Chapter 22 Carbohydrates Carbohydrates n Fun Facts n

Chapter 22 Carbohydrates

Carbohydrates n Fun Facts: n n Photosynthesis converts more than 100 billion metric tons of CO 2 and H 20 into carbohydrates annually. Non-photosynthetic cells can make there own glucose from amino acids, fats and other breakdown products.

Carbohydrates n Fun Facts 2 n Mole Ratios 1 C, 2 H, 1 O n n Empirical Formula = CH 2 O monosaccharides have from 3 to 8 carbons aldose: linear sugar with an aldehyde group ketose: linear sugar with a ketone group

Carbohydrates n Fun Facts 3 n Three classes of carbohydrates n Monosaccharides n n Disaccharides n n 3 to 8 carbons with carbonyl and alcohol FG 2 monosaccharides connected with a ketal or acetal connection Polysaccharides n Multiple ketal or acetal connections

Monosaccharides n Monosaccharides are classified by their number of carbon atoms

Monosaccharides n And they differ by the type of carbonyl present n n Aldehyde Ketone

Monosaccharides n There are only two trioses n often aldo- and keto- are omitted and these compounds are referred to simply as trioses

Monosaccharides n Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

Monosaccharides n Fischer projection: a two dimensional representation for showing the configuration of tetrahedral stereocenters n n horizontal lines represent bonds projecting forward vertical lines represent bonds projecting to the rear

D, L Monosaccharides n Emil Fischer decided on of D- and Lassignments for the enantiomers of glyceraldehyde n n D-monosaccharide: the -OH is on the right L-monosaccharide: the -OH is on the left

D, L Monosaccharides n the most common D-tetroses and D-pentoses

D, L Monosaccharides n the three common D-hexoses

Amino Sugars n Amino sugars contain an -NH 2 group in place of an -OH group

Cyclic Structure n Aldehydes and ketones react with alcohols to form hemiacetals n cyclic hemiacetals form readily as five- or sixmembered ring

Haworth Projections n D-Glucose forms these cyclic hemiacetals

Haworth Projections n n n a five- or six-membered cyclic hemiacetal is represented as a planar ring groups lie either above or below the plane the new carbon stereocenter is called an anomeric carbon stereoisomers that differ in configuration only at the anomeric carbon are called anomers the anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2

Haworth Projections n Terminology of carbohydrate chemistry, n n n b means that the anomeric -OH is on the same side of the ring as the terminal -CH 2 OH a means that the anomeric -OH is on the side of the ring opposite the terminal -CH 2 OH a six-membered hemiacetal ring is called a pyranose, pyranose and a five-membered hemiacetal ring is called a furanose

Haworth Projections n n aldopentoses also form cyclic hemiacetals the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

Haworth Projections n D-fructose also forms a five-membered cyclic hemiacetal

Mutarotation n Mutarotation: the equilibrium interconversion of a- and b-anomers in aqueous solution

Chair Conformations Pg 475 n Lets leave this out. I will be very happy if you can draw Fisher and Hayworth forms.

Physical Properties n Monosaccharides are colorless crystalline solids, very soluble in water n sweetness relative to sucrose:

Chemical Properties n Monosaccharides n Hemiacetal into acetal – glycosidic bond n n n A glycosidic bond slows mutarotation to snails pace. Acid is needed to break acetal or ketal Aldose’s reduce Cu 2+, Fe 3+, and cold Mn. O 4 n n Only works with the linear aldehyde form Hemiacetals are in equilibrium with aldehyde form n n n Called reducing sugars Glycosides cannot reduce these Carbonyl can be reduced

Formation of Glycosides n Treatment of a monosaccharide with an alcohol gives an acetal

Glycosides n n a cyclic acetal derived of a monosaccharide is called a glycoside the bond from the anomeric carbon to the -OR group is called a glycosidic bond mutarotation is VERY SLOW in a glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide

Oxidation to Aldonic Acids n n n the aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion the oxidation product is called an aldonic acid reducing sugar (it reduces the oxidizing agent)

