Welcome to Class 7 Introductory Biochemistry Class 7: Outline and Objectives l Monosaccharides l Aldoses, ketoses; hemiacetals; epimers l Pyranoses, furanoses 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 l Bioenergetics: ATP and coupled reactions l Phosphoryl group transfers l Concentration dependence of G 1
Monosaccharides terminal carbon (C1) is carbonyl (aldehyde) second carbon (C2) is carbonyl (ketone) The most common monosaccharides figure 7-1 Stereoisomers of glyceraldehyde Monosaccharides are chiral. A molecule with n chiral centers can have 2 n 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 7-2 2
D-aldoses (aldehydes) The more commonly occurring aldoses are shown in red boxes figure 7-3 D-ketoses (ketones) (achiral) The more commonly occurring ketoses are shown in red boxes figure 7-3 3
Epimers of Glucose If two sugars differ only in the configuration around one carbon atom, they are called epimers. 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) and in general they have different chemical and physical properties. figure 7-4 Hemiacetals and hemiketals Hemiacetals and hemiketals are molecules with hydroxyl and ether groups on the same carbon. They result from the reaction between aldehyde or keto groups and alcohol. The reaction is freely reversible. figure 7-5 4
Cyclic forms of monosaccharides 1% 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 molecule. The reaction is freely reversible. A new asymmetric C atom (anomeric carbon) is formed in the process of forming a cyclic hemiacetal, making two isomeric forms (anomers) possible, designated α and β. 33% (at equilibrium) 66% figure 7-6 The actual conformation of a pyranose ring is not flat, but assumes a chair-like shape D-Glucose is the aldose that most commonly occurs in nature as a monosaccharide. figure 7-7, 7-8 5
Why more beta than alpha D-glucopyranose? D-Glucopyranose adopts only one of the two possible chair forms where all pyranose substituents are arranged equatorially. α-d-glucopyranose has 4 equatorial and 1 axial substitutions on the pyranose ring whereas β-d- Glucopyranose has 5 equatorial substituents on the pyranose ring. Minimization of steric hindrance favors equatorial positions for the highest number of pyranose substituents. The anomeric effect involving stabilization of the axial configuration of the hydroxyl group on the anomeric carbon through molecular orbital overlap of the oxygen lone pairs and the anomeric carbon bond with its OH group is not enough to stabilize the alpha form and therefore in the case of D- glucopyranose sterics trumps the anomeric effect. 33% 66% Haworth Perspectives of Cyclic Sugars Substituents that appear on the right side in Fischer projections are below the plane of the ring in Haworth perspectives. If the hydroxyl group of anomeric carbon is on the same side of the ring as the hyrdoxyl group of the highest numbered asymmetric carbon (e.g., C5 of a hexose), the anomer is defined as α (opposite side β anomer). But, this is not always easy to see. A practical rule, which works for both D- and L-pyranoses and furanoses, is that if the hydroxyl group on the anomeric carbon is trans to the terminal CH 2 OH in the Haworth perspective drawing, the sugar is an α anomer; if it is cis to the terminal CH 2 OH, it is a β anomer. β-d-ribofuranose α-d-fructofuranose 2 α 5 HO 4 1 β 3 2 OH or HO OH H β or β H HO α-d-glucopyranose β-d-glucopyranose 6
Mutarotation! 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 (aldehyde or ketone) form of the respective monosaccharide until equilibrium between the two forms is reached. This is called 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. Pyranoses and furanoses Glucose: almost exclusively pyranose Fructose: 67% pyranose, 33% furanose figure 7-7 7
Sugars as reducing agents Hemiacetals are easily converted to aldehydes; aldehydes are easily oxidized to acids. The oxidation of the aldehyde involves transfer of two electrons to an acceptor, which becomes reduced. Therefore, monosaccharides are reducing sugars. (Ketones, as well as aldehydes, react with oxidants, but ketones react more slowly, and the products of ketose oxidation include glycolaldehyde, derived from C1 and C2). + H 2 O + 3H + figure 7-10 Sugars as reducing agents Hemiacetals are easily converted to aldehydes; aldehydes are easily oxidized to acids. The oxidation of the aldehyde involves transfer of two electrons to an acceptor, which becomes reduced. Therefore, monosaccharides are reducing sugars. Reducing sugars can be detected in solution by adding some colorless substance, such as AgNO 3, which is reduced to a colored product, such as Ag. + H 2 O + 3H + figure 7-10 8
Chemical oxidation products of glucose figure 7-3 Blood glucose determination Oxidized glucose (gluconate) has a strong tendency to internally esterify >> lactone formation. This helps to drive the reaction by lowering [product]. + OH Assay: a peroxidase reaction uses the H 2 O 2 produced by glucose oxidase to convert a colorless compound into a colored one, which absorbs light at a particular wavelength. figure 7-9 9
Oxidation at other carbons is more difficult, but such oxidation products do occur in nature C6 C1 (the oxidized carbon is shown in color) figure 7-9 Hemiacetals and hemiketals can be esterified with alcohols to form acetals and ketals In contrast to hemiacetals and hemiketals, acetals and ketals are relatively stable. figure 7-5 10
