ARBYDRATES (SUGARS)
ARBYDRATES: 1. Most Abundant Molecules on Earth: (100 MILLIN METRI TNS f 2 And 2 0 onverted To ellulose and ther Plant Products/Year) 2. FUNTINS: Diet, Energy, Structural, Signalling 3. YDRATE F ARBN 6 12 6
3 Major lasses of arbohydrates: 1. Monosaccharides (simple sugars): single polyhydroxy aldehyde or ketone 2. ligosaccharides: short chains of monosaccharides linked by glycosidic bonds 3. Polysaccharides: long chains (hundreds or thousands) of monosaccharides linked by glycosidic bonds
- - - - - - Glyceraldehyde (aldose) = - - Dihydroxyacetone (ketose)
3,4,5,6,7 carbon simple sugars are found in cells (trioses tetroses, pentoses, hexoses, heptoses) These may be aldoses or ketoses; i.e., glucose is an aldohexose; fructose is a ketohexose; ribose is an aldopentose. - - - - - - - - - - Glucose - - = - - - - - - - - Fructose - - - - - - - - Ribose
All monosaccharides except dihydroxyacetone contain one or more chiral carbons and thus occur as enantiomers (D or L) - - - - D-Glyceraldehyde - - - - L-Glyceraldehyde
When determining if D or L, look at the chiral carbon farthest from the carbonyl carbon. If it has the same configuration as D-glyceraldehyde ( group on the right) then the sugar is a D sugar. If is on the left, the sugar is an L sugar. Most natually occuring sugars are in the D-form.
arbon atoms of a sugar are numbered beginning at the end of the chain nearest the carbonyl group. Epimers are sugars that differ only in the configuration about one carbon atom. 1 - - 2 - - galactose - - 3 - - 4 glucose - - - - - - 5 - - 6 - - - -
Monosaccharides with 5 or more carbons exist in aqueous solution as cyclic structures. These rings will be either 5-membered (resemble furan) or 6-membered (resemble pyran) 2 pyran furan
Pyranose ring formation is a reaction between an aldehyde and an alcohol to form a hemiacetal: (example: glucose) - - - - - 4- - - - - 1 2 3 5 6 2 6 5 4 1 3 2 *If the is on the right in the straight chain Fischer projection, it will be down in the ring
Formation of a hemiacetal creates a new chiral carbon and hence isomers referred to as anomers (differ only in configuration about the anomeric carbon) 2 2 2 α-d-glucopyranose β-d-glucopyranose
Ketones react with alcohols to form hemiketals. In the case of fructose, a furanose ring is formed. - - 1 = 2 - - 3 - - 4 - - - - 5 6 2 5 6 2 5 4 6 4 2 α-d-fructofuranose 3 1 2 3 2 1 2 β-d-fructofuranose
Process of interconversion between alpha, linear and beta forms is called mutarotation Glucose interconverts in solution until equilibruim is reached. ere it exists as 37% alpha and 67% beta. nly a negligible amount is in the linear form.
Because of the -- and -- bond angles, the pyranose and furanose rings are not planar; instead, they can assume one of two puckered conformations: axis axis Boat hair Ring substituents are oriented either axially or equatorially
Reactions of Monosaccharides: 1. an be oxidized by mild oxidizing agents (i.e., ferric or cupric ions). arbonyl goup is oxidized to a carboxylic acid. - - - - - - - - - - D-glucose 2u 2+ 2u + D-gluconic acid (an aldonic acid) - - - - - - - - - - -
2. xidation of the primary alcohol group yields a uronic acid: - - - - - - - - - - D-glucose - - - - - - - - D-glucuronic acid
3. Aldoses can be oxidized at both 1 and 6 to yield aldaric acids: - - - - - - - - - - D-glucose - - - - - - - - - D-glucaric acid
Sorbitol, mannitol (gum sweeteners); glycerol (component of lipids) 4. Aldoses and ketoses can be reduced to yield alditols: - - - - - - - - D-Xylose 2 - - - - - - - - D-Xylitol
ther Biologically Important Monosaccharides: Deoxy- sugars: Where on group has been replaced by a atom, i.e., deoxyribose (found in DNA). Amino sugars: Where the group at 2 is replaced by an amino group. These are found in many oligo- and polysaccharides, i.e., chitin. Sugar esters: Phosphate esters are important metabolic intermediates, i.e., ATP and GTP are phosphorylated on the 5 carbon of the ribose ring.
