Carbohydrates Lecture Notes PDF

Carbohydrates Dr Rich Boden BIOL131(Z): Cells: The Building Blocks of Life (2024/25) @bodenlab  biomacromolecules large polymeric molecules: polysaccharides nucleic acids proteins lipids aren’t quite polymers bu...

Carbohydrates Dr Rich Boden BIOL131(Z): Cells: The Building Blocks of Life (2024/25) @bodenlab  biomacromolecules large polymeric molecules: polysaccharides nucleic acids proteins lipids aren’t quite polymers but they form large macromolecular structures viz. membranes.  polymers many building blocks (monomers) linked by covalent bonds to form chains. homopolymers have identical monomers heteropolymers have different monomers polymers can be branched or linear biological molecules are formed by condensation reactions: A-OH + B-OH → AB + H2O …and thus are degraded by hydrolysis reactions (which require enzymes, heat, acid/base etc to catalyse the reaction): AB + H2O → A-OH + B-OH  carbohydrates metabolically consumed as energy sources in catabolism and as carbon skeletons in anabolism (see later lectures). structural roles in some Eukarya (the Viridiplantae, the Fungi, some Metazoa) and many Bacteria and Archaea. involved in cell recognition – important in immunology and in theories like tissue rejection – either directly as polysaccharides alone or as part of glycoproteins and glycolipids. cf. A, B, AB and O blood type antigens in Homo sapiens subsp. sapiens. important in cryoprotection (Gr. neut. n. κρῠ́ος (krúos), cold) and xeroprotection (Gr. masc. adj. ξηρός (xērós), dry) – compatible solutes. Lepidoselaginella lepidophylla (previously “Selaginella subgen. Gymnogynym sect. Lepidophyllae lepidophylla”) enters a dormant state by generating high concentrations of trehalose in the cells which protect the contents during desiccation, during which the leaves curl up. On adding water, they unfurl over a c. 3h time period. Eukarya > Viridiplantae > Streptophyta > Lycopodiopsida > Selaginellales > Sellaginellaceae > Gymnogynoideae trehalose (disaccharide of two D-(+)-glucose monomers linked by (1→1) glycosidic bond) relative Substance sweetness Sweetness What it is relative to sucrose lactose 0.14 disaccharide from milk maltose 0.33 disaccharide from malting barley D-(+)-glucose 0.74 hexose from blood xylitol 1.00 sugar alcohol (a tetrol) sucrose 1.00 (ref.) disaccharide from sugar cane/sugar beet D-(-)-fructose 1.17 hexose from fruit Na cyclamate 26 sulfamate of cyclohexane steviol glycosides (mix) 40-300 glycosides of steviol (a diterpene) aspartame 180-250 dipeptide (methyl-L-aspartate and L-phenylalanine) Na saccharin 300-675 sulfimide of benzoic acid thaumatin 2,000 protein lugduname >230,000 modified from guanine bases.  sugars - groupings monosaccharides can be split by size: 3C = trioses 4C = tetroses 5C = pentoses 6C = hexoses 7C = heptoses (etc) AND by function – aldoses (contain aldehyde C=O groups) and ketoses (contain ketone C=O) groups. can put them together and say “aldohexoses” for example. aldoses are reducing sugars – not all polymers of aldoses are reducing – depends if the aldehyde-derived moiety is still exposed or not.  reducing sugars reduction is gain of electrons – a reducing sugar therefore reduces something else and becomes oxidised (aldehyde to carboxylate) e.g. Benedict’s qualitative reagent contains Cu(II) ions at high pH – aldehyde groups in reducing sugars reduce this to red cuprous oxide oxidised = VIOLET; reduced = RED R-CHO + 2Cu2+ + 5OH- → R-COO- + Cu2O + 3H2O Same idea used to be used for making silver mirrors – still used for applying silver reflective surfaces on laboratory glassware – using Tollen’s reagent (contains [Ag(NH3)2]+ complex, which reducing sugars like D-(+)-glucose will reduce to elementary silver (AgO)). Same is used in histology e.g. silver-staining of glycoproteins. reducing sugars will reduce some dyes etc to make them change colour.  sugars – aldoses and ketose examples aldoses = have aldehyde moieties → reducing sugars 3C = aldotrioses e.g. D-(+)-glyceraldehyde 4C = aldotetroses e.g. D-erythrose, D-threose 5C = aldopentoses e.g. D-arabinose, D-xylose, D-ribose 6C = aldohexoses e.g. D-(+)-glucose, D-galactose ketoses = have ketone moieties → non-reducing sugars 3C = ketotrioses e.g. dihydroxyacetone 4C = ketotetroses e.g. D-erythrulose 5C = ketopentoses e.g. D-ribulose, D-xylulose 6C = ketohexoses e.g. D-(-)-fructose, D-sorbose D-(+)-glucose versus D-(-)-fructose terminal carbonyl = aldehyde! non-terminal carbonyl = ketone! D-(+)-glucose D-(-)-fructose 4 asymmetric carbons (*) 3 asymmetric carbons (*) aldohexose ketohexose  hexoses in solution don’t stay as chains! β-D-(+)-glucose D-(+)-glucose α-D-(+)-glucose loop forms a hemiacetal 0.25 % of total in solution loop forms a hemiacetal 49.875 % of total in 49.875 % of total in solution solution fructose does the same but with hemiketal rings – these are not reducing unless at high pH. glycosidic bonds α-D-(+)-glucose – two molecules 1 4 glycosidic bonds = ether linkages maltose – has a (1→4) glycosidic bond hemiacetal is still present so still a reducing sugar – trehalose has the same monomers but a 1→1 glycosidic bond and is NOT a reducing sugar – can you see why? glycosides and glucosides: biological safety caps! salicin Found in Salix alba a glucoside of salicylic acid [L. fem. n. salix, a willow tree; L. and D-(+)-glucose fem. adj. alba, dull white in colour] arbutin Found in Arbutus spp. a glucoside of [L. neut. n. arbutum, a strawberry; hydroquinone and D-(+)- N.L. fem. n. arbutus, plant with glucose strawberry-like fruit] amygdalin a glycoside of Found in Prunus subgen. mandelonitrile and Amygdalus spp. gentiobiose – former [L. fem. n. prunus, a plum tree; L. hydrolyses into cyanide fem. n. amygdalus, an almond tree] and benzaldehyde  polymers of sugars are saccharides these can have defined length – disaccharides, trisaccharides etc. undefined length but are short – oligosaccharides undefined length but are long – polysaccharides work the same as disaccharides just adding more monomers by glycosidic bonds to form a chain. linear homopolymers e.g. amylose, chitin, cellulose branched homopolymers e.g. amylopectin, glycogen, Floridean starch linear heteropolymers e.g. agarose storage polysaccharides amylose D-(+)-glucose with α-(1→4) glycosidic bonds. Linear. amylopectin chloroplasts and D-(+)-glucose with α-(1→4) amyloplasts of the glycosidic bonds. Clusters Viridiplantae of branches from α-(1→6) glycosidic bonds. Floridean starch D-(+)-glucose with α-(1→4) glycosidic bonds. cytoplasm of the Infrequent branches from α- Rhodophyta (1→6) glycosidic bonds. storage polysaccharides glycogen D-(+)-glucose with α-(1→4) glycosidic bonds. Frequent cytoplasm of the Metazoa branches from α-(1→6) and in some Bacteria glycosidic bonds. Protein in the middle of the granule is glycogenin, the primer that initiates formation (technically makes glycogen a glycoprotein!). structural polysaccharides chitin N-acetyl-D-glucosamine with β- (1→4) glycosidic bonds. exoskeletons of the Insecta Strong hydrogen between adjacent and Crustacea, beaks and polymers AND between monomers gives pens of Cephalopoda and structural strength. Hydrophobic cell walls of the Fungi interactions between polymers adds more strength. cellulose D-(+)-glucose with β-(1→4) cell walls of the glycosidic bonds. Viridiplantae and Oomycota Not-as-strong hydrogen between adjacent polymers AND between monomers gives structural strength.  you should be able to state if a sugar is an e.g. aldo[size]ose or keto[size]ose based on a diagram. explain why a reducing sugar can reduce things and give monosaccharide and polysaccharide examples. describe the formation of glycosidic bonds of any given numbering using hexose sugars. describe the role of glycosides and glucosides in acting as ‘safety caps’ on reactive metabolites. compare and contrast the storage polysaccharides of the Viridiplantae, Rhodophyta and Metazoa in terms of structure and properties. compare and contrast structural polysaccharides and what gives them their rigidity. describe the biological roles of monosaccharides in metabolism and as cryo/xeroprotectants.  self-test: 1) what are the key properties of an aldopentose? 2) label two D-(+)-glucose molecules to show a (1→3) glycosidic bond. 3) what are the three storage polysaccharides found in the Viridiplantae and the Rhodophyta and what are their differential structures? 4) draw the repeating units of chitin to explain how the structure gives the rigidity of the overall polymer.  self-test: 1) what are the key properties of an aldopentose? 5 carbon atoms and a terminal aldehyde moiety. 1) label two D-(+)-glucose molecules to show a (1→3) glycosidic bond. 3) what are the three storage polysaccharides found in the Viridiplantae and the Rhodophyta and what are their differential structures? amylose – no branches, D-(+)-glucose monomers linked by α-(1→4) glycosidic bonds. Hard for digestive enzymes to attack as only one start and end. amylopectin – branches in clusters – has one start and MANY ends so easy for digestive enzymes to attack. D-(+)-glucose monomers linked by α-(1→4) glycosidic bonds with branches initiating at α-(1→6) glycosidic bonds. Overall branching is less frequent that in Floridean starch, which has the same overall structure just less branching and no branch-clustering. [cont]  self-test: 4) draw the repeating units of chitin to explain how the structure gives the rigidity of the overall polymer. Hydrogen bonds (dotted lines) within each polymer keep the ‘rows’ of the chitin sheet rigid and straight. Hydrogen bonds (dotted lines) between each polymer keep ‘columns’ of the sheet rigid and straight, as do hydrophobic interactions (dashed lines).