CARBOHYDRATES
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Transcript of CARBOHYDRATES
CARBOHYDRATESCARBOHYDRATES
Carbohydrates - the most abundant class of biological molecules on Earth
Empirical formula - (CH2O)n, where n ≥ 3 -consist of three main elements: carbon, oxygen and hydrogen; -have aldehyde or ketone functional groups and multiple hydroxyl groups; -can also contain nitrogen, sulfur and phosphorus
Carbohydrates - polyhydroxyaldehydes or polyhydroxyketones or substances that yield these compounds when hydrolyzed
The simplest carbohydrates
•Monosaccharides - one monomeric unit •Disaccharides – 2 monosaccharides (alike or
different) Monosaccharides and disaccharides have sweet taste.
They are called sugars•Oligosaccharides - ~2-6 monosaccharides•Polysaccharides - > 6 monosaccharides (glycogen
in animals; starch, cellulose in plants) - Homopolysaccharides contain only one type of
monosaccharides - Heteropolysaccharides consist of more than one type of monosaccharides
•Glycoconjugates - linked to proteins or lipids
Classification of carbohydrates
Monosaccharides - the simplest carbohydrates with a single aldehyde or ketone unit and multiple hydroxyl groupsClassification of monosaccharides
-trioses – the smallest monosaccharide (C3H6O3); -tetroses (C4H8O4); -pentoses (C5H10O5); -hexoses (C6H12O6); -heptoses (C7H14O7)
1. According to the amount of carbon atoms (only monosaccharides of three to seven carbons are commonly found in the biosphere).
Monosaccharides
Aldoses - polyhydroxy aldehydesKetoses - polyhydroxy ketones
Oxidized carbon: aldoses - C-1, ketoses - usually C-2
2. Depending on whether an aldehyde or ketone group is present monosaccharides can be aldoses and ketoses
aldose ketose
1
2
aldose ketose
1
2
3. Depending on the spatial orientation of the –H and –OH groups attached to the carbon atom adjacent to the terminal primary alcohol group monosaccharides can exist as a D- or L- isomers D-isomer - OH group is written to the right of this carbon in the projection formula
L and D are mirror image of each other - enantiomers
L-isomer – OH group is written to the left of this carbon in the projection formula
4. Depending on whether the monosaccharide rotates the plane of polarized light to the right (+) or to the left (-) monosaccharides are divided into (+) and (-) isomers or dextrorotatory and levorotatory isomers
Ordinary light vibrating in all possible planes.
Plane-polarized light vibrating in single plane
5. Monosaccharides can exist in cyclic structure Depending on whether the cyclic structure of monosaccharide is related to that of furan or pyran monosaccharide can be classified asfuranose or pyranose
6. Depending on the orientation of –H and –OH groups about specific chiral carbon atom in the cyclic form monosaccharides can have alpha () or beta () configuration
Importance of carbohydrates1. Energetic role – carbohydrates are very effective energy-yielding nutrients 2. Structural role - carbohydrates are building material (cell wall in bacteria and plants; connective tissue in animals) Cellulose forms fibers of wood, cotton clothing, paper 3. Pentoses – components of nucleic acids4. Glycoproteins – receptors, cell recognition. Many important macromolecules in living systems are glycoproteins
Fisher projection formulasAldehyde C-1 is drawn at the top of a Fischer projectionThe –H and –OH groups are written to the right or to the left (D and L isomers)Any two monosaccharides that differ only in the configuration around a single carbon atom are called epimers
Fisher projection formulas of glucose
D-glucose – OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group L-glucose - OH group is written to the left of the carbon atom adjacent to the terminal primary alcohol group L and D glucoses are enantiomers (differ at every chiral carbon and are mirror image of each other)
Family of D-aldoses D-Sugars predominate in nature
•Aldoses shown in blue are most important in chemistry and biochemistry
D-confi-guration - OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group
•Epimers - sugars that differ at only one of several chiral centers (e.g., D-galactose is an epimer of D-glucose at C-4)
