Carbohydrates Lecture Notes - PDF

Carbohydrates Intro Video Lecture  Monosaccharides Have the general formula Cn(H2O)n Sugars: Their Disaccharides Simplest polysaccharide liked by glycosidic bonds Structures & Oligosaccharides Stereochemistry Thr...

Carbohydrates Intro Video Lecture  Monosaccharides Have the general formula Cn(H2O)n Sugars: Their Disaccharides Simplest polysaccharide liked by glycosidic bonds Structures & Oligosaccharides Stereochemistry Three of more sugars linked by glycosidic bonds Polysaccharides Formed when many monosaccharides are bonded together  Monosaccharides Building blocks of all carbohydrates Aldose: Monosaccharide containing an aldehyde group as part of its structure Ketose: Monosaccharide containing a ketone group as part of its structure Triose - Simplest monosaccharide that contains three carbon atoms Examples – Glyceraldehyde and dihydroxyacetone Stereoisomerism in Monosaccharides Enantiomers: Mirror-image, nonsuperimposable stereoisomers Example - D-glyceraldehyde and L-glyceraldehyde are enantiomers of each other Possibility of stereoisomerism increases as the number of carbon atoms increases Fischer Projections to Assign Configurations Most highly oxidized carbon is written at the top and is designated C-1 Other carbon atoms are numbered in sequence from top to bottom 1 D configuration 2 3 —OH is on the right of the highest-numbered 4 chiral carbon 5 L configuration 6 —OH is on the left of the highest-numbered chiral carbon Epimers Diastereomers that differ from each other in the configuration at only one chiral carbon  D-Aldoses with 3-6 Carbons Boxed carbohydrates are those you should be familiar with  D-Ketoses with 3-6 Carbons Boxed carbohydrates are those you should be familiar with 8 Cyclic Sugars with Five or Six Carbon Atoms Cyclization of sugars takes place because of an intramolecular reaction between functional groups on distant carbons. Hemiacetal (Pyranose) Hemiketal (furanose) In both cases shown above, the carbonyl carbon becomes a chiral center called the anomeric carbon  Cyclic Structure: Anomers One of the possible stereoisomers formed when a sugar assumes the cyclic form Designated α and β β- means that —OH on the anomeric carbon is cis to the terminal —CH2OH, α- means that it is trans ~36.4% ~63.6% Anomers can freely interconvert in aqueous solutions between each other, so at equilibrium, D-glucose is a mixture of the beta anomer at 63.6%, and the alpha is about 36.4%, the linear form is in a very small amount. While this interconversion can happen it is slow, because bonds need to be broken and reform.  Haworth Projection Formulas Cyclic structures are shown in perspective drawings as planar five- or six-membered rings viewed nearly edge on Furanose: Cyclic sugar with a five-membered ring Rings are very nearly planar Pyranose: Cyclic form of a sugar containing a six-membered ring Rings exist in solution in the chair conformation Haworth Representations of Sugar Structures Carbohydrates & Glycoproteins POGIL Section 14 Fisher to Haworth Projection ORGANISMS CONTAIN A VARIETY OF SUGARS  Pre-Activity Q4 Functions of Carbohydrates Major energy sources Oligosaccharides Play a key role in processes that take place on the surfaces of cells, such as cell–cell interactions and immune recognition Polysaccharides Essential structural components of several classes of organisms  Question 3 Aldose vs. Ketose  Formation of Glycosidic Bonds Glycoside: Compound formed when a hemiacetal carbon reacts with an alcohol to give a full acetal Resulting bond is called a glycosidic bond The basis for the formation of oligosaccharides and polysaccharides Glycosidic linkages take many forms α- or β-glycosidic linkage Two Different Disaccharides of α-D-Glucose 6 6 6 6 5 5 5 5 4 1 4 1 4 1 4 1 2 2 2 2 3 3 3 3 A Disaccharide of β-D-Glucose 6 5 4 1 3 2 6 5 4 1 3 2 Variation in Glycosidic Linkages Lead to the formation of linear and branched-chain polymers The Reducing End Formation of a glycosidic bond renders a sugar nonreducing Reducing end – the end of a disaccharide or polysaccharide chain with a free anomeric carbon Free anomeric carbon Reducing Sugars Turn and Talk Using your understanding of anomers, identify the conformation (alpha or beta) of the individual monomer and the linkage (C#) between the two. NAc = N-acetyl Some Important Oligosaccharides Sucrose Lactose Maltose Sucrose Most abundant disaccharide Disaccharide formed