Carbohydrates What are they? Formula = (C 2 O) n where n > 3 Also called sugar Major biomolecule in body What do cells do with carbs? Oxidize them for energy Store them to oxidize later for energy Use them as components of larger molecules Sugars attached to proteins and lipids Where do carbs come from? 1. Plants fix CO 2 from air to synthesize sugars 6CO 2 + 6 2 O ---> C 6 12 O 6 + 6O 2 2. Other organisms eat sugar or make sugar from smaller organic compounds
Carbohydrates What carbs are found in cells? 1. Monosaccharides single sugar C 2 O O O O Glc O O 2. Oligosaccharides (disaccharides most abundant) glycosidic bonds often linked to proteins - glycoproteins or to lipids - glycolipids 3. Polysaccharides large polymers structural - cellulose (linear) storage - starch (branched) C 2 O O O O C 2 O O O O Glc O O C 2 O O O O O Glc Maltose, a disaccharide O C 2 O O O O Glc Glc n O α-amylose, component of starch
Monosaccharides An aldehyde or ketone with > 1 hydroxyl group Colorless, crystalline, usually sweet One carbonyl group aldose ketose
Monosaccharides Aldoses Most common in nature - hexoses (6-carbon sugars) Components of nucleotides and nucleic acids Six carbons D isomers Five carbons D isomers
Monosaccharides Other Aldoses
Monosaccharides Ketoses
Monosaccharides Some exceptions to rule on D isomers only L-Arabinose and sugars in glycoconjugates
Monosaccharides Epimers - Two sugars that differ only in configuration around one carbon atom
Monosaccharides Why are these particular sugars utilized? Evolution Glucose forms especially stable structures Monosaccharides can spontaneously cyclize C1 not chiral in open chain form, but it is in pyranose Stereoisomers differing only at C1 are called α and β anomers C1 is anomeric carbon - carbon that used to be the carbonyl C
Monosaccharides
Sugars can be reducing agents Monosaccharides Fehling s reaction Cuprous ion (Cu + ) forms red precipitate Measure ppt. - estimate [glc] reduction oxidization Measure blood glc by measuring amount of 2 O 2
Glucose meters Captain glucose and meter boy
Oligosaccharides & Polysaccharides Glycosidic bonds ydroxyl on anomeric carbon is reactive and can condense with an alcohol to form a glycosidic bond Reducing end (hemiacetal) - still has free anomeric carbon Nonreducing end - no anomeric carbon
Disaccharides Occurs naturally only in milk Table sugar Formed by plants, not us No anomeric carbon Glycosidases hydrolyze glycosidic bonds, specific for particular linkages Lactose intolerance: lack lactase, the enzyme that can break down lactose
Polysaccharides omopolysaccharides - starch, glycogen (storage); cellulose, chitin (structure) eteropolysaccharides - bacterial cell membrane, extracellular space in animal cells
2 types of glc polymers Amylose - linear, α1 4 Polysaccharides - Storage Amylopectin - highly branched, α1 4 glycosidic, α1 6 branch points
Polysaccharides - Storage Starch - α1 4 glycosidic, α1 6 branch points; 1 branch/24-30 residues Glycogen - α1 4 glycosidic, α1 6 branch points, more extensively branched: 1 branch/8-12 residues nonreducing ends - glc can be mobilized quickly Lots in liver, some in muscle Liver stores glycogen equivalent to glc concentration of 400 mm (blood glc ~5 mm)
Polysaccharides - - Structure Cellulose fibrous, tough, water insoluble, plant cell wall, humans can t digest linear, unbranched, lots of glc linked by (β1 4) glycosidic linkages Chitin tough, water insoluble, component of hard exoskeleton (insect, lobster, crab), indigestible by humans linear, uses modified sugars (N-acetyl-D-glucosamine) with (β1 4) linkages
Starch - floppy left-handed helix Polysaccharides- structure Cellulose - rigid chain
Polysaccharides Glycogen, starch ingested in diet hydrolyzed by α-amylases (in saliva and intestinal secretions) that break α1 4 linkages Most animals cannot use cellulose as fuel because no enzyme to hydrolyze β1 4 linkages Termites readily digest cellulose (wood) because their intestinal tract harbors microorganism that secretes cellulase (breaks β1 4 linkages) Only vertebrates able to use cellulose as food are cattle and other ruminants (sheep, goats, camel, giraffes) that have extra stomach compartment (rumen) that has bacteria/protists that secrete cellulase