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 groups

Classification 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 as

furanose 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 carbohydrates

1. 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 acids

4. Glycoproteins – receptors, cell recognition. Many important macromolecules in living systems are glycoproteins

Fisher projection formulas

Aldehyde C-1 is drawn at the top of a Fischer projection

The –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 liver

Fructose (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 compounds

Acitals 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 mutarotation

Anomers - 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 galactose

Microorganisms in the colon ferment undigested lactose to lactic acid generating methane (CH4) and hydrogen gas (H2)

Symptoms: gut distention; annoying problem of flatulence

Undigested lactose and lactic acid are osmotically active and draws water into the intestine resulting in diarrhea

The 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

Cellobiose

Consists of two -D-glucose joined by -1,4-glycosidic bond Cellobiose is the component of cellulose

There 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 sugar

A hemiacetal structure also react

Disaccharides 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 test

Seliwanow’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 bond

Molecular 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 cells

Present 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