When two or more sugars are joined together to form polysaccharides they are linked through a glycosidic bond. The anomeric carbon of one sugar reacts with a hydroxyl group on another sugar. This represents the reaction of an alcohol and a hemiacetal to form an acetal.
2 + 2
2 2 6 5 4 3 2 + 1 2 0 2 2 6 5 4 1 3 2
Because the reacting anomeric carbon was in the alpha isomer, and it reacted with the on 4, this is an α,1 4 glycosidic linkage. This is the disaccharide maltose. The anomeric carbon involved in the bond is now non-reducing. 2 1 4 2 0 2
This is an example of a β, 1 4 glycosidic bond. This is cellobiose. (2 glucose residues-like maltose except for the linkage) 2 2
Lactose (galactose-β-1,4-glucose). The principle carbohydrate in milk. Broken down by the enzyme lactase. Lactose intolerance results from a defect in the ability to produce lactase. 2 2
Sucrose (glucose α-1,2-fructose). ommon table sugar. A primary product of photosynthesis. leaved by invertase. Sucrose is a NN-REDUING sugar (both anomeric carbons linked by a glycosidic bond). 2 1 2 1 2 3 4 5 2 6
Naming sugars: 2 -α-d-glucopyranosyl-(1 2 4)-α-D-glucopyranose 1. Nonreducing end to the left. 2. preceeds name of the 1st sugar 3. onfiguration of anomeric to the left given next 4. Pyranosyl or Furanosyl given next 5. Type of bond indicated next
Structurally important polysaccharides: ellulose: Accounts for 1/2 of the carbon in the biosphere. A linear polymer of glucose linked β-1,4. β-linked glucose forms straight extended chains; several chains -bond to form fibers of great tensile strength (cotton, wood). Animals cannot digest cellulose. hitin: Principle structural component of the exoskeletons of invertebrates. A homopolymer of β-1,4 linked N-acetyl-D-glucosamine. 2 N3
Storage Polysaccharides: Starch: Synthesized by plants in two forms: amylose and amylopectin. Both are glucose polymers linked α-1,4. Amylose is a linear molecule; amylopectin is a branched molecule with α-1,6 branch points every 24-30 glucose residues. Glycogen: The storage polysaccharide of animals. Structure is identical to amylopectin except that it is more highly branched, with branch points every 8-12 glucose residues.
Amylose (α-1,4) Amylopectin (α-1,4 with α-1,6 branch points )
Dextrans: onsist of a branched polymer of D-glucose units. The main chain linkage is α-1,6 but 1,2; 1,3; or 1,4 branch points can occur. Dextrans are a main component of dental plaque. Dextrans are used in research as a support medium for column chromatography. Glycosaminoglycans: onsist of alternating units of uronic acids and hexosamino residues. Examples are hyaluronic acid (lubricant in joints and a component of cartilage and tendons) and heparin (anticoagulant). These are often linked to extracellular proteins to form proteoglycans.
Many Proteins (Glycoproteins) and Lipids (Glycolipids) have complex arrays of covalently attached oligosaccharides which serve a variety of functions: ydrophilicity of sugars alters the polarity and thus the solubility ligosaccharide chains may affect the folding of peptides and influence tertiary stucture Bulkiness of sugar chains may protect from degradation (R target for degradation). Sugars provide recognition sequences for receptor-ligand interactions Sugars may target molecules for transport to a particular site in/out of the cell