Formation of the acetal disaccharide maltose Dehydration Formation of an acetal from a hemiacetal and an alcohol (hydroxyl group). Wavy lines: Anomer not specified (could be α or β) O-glycosidic bond figure 7-10 Common disaccharides Reducing sugars have a free anomeric carbon. Non-reducing sugars have no free anomeric carbons. Non-reducing sugars are named pyranosides or furanosides. figure 7-11 11
Naming Conventions 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. or β-d-fructofuranosyl α-d-glucopyranoside Fru(β2 1α)Glc figure 7-11 H O α 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 Fru(β2 1α)Glc(6 1α)Gal Polysaccharides (glycans) figure 7-12 12
Some polysaccharides Glucose l Starch (plants) l Amylose: α1 4 l Amylopectin: α1 4, α1 6 l Glycogen (animals, bacteria): α1 4, α1 6 (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 Starch Maltose figure 7-13 a,b,c 13
Structure of starch Starch granules figure 7-19a,b Starch Maltose What is the advantage of storing glucose as a polymer? figure 7-13a,b,c 14
Starch Maltose What is the advantage of having only one reducing end? figure 7-13a,b,c Starch Maltose figure 7-13a,b,c What is the advantage of having many non-reducing ends (branching)? 15
Cellulose 180 flip Cellulose accounts for over half of the carbon in the biosphere. The disaccharide unit of cellulose is called cellobiose. figure 7-14, 7-20 Chitin 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 group at C2, chitin is the next most abundant polysaccharide in the biosphere. figure 7-16a 16
Peptidoglycans in bacterial cell walls Penicillin interferes with cell wall formation by preventing the synthesis of cross-links. (Alexander Fleming) What is the advantage of having unusual (D-) amino acids? figure 20-30 Proteoglycans (more carbohydrate than protein) Glycosaminoglycans unbranched polysaccharides of alternating uronic acid (oxidized at C6) and GlcNAc or GalNAc residues (often sulfated) 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 7-22 17
Glycoproteins (more protein than carbohydrate) GlcA-GlcNS GlcA-GalNAc Immunoglobin Plasma membrane protein Almost all secreted and membrane-associated proteins of eukaryotic cells are glycosylated. figures 5-22b, 7-26 Glycoproteins (more protein than carbohydrate) Immunoglobin Plasma membrane protein Almost all secreted and membrane-associated proteins of eukaryotic cells are glycosylated. figures 5-21b, 7-26 18
Glycoproteins figure 7-30 What is the advantage of having so much potential variation? Glycoproteins 19
Introduction to Bioenergetics The equilibrium constant for a reaction, K' eq, is mathematically related to G' º A + B C + D Standard free energy change (1 M concentrations, etc.): [A], [B], [C], [D] are the molar concentrations of the reaction components at equilibrium. = [C][D] [A][B] If [C][D] > [A][B] at equilibrium, then lnk' eq is positive, and therefore G' º is negative. This means if initially all reactants are present at 1 M concentration, the reaction would go from A + B to C + D before and until equilibrium is reached. 20
The actual G of a reaction depends on reactant and product concentrations as well as G'º A + B C + D If the reactants are initially present not at 1 M, but at different concentrations (nonstandard conditions): The criterion for the direction of net spontaneous reaction is G, not G' º. A reaction with a positive G' º can go forward as long as G is negative. This is the case when becomes negative ([C][D] < [A][B]), for example when products C and D are constantly removed as soon as they are formed. Standard free energy changes are additive If the two reactions can be effectively coupled, a reaction with a large negative G' º can drive a reaction with a positive G' º. The pathway in a coupled reaction from A to C is different from the individual reactions A to B (1) and B to C (2). 21
Standard free energy changes are additive Glucose + P i Glucose 6-P + H 2 O ATP + H 2 O ADP + P i Glucose + ATP ADP + Glucose 6-P ΔG' º = 13.8 kj/mol ΔG' º = 30.5 kj/mol ΔG' º = 16.7 kj/mol Glucose phosphorylation with P i is endergonic. ATP hydrolysis to ADP and P i is highly exergonic. ATP hydrolysis coupled to glucose phosphorylation is exergonic. Energy coupling Example: glucose phosphorylation Energy coupling occurs through shared intermediates (P i in this case). figure 1-27b 22
Nucleotides and nucleosides Adenine D-Ribose Nucleoside Nucleotide = Nucleoside-P Nucleoside-diP Nucleoside-triP Adenosine triphosphate (ATP) Hydrolysis of the γ- and β-phosphates is highly exergonic. γ β α (phosphate groups are usually complexed with Mg 2+ ) figures 1-26, 13-12 23
ATP hydrolysis P i inorganic phosphate Factors favoring hydrolysis: 1. Relief of electrostatic repulsion 2. P i is stabilized by resonance 3. Mass action favors hydrolysis (high [H 2 O]) figure 13-11 24
In intact cells, G for ATP hydrolysis is often much more negative than G' º ( 30.5 kj/mol), ranging from 50 to 65 kj/mol. This is because [ATP]/[ADP][Pi] > 1.0 in cells 25
Energy released by hydrolysis of biological phosphate compounds figure 13-19 Hydrolysis of phosphocreatine Phosphocreatine has a high phosphoryl group transfer potential. It can drive the formation of ATP from ADP. figure 13-15 26
ATP can provide energy by group transfer even when there is no net transfer of P Derivation of energy from ATP hydrolysis generally involves covalent participation of ATP in the reaction. Formation of glutamine by condensation of glutamate with NH 3 is endergonic (positive ΔG' º). Formation of γ-glutamyl P by transfer of P from ATP is exergonic (negative ΔG' º). Formation of glutamine by displacement of P from γ-glutamyl P by NH 3 is exergonic (negative ΔG' º). The net coupled reaction is exergonic (negative ΔG' º). figure 13-18 27