sugar. 57 Cellobiose • A disaccharide that is obtained from the hydrolysis of cellulose • Two -D-glucose molecules are joined by a β-1,4 link • It is both an acetal (at C1 in the left-side glucose) and a hemiacetal (at C1 in the right-side glucose). • The hemiacetal group on the right can react with an oxidizing agent, thus maltose can be a reducing sugar. 58 Oligosaccharides: Raffinose • What are oligosaccharide? • Example: raffinose • It is found in Found in beans and vegetables like cabbage, brussel, sprouts, broccoli, asparagus. Homework Humans lack the alpha-galactosidase enzyme that is needed to break down raffinose, but intestinal bacteria can ferment it into hydrogen, methane, and other gases. 1. Recognize the monosaccharides that make up raffinose. 2. What is the monosaccharide that is attached to what disaccharide? Oligosaccharides as drugs • Streptomycin and erythromycin (antibiotics) • Doxorubicin (cancer chemotherapy) • Digoxin (cardiovascular disease) Polysaccharides • Homopolysaccharide: a polymer that consists of only one type of monosaccharide • Heteropolysaccharide a polymer that consists of more than one type of monosaccharide • Glucose is the most common monomer 61 Cellulose 62 Cellulose • Animals lack cellulases enzymes that hydrolyze cellulose to glucose. • Cellulases are found in the bacteria that inhabit the digestive tracts of insects and grazing animals, such as cattle and horses. 63 H-bonds in cellulose 64 Application: Fibers and health • Stimulate peristaltic action • Bind to toxic substances in foods they get exported from the body, thus preventing them from damaging the lower intestine or being reabsorbed there • Bind carcinogens, thus prevent cancer • Bind cholesterol resulting in less amount in the blood • Fewer calories 65 Starch Starches are polymers of α-D-glucose that occur in plant cells, usually as starch granules in the cytosol. 66 Types of starches Amylose α (1-4) 67 Types of starches Amylose forms a helix with six residues per turn. Iodine molecules can fit inside the helix to form a starch–iodine complex (dark-blue color) 68 Types of starches Amylopectin No known preferred conformation for amylopectin Branch points occur about every 25 residues α (1-6) α (1-4) Amylopectin and glycogen react with iodine to produce a red-brown product, not blue. 69 Amylases • Starches are storage molecules • Both plants and animals contain enzymes that hydrolyze starches (α- and β-amylase ) • Both enzymes attach α(1-4) linkages α- amylase β-amylase Endoglycosidase Exoglycosidase Acts on glycosidic linkages along the chain Acts on the nonreducing end of the polymer Produces glucose and maltose Produces maltose (glucose dimer) 70 Amylase action on amylose and amylopectin • Amylose can be completely degraded to glucose and maltose by the two amylases • Amylopectin is not completely degraded by the two amylases because the branching linkages are not attacked. • Debranching enzymes degrade the α(1-6) linkages 71 Glycogen • glycogen is more highly branched than starch • Branch points occur about every 10 residues in glycogen • α (1-4) linkage in main chain • α (1-6) linkage at branches • The average chain length is 13 glucose residues • 12 layers of branching 72 Glycogen • Glycogen is found in granules in animal cells particularly in well-fed liver and muscle cells • Some athletes (long-distance runners) try to build up their glycogen reserves before a race by eating large amounts of carbohydrates. • Degradative enzymes remove glucose units from glycogen when energy is needed • Glycogen phosphorylase cleaves one glucose at a time from the non-reducing end of a branch to produce glucose-1phosphate 73 Glycogen • Debranching enzymes complete the breakdown of glycogen. • The number of branch points is important because: • 1. A more branched polysaccharide is more water soluble and does not crystallize. • 2. Easy access to glucose residues: when energy is needed quickly, the glycogen phosphorylase has more potential targets if there are more branches, allowing a quicker mobilization of glucose. 74 Amylopectin versus glycogen • Branching is more in glycogen: Branch points occur about every 10 residues in glycogen and about every 25 residues in amylopectin. • Source 75 Pectin • A polysaccharide of plant cell wall • A polymer made up mostly of D-galacturonic acid • D-galacturonic acid is a galactose derivative • A gelling agent in yogurt, fruit preserves, jams, and jellies. 76 Chitin Has a structural role Mechanical strength because of H-bonds between strands. A major structural component of the exoskeletons of invertebrates such as insects and crustaceans (a group that includes lobsters and shrimp), and the cell walls of algae, fungi, and yeasts. 77 Dextran • A storage polysaccharide • Yeast and bacteria • -(1-6)-D-glucose with branched chains • Branches: 1-2, 1-3, or 1-4 Are polysaccharides reducing? • A sample that contains only a few molecules of a large polysaccharide, each molecule with a single reducing end, might well produce a negative test because there are not enough reducing ends to detect. Glycoseaminoglycans (GAGs) Polysaccharides that have a repeating disaccharide One of the sugars is an amino sugar At least one of the sugars has a negative charge due to the presence of a sulfate group or a carboxyl group GAGs are linked to proteins to form proteoglycans Heparin is a natural anticoagulant Hyaluronic acid is a component of the vitreous humor of the eye and of the lubricating fluid of joints 80 Glycoseaminoglycans (GAGs) GAGs are highly polar and attract water, thus act as a lubricant or as a shock absorber 81 Localization and function of GAG GAG Hyaluronate Localization synovial fluid, vitreous humor, ECM of loose connective tissue Chondroitin sulfate cartilage, bone, heart valves Comments the lubricant fluid , shock absorbing As many as 25,000 disaccharide units most abundant GAG Heparan sulfate basement membranes, components of cell surfaces contains higher acetylated glucosamine than heparin Heparin component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin A natural anticoagulant Dermatan sulfate skin, blood vessels, heart valves Keratan sulfate cornea, bone, cartilage aggregated with chondroitin sulfates Only one not having uronic acid Proteoglycans • Lubricants • Structural components in connective tissue • Mediate adhesion of cells to the extracellular matrix • Bind factors that stimulate cell proliferation Polysaccharides in bacterial cell wall • Heteropolysaccharides are major components of bacterial cell walls Lactic acid • N-Acetylmuramic acid is found only in prokaryotic cell walls; it does not occur in eukaryotic cell walls. 84 Polysaccharides in bacterial cell wall • Polysaccharides are cross-linked by peptides. 85 Polysaccharides in bacterial cell wall Peptidoglycan is the material that results from the crosslinking of polysaccharides by peptides 86 Glycoproteins • Glycoproteins contain carbohydrate residues attached to the polypeptide chain • Example: antibodies. 87 Glycoproteins • The carbohydrates of glycoproteins are linked to the protein component through either O-glycosidic or Nglycosidic bonds – The N-glycosidic linkage is through the amide group of asparagine (Asn, N) – The O-glycosidic linkage is to the hydroxyl of serine (Ser, S), threonine (Thr, T) or hydroxylysine (hLys) 88 Significance of protein-linked sugars • Soluble proteins as well as membrane proteins • Purpose: • Protein folding • Protein targeting • prolonging protein half-life • Cell-cell communication • Signaling Carbohydrates as antigenic determinants-ABO blood groups • Carbohydrates act as antigenic determinants, the portions of an antigenic molecule that antibodies recognize and bind. Glycoprotein and blood transfusion 91 Sialic acid • N-acetylneuraminate • Precursor: the amino sugar, neuraminic acid • Location: a terminal residue of oligosaccharide chains of glycoproteins and glycolipids. Low carbohydrate diet • More carbohydrates, more insulin, more fat synthesis and storage and inhibition of fat burning 93

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