Carbohydrates - Monosaccharides

Carbohydrates A. Definition B. Roles C. Monosaccharides-Chemistry 1. Chirality a. One or more asymmetric carbons b. Linear and ring forms 2. Derivatives: the chemistry of carbohydrates a. Oxidation b. Reduction c. d. Ester formation Amino sugars i. ii. i. ii. C1 C6 C1/C2 Other carbons 3. Polymerization a. The Glycosidic Bond b. Non-covalent bonds in macro-molecular structure D. Oligosaccharides 1. 2. 3. 4. Glycoproteins & glycolipids O-linked N-linked Sequence determination-ABO E. Polysaccharides 1. Polymers of glucose 2. Polymers of disaccharides Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates ① • Oxidation – C1 – C6 ② ③ ④ • Reduction – C1/C2 – Other carbons • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure 1 Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” • Oxidation of aldehyde/ketone to acid (2e– loss); reaction from the C1 of aldoses – named as “onic” acids (“onate” for conjugate base) – these sugars can reduce Cu2+ to Cu+ (Fehling’s/Benedict’s test) or Ag+ to Ag0 (Tollens’ test) – Example: gluconic acid Blue This reaction is called Benedict’s test H2O Red (ppt) The copper is reduced and the sugar is oxidized. So, the sugar is called a Reducing Sugar. Reducing sugars have a free anomeric carbon. Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” Colorimetric Glucose Analysis d-D-Gluconolactone b-D-Glucose CH2OH CH2OH O OH OH HO OH O Glucose oxidase OH HO O2 O OH H2O2 NH NH2 OCH3 OCH3 2 H2 O Peroxidase OCH3 OCH3 NH NH2 Oxidized o-dianisidine (bright orange) Reduced o-dianisidine (faint orange) • Enzymatic methods are used to quantify reducing sugars such as glucose. – The enzyme glucose oxidase catalyzes the conversion of glucose to glucono-d-lactone and hydrogen peroxide. – Hydrogen peroxide oxidizes organic molecules into highly colored compounds. – Concentrations of such compounds is measured colorimetrically. • Electrochemical detection is used in portable glucose sensors. 2 Carbohydrates ① Monosaccharides: Derivatives Oxidation: These make “sugar acids” or “acid sugars” Oxidation of alcohol to acid (4e– loss) – Reaction for C6 groups; like C1 oxidation, normally on aldoses – Named as uronic acids (“uronate” for conjugate base) – Many L-sugars are Uronic acids – Examples: D-glucuronic acid, L-iduronic acid CHO CH2OH D-Idose L-Idose L-Iduronic acid Carbohydrates ② Monosaccharides: Derivatives Reduction: These make “sugar sugar alcohols alcohols” or “deoxysugars” deoxysugars Reduction of aldehyde/ketone to alcohol (2e– gain) – Only carbon not already an alcohol is the anomeric carbon (C1/C2) – Named as sugar “ol” or “itol” – Examples: glycerol, mannitol, glucitol (sorbitol) Reduction of alcohol carbon to methyl/methylene (2e– gain) – Can react at any except the anomeric carbon – Named as “x-deoxy” sugar with x being the reduced carbon – Many are L-sugars, and have specific trivial names – Examples: 2-deoxyribose, L-Fucose (Fuc) and L-Rhamnose (Rha) Fuc is also 6-deoxy L-Gal Rha is also 6-deoxy L-Man OH OH b-D-Gal Notice that for L-sugars the b-anomer is down and a-anomer is up b-D-Man 3 Carbohydrates ② Monosaccharides: Derivatives sugar alcohols Reduction: These make “sugar alcohols” or “deoxysugars” deoxysugars Reduction of aldehyde/ketone to alcohol (2e– gain) – Only carbon not already an alcohol is the anomeric carbon (C1/C2) – Named as sugar “ol” or “itol” – Examples: glycerol, mannitol, glucitol (sorbitol) Reduction of alcohol carbon to methyl/methylene (2e– gain) – Can react at any except the anomeric carbon – Named as “x-deoxy” sugar with x being the reduced carbon – Many are L-sugars, and have specific trivial names – Examples: 2-deoxyribose, L-Fucose (Fuc) and L-Rhamnose (Rha) Fuc is also 6-deoxy L-Gal Rha is also 6-deoxy L-Man OH OH b-D-Gal Notice that for L-sugars the b-anomer is down and a-anomer is up b-D-Man Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates ① • Oxidation – C1 – C6 ② -onic -uronic • Reduction -tol – C1/C2 – Other carbons deoxy- ③ ④ • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure 4 Carbohydrates ③ Monosaccharides: Derivatives Esters: condensation of alcohol (sugar) and an acid • Most important sugar esters use a phosphoric acid • These are called phospho-sugars or sugar phosphates • Examples: nucleotides, Glc 6-P, Fru 1,6-P2, glycerol 3-phosphate D- a-D-Glucose 6phosphate b-D-Fructose 1,6-bisphosphate Carbohydrates ④ Monosaccharides: Derivatives Amines: condensation of alcohol (sugar) and ammonia • • • These are called amino-sugars In biology, its ALWAYS at C2 Often, the amino group will be further modified by an acetyl group (CH3CO–) making an amide • Named as sugar“amine” and when acetylated, the “N-acetyl” comes first. When abbreviation used “NAc” it comes after. • Examples: glucosamine, N-acetyl-glucosamine (GlcNAc) (GlcNAc) 5 Carbohydrates Monosaccharides: Derivatives Important Hexose Derivatives ③ ④ ④ ①&④ ② R=pyruvyl ①&④ ① Carbohydrates Monosaccharides: Derivatives Muramic acid ①&④ Sialic Acid (Sia) (aka NeuNAc) Combined acid (C1) + amine (C2) =O Combined acid (C3) + amine (C2) Pyruvic acid = H3C-C-COO– 6 Carbohydrates Monosaccharides: Chemistry • Chemical Features: – Chirality • One or more asymmetric carbons • Linear and ring forms – Derivatives: the chemistry of carbohydrates • Oxidation – C1 – C6 • Reduction – C1/C2 – Other carbons • Ester formation • Amino sugars – Polymerization • The Glycosidic Bond • Non-covalent bonds in macro-molecular structure Carbohydrates The Glycosidic Bond 7 Carbohydrates Hemiacetals and Hemiketals are reactive to alcohols in condensation reactions • Hemiacetals condense with alcohols to form Acetals. • Hemiketals condense with alcohols to form Ketals. • These reactions form the basis of the GLYCOSIDIC BOND. MeOH : – Glc • Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon (the hemiacetal/hemiketal) and a hydroxyl carbon (the other sugar). The Glycosidic Bond • The glycosidic bond between sugars is stable and does not readily hydrolyze. – The anomeric carbon involved in the glycosidic linkage is fixed in its chirality and is therefore nonreducing. – The second monomer, with its unreacted hemiacetal, is still reducing. (b-D-adenine riboside) Carbohydrates Hemiacetals and Hemiketals are reactive to alcohols in CONDENSATION reactions • Hemiacetals condense with alcohols to form Acetals. • Hemiketals condense with alcohols to form Ketals. • These reactions form the basis of the GLYCOSIDIC BOND. • Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon (the hemiacetal/hemiketal) and a hydroxyl carbon (the other sugar). The Glycosidic Bond • The glycosidic bond between sugars is stable and does not readily hydrolyze. – The anomeric carbon involved in the glycosidic linkage is fixed in its chirality and is therefore nonreducing. – The second monomer, with its unreacted hemiacetal, is still reducing. (b-D-adenineriboside) 8 Carbohydrates Disaccharides Carbohydrates Disaccharides: • Disaccharides can be named by the organization and linkage or a common name. – The disaccharide formed upon condensation of two glucose molecules via a 1 à 4 bond is described as α-D-glucopyranosyl-(1à4)-D-glucopyranose. – The common name for this disaccharide is maltose. Still undergoing If you only know the sugar and the linkage mutarotation Non-reducing à reducing If you know the exact structure of the sugar As we make sugar-polymers, the convention is to have the non-reducing sugar to the LEFT and the reducing end at the RIGHT. 9 Carbohydrates Disaccharides: Here is likely the first disaccharide you encountered in your life: Lactose. Non-reducing à reducing Still undergoing mutarotation Carbohydrates Disaccharides: Here is likely the disaccharide you ingest the most: Sucrose. b Notice that these are drawn upside down Trehalose • Nonreducing Disaccharides • • Two sugar molecules can be also joined in a glycosidic bond between two anomeric carbons. The product has two acetal groups and no hemiacetals or hemiketals. There are no reducing ends; this is a nonreducing sugar. Review of Glycosidic Bonds 10 Carbohydrates Disaccharides: Here is likely the disaccharide you ingest the most: Sucrose. b Notice that these are drawn upside down Trehalose • Nonreducing Disaccharides • • Two sugar molecules can be also joined in a glycosidic bond between two anomeric carbons. The product has two acetal groups and no hemiacetals or hemiketals. There are no reducing ends; this is a nonreducing sugar. Review of Glycosidic Bonds Carbohydrates Oligosaccharides 11 Carbohydrates Oligosaccharides: Glycoconjugates: Glycolipids Here is an oligosaccharide that we encountered with sphingoglycolipids, or for short: Glycolipids. GM1 = Gal-b(1à3)GalNAc-b(1à4)Gal-b(1à4)Glc-b(1àCeramide) Sia-b(2à3) Dysfunction in Ganglioside Recycling Leads to a Variety of Medical Disorders 12 Carbohydrates Oligosaccharides: Glycoconjugates: Glycoprotein When you attach an oligosaccharide to a protein: Glycoproteins. • A protein with small oligosaccharides attached: – Carbohydrate is attached via its anomeric carbon to amino acids on the protein. • Common connections occur at Ser, Thr, and Asn. – About half of mammalian proteins are glycoproteins. – Generally, bacteria do not glycosylate their proteins. – Carbohydrates play role in protein-protein recognition. – Viral proteins are heavily glycosylated; this helps evade the immune system. • Proteins whose role is to bind specific carbohydrates/oligosaccharides = Lectins (or selectins) – Lectins important for many biological functions • Recruitment of leukocytes to sites of inflammation • Sperm-egg recognition • Virus-target cell interaction • Attachment of flora (microbiome) in gut • Nervous system development • Serum-protein turnover (sialic acid) • Targeting proteins to lysosomes for degradation (Man) How are these sugars attached? Carbohydrates Oligosaccharides: TWO classes attached to proteins, O-linked and N-linked • First, these polymers, unlike any others we have studies, are BRANCHED • Second, like glycolipids, O-linked sugars are attached via an Oglycosidic bond – Use a Ser or Thr in the sequence (recall glycophorin) – Usually smaller than N-linked – Synthesized one at a time by specific glycosyltransferases (specific for sugar, linkage, and chirality) • Third, N-linked sugars are attached via an N-glycosidic bond – Use an Asn – The Asn residues are within a 3 AA sequence context: NXS/T – All the same at the core Let’s look more closely at O-linked oligosaccharide synthesis 13