Oxidation to Uronic Acids n Enzyme-catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic acid n enzyme-catalyzed oxidation of D-glucose, for example, yields D-glucuronic acid

Reduction to Alditols n The carbonyl group can be reduced to a hydroxyl group by Na. BH 4 and H 2/Pd n the reduction product is called an alditol

Reduction to Alditols n n n sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae it is about 60 percent as sweet as sucrose these three alditols are also common in the biological world

D-Glucuronic Acid n n n D-glucuronic acid exists in the plants and animals in humans, it is an important component of the acidic polysaccharides of connective tissues it is used to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

Phosphate Esters n Mono- and diphosphoric esters are intermediates in metabolism of monosaccharides n the first step in glycolysis is conversion of D-glucose to a-D-glucose 6 -phosphate

Disaccharides n Sucrose n n most abundant disaccharide sucrose is a nonreducing sugar (why)

Disaccharides n Lactose n n n lactose is the principal sugar present in milk it consists of D-galactopyranose bonded by a b-1, 4 glycosidic bond to carbon 4 of D-glucopyranose lactose is a reducing sugar (why)

Disaccharides n Maltose n n n present in malt two D-glucopyranose joined by an a-1, 4 -glycosidic bond maltose is a reducing sugar (Why)

Polysaccharides n Polysaccharide: lots of monosaccharide units n n Also called glycans Can be a or b linked anomers n n One we can digest “a” The other we cannot “b”

Polysaccharides - a n Starch: an energy storage polymer of Dglucose found in plants n n n starch can be separated into amylose and amylopectin amylose is D-glucose units joined by a-1, 4 -bonds Amylopectin - D-glucose units joined by a-1, 4 bonds; at branch points, new chains every 24 to 30 units are started by a-1, 6 -glycosidic bonds

Polysaccharides - a n Glycogen is the energy-reserve carbohydrate for animals n n glycogen - glucose units joined by a-1, 4 - and a-1, 6 -glycosidic bonds (branches occur every 8 to 12 residues - more compact than starch) the total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle

Polysaccharides - a n Why Store sugar as starch or glycogen? n Osmolarity n n n Individual sugars would be 0. 4 M Polymers (mostly insoluable) 10 -8 M Cells would burst with water running into the to equilibrate osmotic pressure!

Polysaccharides - b n Cellulose is a linear polysaccharide of Dglucose units joined by b-1, 4 -glycosidic bonds n n it has an average molecular weight of 400, 000 g/mol, approximately 2200 glucose units cellulose molecules act like stiff rods and align themselves side by side into well-organized waterinsoluble fibers in which the OH groups hydrogen bond with each other rather than water. this arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength it is also the reason why cellulose is insoluble in water

Polysaccharides - b n Cellulose (cont’d) n animals cannot digest cellulose n n no contain b-glucosidases, enzymes that catalyze hydrolysis of b-glucosidic bonds we have only a-glucosidases; hence we can digest starch and glycogen many bacteria and microorganisms have bglucosidases and can digest cellulose termites have such bacteria in their intestines and can use wood as their principal food

Acidic Polysaccharides n Acidic polysaccharides: contain carboxyl groups and/or sulfuric ester groups n n play important roles in the structure and function of connective tissues there a large number of highly specialized forms of connective tissue n n such as cartilage, bone, synovial fluid, skin, tendons, blood vessels, intervertebral disks, and cornea most connective tissues are made up of collagen, a structural protein, in combination with a variety of acidic polysaccharides

Acidic Polysaccharides n Hyaluronic acid n Found in embryonic tissues, synovial fluid, lubricant of joints in the body, and the vitreous of the eye

Acidic Polysaccharides n Heparin: a heterogeneous mixture of variably sulfonated polysaccharide chains, ranging in molecular weight from 6, 000 to 30, 000 g/mol

Acidic Polysaccharides n Heparin (cont’d) n n n heparin is synthesized and stored in mast cells of various tissues, particularly the liver, lungs, and gut the best known and understood of its biological functions is its anticoagulant activity it binds strongly to antithrombin III, a plasma protein involved in terminating the clotting process

Carbohydrates End Chapter 19