Family of D-ketoses
D-confi-guration - OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group
Glucose, Galactose and FructoseGlucose, galactose and fructose - the most important carbohydrates in nature
Glucose (grape sugar, dextrose) Aldohexose Exist in free state in animal and plant tissue Component of sucrose, maltose, lactose, starch, cellulose, glycogen Key sugar for body (blood sugar) 3.3-5.5 mmol/l in blood plasma Oxidation in body – important source of energy
Galactose Aldohexose Component of lactose, pectin, glycolipids and glycoproteins playing the structural role (cell membranes, connective tissue) Abundant in milk Can be converted to glucose in liverFructose (levulose) Ketohexose Constituent of sucrose Abundant in fruit juices and honey Very sweet (about twice as sweet as glucose) Can be converted to glucose in liver
•(a) Six-membered sugar ring is a “pyranose”
•(b) Five-membered sugar ring is a “furanose”
Cyclic structure of
mono-saccharides
Hemiacetals and acetalsCompounds derived from
aldehydes that contain an alkoxy and a hydroxy group on the same carbon atom are called hemiacetals
Compounds derived from aldehydes that contain two alkoxy groups on the same carbon atom are called acetals
Hemiacetals are unstable compoundsAcitals are stable in alkaline solutions but unstable in acidic solutions
MutarotationIn solution aldehyde group reacts with hydroxyl group attached to
carbon 5 and cyclic form is formed D-glucose gives two cyclic forms - -D-glucopyranose and -D-gluco-pyranose (different orientation of –H and –OH groups about carbon 1)There is equilibrium between open chain, -D-glucopyranose (36 %) and -D-glucopyranose (64 %) in solution (they can be interconverted) Process of interconversion is called mutarotationAnomers - two cyclic isomers differ only in their stereo arrangement about the carbon involved in mutarotation
Cyclic structures of monosaccharides are intramolecular hemiacetals
When monosaccharide hemiacetal reacts with an alcohol the product is an acetal
Acetal structure is called glycoside
When -D-glucopyranose react with alcohol (CH3OH) two optically active isomers (glycosides) are formed – methyl -D-glucopyranoside and methyl -D-glucopyranoside
Methyl -D-glucopyranoside and methyl -D-glucopyranoside are acetals (stable)
All carbohydrates other than monosaccharides are glycosides
Structure of Galactose and FructoseGalactose is epimer of glucose (differ from each other at
only one of several chiral centers) Galactose: aldohexose; exists in open chain and two cyclic pyranose forms
Fructose: ketohexose; exists in open chain and cyclic pyranose forms
PentosesThe most important pentoses are ribose and deoxyribose (constituents of nucleic acids)
Ribulose - important ketopentose-precursor in synthesis of ribose in organism -captures carbon dioxide in photosynthesis
DisaccharidesDisaccharides are carbohydrates composed of two monosaccharides residues united by glycosidic linkage
Glucose is linked via its 1-st carbon atom hydroxyl group to the hydrohyl group on C4 of the second glucose by -1,4-glycosidic bond
Disaccharides contain acetal structure and some also contain a hemiacetal structure
Maltose
Enzyme maltase hydrolyses maltose into two glucose molecules
LactoseConsists of -D-galactose and -D-glucose joined by -1,4-glycosidic bond
Lactose (milk sugar) - the main sugar of milk In the intestine it is decomposed to galactose and glucose by the enzyme lactase
Lactose intolerance (hypolactasia) - deficiency of the enzyme lactase, which cleaves lactose into glucose and galactoseMicroorganisms in the colon ferment undigested lactose to lactic acid generating methane (CH4) and hydrogen gas (H2) Symptoms: gut distention; annoying problem of flatulenceUndigested lactose and lactic acid are osmotically active and draws water into the intestine resulting in diarrheaThe gas and diarrhea hinder the absorption of other nutrients (fats and proteins) Treatment:
- to avoid the products containing lactose; - the enzyme lactase can be ingested
Intolerance to MilkMany people are unable to metabolize the milk sugar lactose and experience gastro-intestinal disturbances if they drink milk
Sucrose
