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Welcome to Class 7 Class 7: Outline and Objectives l Monosaccharides l Aldoses, ketoses; hemiacetals; epimers l Pyranoses, furanoses Welcome to Class 7 l Mutarotation, anomers l Disaccharides and glycosidic bonds l Polysaccharides l Starch, glycogen, cellulose, chitin l Bacterial cell walls (peptidoglycans) l Glycoconjugates: Proteoglycans and glycoproteins Introductory Biochemistry l Bioenergetics: ATP and coupled reactions l Phosphoryl group transfers l Concentration dependence of ∆G Monosaccharides Stereoisomers of glyceraldehyde terminal carbon (C1) is carbonyl (aldehyde) second carbon (C2) is carbonyl (ketone) Monosaccharides are chiral. A molecule with n chiral centers can have 2n possible stereoisomers. The chiral center most distant from the carbonyl carbon defines D- and L-forms. L- and D- isomers of the same compound are mirror images (enantiomers). Enantiomers of compounds with more than one chiral center have all chiral centers reversed. figure 7-1 figure 7-2 The most common monosaccharides 1 D-aldoses (aldehydes) D-ketoses (ketones) (achiral) figure 7-3 The more commonly occurring aldoses are shown in red boxes figure 7-3 The more commonly occurring ketoses are shown in red boxes Hemiacetals and hemiketals Epimers of Glucose If two sugars differ only in the configuration around one Hemiacetals and hemiketals are molecules with hydroxyl and ether groups carbon atom, they are called epimers. on the same carbon. They result from the reaction between aldehyde or keto groups and alcohol. The reaction is freely reversible. D-Mannose and D-Galactose are both epimers of D-Glucose. D-Mannose and D-Galactose are not epimers of one another. Although epimers are isomeric, they are not mirror images (enantiomers) figure 7-5 and in general they have different chemical and physical properties. figure 7-4 2 Cyclic forms of monosaccharides The actual conformation of a pyranose ring is not flat, but assumes a chair-like shape Monosaccharides contain both aldehyde or keto groups and hydroxyl groups. In aqueous solutions, most monosaccharides occur as cyclic structures. They result from hemiacetal or hemiketal formation between aldehyde or keto groups and hydroxyl groups on the same 1% molecule. The reaction is freely reversible. A new asymmetric C atom (anomeric carbon) is formed in the process of forming a cyclic D-Glucose is the aldose that hemiacetal, making two most commonly occurs in isomeric forms (anomers) nature as a monosaccharide. possible, designated α and β. 33% (at equilibrium) 66% figure 7-6 figure 7-7, 7-8 Why more beta than alpha D-glucopyranose? Haworth Perspectives of Cyclic Sugars ● Substituents that appear on the right side in Fischer projections are below D-Glucopyranose adopts only one of the two the plane of the ring in Haworth perspectives. possible chair forms where all pyranose ● If the hydroxyl group of anomeric carbon is on the same side of the ring as substituents are arranged equatorially. the hyrdoxyl group of the highest numbered asymmetric carbon (e.g., C5 α-D-Glucopyranose has 4 equatorial and 1 axial of a hexose), the anomer is defined as α (opposite side ≡ β anomer). But, substitutions on the pyranose ring whereas β-D- this is not always easy to see. Glucopyranose has 5 equatorial substituents on ● A practical rule, which works for both D- and L-pyranoses and furanoses, the pyranose ring. Minimization of steric hindrance 33% is that if the hydroxyl group on the anomeric carbon is trans to the terminal favors equatorial positions for the highest number CH2OH in the Haworth perspective drawing, the sugar is an α anomer; if it of pyranose substituents. The anomeric effect is cis to the terminal CH2OH, it is a β anomer. involving stabilization of the axial configuration of 5 HO the hydroxyl group on the anomeric carbon α-D-Fructo- β-D-Ribo- 4 1 β through molecular orbital overlap of the oxygen furanose 2 α furanose lone pairs and the anomeric carbon bond with its 3 2 OH group is not enough to stabilize the alpha form and therefore in the case of D- glucopyranose sterics trumps the anomeric HO OH H 66% or β or β effect. OH H HO α-D-Glucopyranose β-D-Glucopyranose 3 Mutarotation Pyranoses and furanoses ! Although anomers are isomeric, they are not mirror images (enantiomers). In general, they have different physical and chemical properties. Anomers rotate polarized light differently. ! Interconversion between α and β anomers occurs via the linear Glucose: almost (aldehyde or ketone) form of the respective monosaccharide until exclusively pyranose Fructose: 67% pyranose, equilibrium between the two forms is reached. This is called 33% furanose mutarotation. Their equilibrium ratio need not be 1:1! Because anomers rotate polarized light differently, the optical rotation of the solution changes in the process. ! At equilibrium, the linear (aldose or ketose) form is present only in minute amounts. figure 7-7 Sugars as reducing agents Sugars as reducing agents Hemiacetals are easily converted to aldehydes; Hemiacetals are easily converted to aldehydes; aldehydes are easily oxidized to acids. The