when glucose and fructose are bonded together Monosaccharide units - α-D-glucose and β-D-fructose Glycosidic linkage notation - α,β(1 → 2) Not a reducing sugar Both anomeric groups are involved in the glycosidic linkage Common table sugar, which is obtained from the juice of sugarcane and sugar beets Artificial sweeteners Fructose Sucralose Lactose Referred to as the milk sugar Made up of β-D-galactose and D-glucose Galactose is a C-4 epimer of glucose Glycosidic linkage is β(1 → 4) Reducing sugar Group at the anomeric carbon of the glucose portion is not involved in the glycosidic linkage  MEDICINE Lactose Intolerance Maltose Disaccharide that is obtained from the hydrolysis of starch Consists of two residues of D-glucose joined by an α(1 → 4) linkage Differs from cellobiose solely because of the glycosidic linkage conformation  Homopolysaccharides and Heteropolysaccharides Homopolysaccharides: contain only a single monomeric sugar species – serve as storage forms and structural elements Heteropolysaccharides: contain 2+ kinds of monomers – provide extracellular support Some Important Polysaccharides  Question 4 Cellulose vs. Amylose Cellulose Major structural component of plants, especially of wood and plant fibers Linear homopolysaccharide of β-D-glucose Residues are linked in β(1 → 4) glycosidic bonds Individual polysaccharide chains are hydrogen-bonded together Give plants fiber mechanical strength Animals lack cellulase, but this enzyme is found in some bacteria and insects, which explains why grazing animals like cattle and horses can live on grass and hay, but humans cannot. Chitin Major structural component of the exoskeletons of invertebrates, such as insects and crustaceans Occurs in cell walls of algae, fungi, and yeasts Linear homopolysaccharide with all the residues linked in β(1 → 4) glycosidic bonds Differs from cellulose in the nature of the monosaccharide unit In cellulose, the monomer is β-D-glucose In chitin, the monomer is N-acetyl-β-D-glucosamine Polymeric Structure of Chitin  Starches Polymers of α-D-glucose that occur in plant cells Types can be distinguished from one another by their degrees of chain branching Amylose Linear polymer of glucose, with all the residues linked together by α(1 → 4) bonds Amylopectin Branched chain polymer, with branches starting at α(1 → 6) linkages along the chain of α(1 → 4) linkages  Glycogen Storage polysaccharide of animals and present in all cells Most prevalent in skeletal muscles and in the liver Branched-chain polymer of α-D-glucose Consists of a chain of α(1 → 4) linkages with α(1 → 6) linkages at the branch points Glycogen phosphorylase and debranching enzymes assist in the breakdown of glycogen  Enzymes That Hydrolyze Starch Amylase attack α(1 → 4) linkages Hydrolyzes glycosidic linkages anywhere along the chain Glucose and maltose are the products of the reaction Alpha-glucosidase Debranching enzymes, which occur in both plants and animals, degrade the α(1 → 6) linkages  Structure and Roles of Some Polysaccharides Table 7-2 Structures and Roles of Some Polysaccharides Polymer Type Repeating unit Size (number of Roles/significance monosaccharide units) Starch: Homo- (𝛼1→4)Glc, 50-5,000 Energy storage: in plants Amylose linear Starch: Homo- (𝛼1→4)Glc, Up to 106 Energy storage: in plants Amylopectin with (𝛼1→6)Glc branches every 24-30 residues Glycogen Homo- (𝛼1→4)Glc, Up to 50,000 Energy storage: in bacteria and animal with (𝛼1→6)Glc cells branches every 8-12 residues Cellulose Homo- (𝛽1→4)Glc Up to 15,000 Structural: in plants, gives rigidity and strength to cell walls Chitin Homo- (𝛽1→4)GlcNAc Very large Structural: in insects, spiders, crustaceans, gives rigidity and strength to exoskeletons Dextran Homo- (𝛼1→6)Glc, Wide range Structural: in bacteria, extracellular with (𝛼1→3) adhesive branches Peptidoglycan Hetero-; peptides 4)Mur2Ac(𝛽1→4) Very large Structural: in bacteria, gives rigidity and attached GlcNAc (𝛽1 strength to cell envelope Hyaluronan (a glycosaminoglycan) Hetero-; acidic 4)GlcA(𝛽1→3) Up to 100,000 Structural: in vertebrates, extracellular GlcNAc (𝛽1 matrix of skin and connective tissue; viscosity and lubrication in joints  Heteropolysaccharide: Glycosaminoglycans Type of polysaccharide based on a repeating disaccharide in which: – One of the sugars is an amino sugar – At least one sugar has a negative charge due to the presence of a sulfate or carboxyl group Examples – Heparin is a