Sucrose consists of -D-glucose and -D-fructose joined by -1,2-glycosidic bond Sucrose (table sugar)Abundent in sugar cane and sugar beets In the intestine it is decomposed to fructose and glucose by the enzyme sucrase
CellobioseConsists of two -D-glucose joined by -1,4-glycosidic bond Cellobiose is the component of celluloseThere is no enzyme in the human intestin to split -1,4-glycosidic bond
Reactions of monosaccharidesOxidati
onAldehyde group in monosaccharides is oxidized to monocarboxylic acid (suffix –onic) by mild oxidizing agent (bromine water)The stronger oxidizing agent, nitric acid, oxidizes both carbon one and carbon six to form dicarboxylic acid (suffix –aric)
ReductionMonosaccharides are reduced to polyhydroxy alcohols by reducing agents such as H2/Pt or sodium amalgam (Na(Hg))
Glucose is reduced to glucitol (sorbitol), galactose - to galactitol, mannose - to mannitol To name the alcohol the suffix –itol have to be added
Redox tests for carbohydratesAldehydes can be oxidized by Ag+ and Cu2+ (Ag+ and Cu2+ are reduced) Sugars containing aldehyde group (glucose, galactose) reduce Ag+ and Cu2+ - reducing sugarsTollens, Fehling, Benedict tests can be used to detect reducing sugars
Tollens test
Fehling test
A carbohydrate need not to have a free aldehyde group to be a reducing sugarA hemiacetal structure also reactDisaccharides maltose and lactose have the hemiacetal structures and are therefore reducing sugars In alkaline conditions the ring open to form aldehyde group
Sucrose does not have hemiacetal structure and therefore it is not a reducing sugar
Color reactions (Fehling, Benedict) based on the reductive properties of sugars are used to monitor the glucose concentration in blood and urine
Ketose testSeliwanow’s reagent (resorcimol in HCl) is used to detect the ketoses (fructose)Seliwanow’s reagent produces a red color within 90 sec for a ketose
Polysaccharides
•Homopolysaccharides - contain only one type of monosaccharide
•Heteropolysaccharides - contain residues of more than one type of monosaccharide
Storage polysaccharides – depot of the energy molecule, glucose Structural polysaccharides – provide a protective wall or lubricative coating to cells
Starch and Glycogen
•Glycogen is present primarily in liver and muscles
•Starch and glycogen are stored in the cell in cytoplasmic packages called granules
Storage polysaccharides
•D-Glucose is stored intracellularly in polymeric forms•Plants and fungi – starch. Animals – glycogen •Starch is especially abundant in potatoes, corn and wheat
Starch is a mixture of amylose (unbranched) and amylopectin (branched)
Starch
Amylose - a linear unbranched polymer of D-glucose units linked by -1,4-glycosidic bondMolecular weight from several thousands to 500000
Enzyme -amylase cleaves -1,4-glycosidic bonds
Amylopectin: (1) a main backbone composed of glucose units
linked by -1,4-glycosidic bond (like amylose);
(2) branches connected to the backbone via -1,6-glycosidic bonds (about every 25 glucose residues – 1 branch)
Molecular structures of amylose and amylo-pectin
GlycogenDepot of glucose in animal cellsPresent in liver (10 % of the wet weight) and muscles (about 1 %)
The structure is identical to amylopectin but has more numerous -1,6-glycosidic branches and much higher molecular weight (several millions)
Cellulose
Important in textile and paper industry Unbranched chain Chains of cellulose can associate into bundles called fibrils (very strong) Animals and humans can’t digest cellulose (amilase doesn’t split -1,4-bonds) Wood-rot fungi and some bacteria synthesize cellulase Ruminant animals are able to use cellulose (stomach contain bacteria producing cellulase)
Major structural component of wood and plant fibers Abundant in nature (50 % of the organic matter in biosphere) Polymer of glucose (glucose molecules are connected by -1,4-glycosidic bonds)
OH-groups of cellulose can react with nitric acid or acetic acid or acetic anhydride - nitrocellulose (celluloid) and cellulose acetate are formed
Nitrocellulose and cellulose acetate: important materials in textile industry production of photographic films celluloid shirt collars billiard balls and many other articles