oxidation of the aldehyde involves aldehydes are easily oxidized to acids. The oxidation of the aldehyde involves transfer of two electrons to an acceptor, which becomes reduced. transfer of two electrons to an acceptor, which becomes reduced. Therefore, monosaccharides are reducing sugars. (Ketones, as well as aldehydes, Therefore, monosaccharides are reducing sugars. react with oxidants, but ketones react more slowly, and the products of ketose Reducing sugars can be detected in solution by adding some colorless substance, oxidation include glycolaldehyde, derived from C1 and C2). such as AgNO3, which is reduced to a colored product, such as Ag↓. + H2O + H2O + 3H+ + 3H+ figure 7-10 figure 7-10 4 Chemical oxidation products of glucose Blood glucose determination Oxidized glucose (gluconate) has a strong tendency to internally + OH– esterify >> lactone formation. This helps to drive the reaction by lowering [product]. Assay: a peroxidase reaction uses the H2O2 produced by glucose oxidase to convert a colorless compound into a colored one, which absorbs light at a particular wavelength. figure 7-3 figure 7-9 Oxidation at other carbons is more difficult, but Hemiacetals and hemiketals can be esterified such oxidation products do occur in nature with alcohols to form acetals and ketals C6 C1 (the oxidized carbon is shown in color) In contrast to hemiacetals and hemiketals, acetals and ketals are relatively stable. figure 7-9 figure 7-5 5 Formation of the acetal disaccharide maltose Common disaccharides Reducing sugars have a free anomeric carbon. Formation of an acetal from a hemiacetal and an alcohol (hydroxyl group). Dehydration Non-reducing sugars have no free anomeric carbons. Wavy lines: Anomer not specified (could be α or β) Non-reducing sugars are named pyranosides or furanosides. O-glycosidic bond figure 7-10 figure 7-11 Naming Conventions Polysaccharides (glycans) Reducing oligosaccharides are named ending with the sugar that has the reducing anomeric carbon . Non-reducing oligosaccarides can be named beginning from either end sugar. H α or O β-D-fructofuranosyl α-D-glucopyranoside Fru(β2↔1α)Glc Raffinose α-D-galactopyranosyl-(1→6)-α-D-glucopyranosyl β-D-fructofuranoside Gal(α1→6)Glc(α1↔2β)Fru or β-D-fructofuranosyl α-D-glucopyranosyl-(6→1)-α-D-galactopyranoside figure 7-12 figure 7-11 Fru(β2↔1α)Glc(6→1α)Gal 6 Some polysaccharides Starch l Starch (plants) l Amylose: α1→4 l Amylopectin: α1→4, α1→6 Glucose l Glycogen (animals, bacteria): α1→4, α1→6 Maltose (more branched than starch) l Cellulose: β1→4 Starch and cellulose both consist of recurring units of D-glucose. Their different properties result from different types of glycosidic linkage. l Peptidoglycans (bacterial cell walls) l Chitin (exoskeletons, cell walls): N-acetyl-D-glucosamine β1→4 figure 7-13 a,b,c Structure of starch Starch Maltose Starch granules figure 7-13a,b,c figure 7-19a,b What is the advantage of storing glucose as a polymer? 7 Starch Starch Maltose Maltose figure 7-13a,b,c figure 7-13a,b,c What is the advantage of having only one reducing end? What is the advantage of having many non-reducing ends (branching)? Chitin Cellulose 180° flip Cellulose accounts for over half of the carbon in the biosphere. The disaccharide unit of cellulose is called cellobiose. N-acetyl-D-glucosamine: β1→4 Chitin is the principal structural component of the exoskeletons of arthropods (crustaceans, insects, and spiders) and is present in the cell walls of fungi and some algae. After cellulose, from which it only differs in the acetylated amino figure 7-14, 7-20 group at C2, chitin is the next most abundant polysaccharide in the biosphere. figure 7-16a 8 Peptidoglycans in Proteoglycans bacterial cell walls (more carbohydrate than protein) Glycosaminoglycans ≡ unbranched polysaccharides of alternating uronic acid (oxidized at C6) and GlcNAc or GalNAc residues (often sulfated) Penicillin interferes with cell wall formation by preventing the synthesis of cross-links. (Alexander Fleming) Core proteins + covalently linked glycosaminoglycans ≡ proteoglycans Proteoglycans form the ground substance of connective tissue (cartilage, tendon, skin, blood vessel walls). They have a slimy, mucuslike consistency. figure 20-30 figure 7-22 What is the advantage of having unusual (D-) amino acids? Glycoproteins Glycoproteins (more protein than carbohydrate) (more protein than carbohydrate) GlcA-GlcNS GlcA-GalNAc Immunoglobin Plasma membrane protein Immunoglobin Plasma membrane protein Almost all secreted and membrane-associated Almost all secreted and membrane-associated proteins of eukaryotic cells are glycosylated. figures 5-22b, 7-26 proteins of eukaryotic cells are glycosylated. figures 5-21b, 7-26 9 Glycoproteins Glycoproteins figure 7-30 What is the advantage of having so much potential variation? The equilibrium constant for a reaction, Introduction to Bioenergetics K'eq, is mathematically related to ∆G' º A + B C + D Standard free energy change (1 M concentrations, etc.): [C][D] = [A][B] [A], [B], [C], [D] are the molar concentrations of the reaction components at equilibrium.
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