natural anticoagulant – Hyaluronic acid is a component of the lubricating fluid of joints – Chondroitin sulfates and keratan sulfate Components of connective tissue  Question 5 Glycoproteins Contain carbohydrate residues that are covalently linked to the polypeptide chain Antibodies are glycoproteins Oligosaccharide portion of glycoproteins acts as antigenic determinants, which are portions of molecules that antibodies recognize as foreign and to which they bind  Glycoproteins Have Covalently Attached Oligosaccharides Two types of attachments: – O-linked: a glycoside bond joins the anomeric carbon of a carbohydrate to the —OH of a Ser or Thr residue – N-linked: an N-glycosyl bond joins the anomeric carbon of a sugar to the amide nitrogen of an Asn residue  MEDICINE Glycoproteins Contain carbohydrate residues that are covalently linked to the polypeptide chain Antibodies are glycoproteins Oligosaccharide portion of glycoproteins acts as antigenic determinants, which are portions of molecules that antibodies recognize as foreign and to which they bind  Glycoproteins: Proteoglycans Play an important role in eukaryotic cell membranes – Sugar portions are added to the protein as it passes through the Golgi on its way to the cell surface Proteoglycans: Glycoproteins with a high carbohydrate content – Constantly synthesized and broken down – Accumulation could lead to tragic consequences. Hurler’s syndrome Glycoprotein: Peptidoglycan Major component of bacterial cell walls Polysaccharide that contains peptide crosslinks Other Examples of Glycoproteins Mucins: secreted or membrane glycoproteins – can contain large numbers of O-linked oligosaccharide chains – present in most secretions Proteins of the blood – examples: immunoglobulins (antibodies), follicle- stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone Milk proteins – example: major whey protein α-lactalbumin  Question 6 Carbohydrates and their derivatives exhibit a variety of complex structures Sugars can be Modified or Covalently Linked: Redox Reactions of Simple Sugars 1. Oxidation of an aldose converts its aldehyde group to a carboxylic acid group, thereby yielding an aldonic acid such as gluconic acid (at right). Aldonic acids are named by appending the suffix -onic acid to the root name of the parent aldose. Sugars can be Modified or Covalently Linked: Redox Reactions of Simple Sugars 2. Oxidation of the primary alcohol group of aldoses yields uronic acids, which are named by appending -uronic acid to the root name of the parent aldose, for example, D-glucuronic acid. Uronic acids can assume the pyranose, furanose, and linear forms. Sugars can be Modified or Covalently Linked: Redox Reactions of Simple Sugars 3. Aldoses and ketoses can be reduced under mild conditions—for example, by treatment with NaBH4— to yield polyhydroxy alcohols known as alditols, which are named by appending the suffix -itol to the root name of the parent aldose. - Xylitol is a sweetener that is used in “sugarless” gum and candies. Sugars can be Modified or Covalently Linked: Deoxy Sugars 4. Monosaccharide units in which an OH group is replaced by H are known as deoxy sugars. - 𝛃-D-2-deoxyribose, the sugar component of DNA's sugar– phosphate backbone - L-Fucose is one of the few L sugar components of polysaccharides. Sugars can be Modified or Covalently Linked: Amino Sugars 5. In amino sugars, one or more OH groups have been replaced by an amino group, which is often acetylated. - N-Acetylneuraminic acid, which is derived from N-acetylmannosamine and pyruvic acid, is an important constituent of glycoproteins and glycolipids (proteins and lipids with covalently attached carbohydrate). Lectins Are Proteins That Read the Sugar Code and Mediate Many Biological Processes Lectins: bind carbohydrates with high specificity and with moderate to high affinity Functions: – cell-cell recognition – signaling – adhesion – intracellular targeting of newly synthesized proteins  Question 7 Selectins  Selectins Selectins: Family of plasma membrane lectins that mediate cell-cell recognition and adhesion in a wide range of cellular processes – move immune cells through the capillary wall – mediate inflammatory responses – mediate the rejection of transplanted organs  MEDICINE What Makes Sugar Sweet? TAS1R2 and TAS1R3 encode sweet-taste receptors Binding of a compatible molecule generates a “sweet” electrical signal in the brain – Requires a steric match