Cyclic Structure of Fructose

1. Cyclic Structure of Fructose • As a ketohexose, fructose forms a 5-membered ring when the hydroxyl on C-5 reacts with the carbonly on C-2 -D-Fructose -D-Fructose

2. Dehydration of Carbohydrates: The Molisch Test • Carbohydrates, like most alcohols, undergo dehydration reactions in the presence of concentrated sulfuric acid. Pentoses (five carbon sugars) give furfural, and ketohexoses and aldohexoses give substituted furfurals.

3. Oxidation of Monosaccharides • Recall that Benedict’s reagent (CuSO4) can oxidize aldehydes with adjacent hydroxyl groups • The blue Cu2+ ions in the Benedict’s reagent are reduced to form a brick-red precipitate, Cu2O • Normally, ketones are not oxidized, however ketones with an adjacent hydroxyl group can rearrange to the aldehyde during reaction with Benedict’s reagent • So, both aldoses and ketoses, in open chain form, can be oxidized by Benedict’s reagent to form carboxylic acids • Sugars that can be thus oxidized are called reducing sugars

4. Oxidation to Aldonic Acids • the aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion • the oxidation product is called an aldonic acid • any carbohydrate that reacts with an oxidizing agent to form an aldonic acid is classified as a reducing sugar (it reduces the oxidizing agent)

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

6. D-Glucuronic Acid • D-glucuronic acid is widely distributed in the plant and animal world • in humans, it is an important component of the acidic polysaccharides of connective tissues • it is used by the body to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

7. Reduction of Monosaccharides • Reduction of the carbonyl group of a monosaccharide (in open-chain form) produces a sugar alcohol, or alditol • D-Glucose is reduced to D-glucitol (also called D-sorbitol) using hydrogenation (H2 and a metal catalyst)

8. Reduction to Alditols • 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 (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics • these three alditols are also common in the biological world

9. Formation of glycosides • Recall that an alcohol can react with a hemiacetal to form an acetal (a di-ether) • When an alcohol reacts with a cyclic hemiacetal of a monosaccharide the cyclic acetal product is called a glycoside • The new ether bond is called a glycosidic bond • Monosaccharides are linked together by glycosidic bonds to form disaccharides and polysaccharides • Alkyl glycosides can not undergo mutarotation, and so are not reducing sugars -D-Glucose Methanol Methyl--D-glucoside

10. Formation of Glycosides • a cyclic acetal derived from a monosaccharide is called a glycoside • the bond from the anomeric carbon to the -OR group is called a glycosidic bond • mutarotation is not possible in a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound • glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide • glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -e is replaced by -ide

11. Disaccharides • A disaccharide is formed when a hydroxyl group on one monosaccharide reacts with the anomeric carbon of another monosaccharide to form a glycosidic bond • Each disaccharide has a specific glycosidic linkage (depending on which hydroxyl reacts with which anomer) • The three most common disaccharides are maltose, lactose and sucrose • When hydrolyzed using acid or an enzyme, the following monosaccharides are produced:

12. Maltose • Maltose (malt sugar or corn sugar) consists of two glucose molecules linked by an -1,4-glycosidic bond • It comes from partial hydrolysis of starch by the enzyme amylase, which is in saliva and also in grains (like barley) • Maltose can be fermented by yeast to produce ethanol • Maltose is also used in cereals, candies and malted milk • Because one of the glucose molecules is a hemiacetal, it can undergo mutorotation, and so maltose is a reducing sugar

13. Lactose • Lactose (milk sugar) consists of one glucose molecule and one galactose molecule linked by a -1,4 glycosidic bond • It comes from milk products (about 4-5% of cow’s milk) • Because the glucose is a hemiacetal, it can undergo mutorotation, and so lactose is a reducing sugar

14. Hydrolysis of Lactose • Some people don’t produce enough lactase, the enzyme that hydrolyzes lactose, and so can’t digest lactose • Many adults become lactose intolerant, and develop abdominal cramps, nausea and diarrhea • Lactase can be added to milk products (or taken as a supplement) to combat this problem

15. Sucrose • Sucrose (table sugar) consists of one glucose molecule and one fructose molecule linked by an ,-1,2-glycosidic bond • Sucrose is the most abundant disaccharide and is commercially produced from sugar cane and sugar beets • Because the glycosidic bond in sucrose involves both anomeric carbons, neither monosaccharide can undergo mutorotation, and so sucrose is not a reducing sugar

16. Hydrolysis of Sucrose • Sucrose is hydrolyzed by the enzyme sucrase, which is secreted in the small intestine • The glucose and fructose can then be absorbed into the bloodstream (disaccharides are too large to be absorbed)

17. Fermentation • A fermentation is defined as an energy-yielding metabolic pathway with no net change in the oxidation state of products as compared to substrates • Yeast can ferment glucose, fructose, maltose and sucrose • Ultimately, glucose is converted to pyruvate through glycolosis, and the pyruvate is then converted to CO2 and ethanol by a two-step enzymatic process • The net reaction is: C6H12O6 2C2H5OH + 2CO2

18. Polysaccharides • A polysaccharide is a polymer consisting of hundreds to thousands of monosaccharides joined together by glycosidic linkages • Three biologically important polysaccharides are starch, glycogen and cellulose - all three are polymers of D-glucose, but they differ in the type of glycosidic bond and/or the amount of branching • Starch and glycogen are used for storage of carbohydrates - starch is found in plants and glycogen in animals - the polymers take up less room than would the individual glucose molecules, so are more efficient for storage • Cellulose is a structural material used in formation of cell walls in plants

19. Plant Starch (Amylose and Amylopectin) • Starch contains a mixture of amylose and amylopectin • Amylose is an unbranched polymer (forms -helix) of D-glucose molecules linked by -1,4-glycosidic bonds • Amylopectin is like amylose, but has extensive branching, with the branches using -1,6-glycosidic bonds

20. Glycogen and Cellulose • Glycogen (animal starch) is like amylopectin, except it’s even more highly branched - animals store glycogen in the liver (about a one-day supply in humans) and use it to maintain fairly constant blood sugar levels between meals • Cellulose is an unbranched polymer of D-glucose molecules linked by -1,4-glycosidic bonds - cellulose forms -sheets of parallel strands held together by hydrogen bonding - we don’t have the enzyme to break down cellulose - some animals have microorganisms that do have the enzyme

21. Iodine Test for Starch • The presence of starch can easily be identified using iodine (I2) • Rows of iodine atoms form in the core of the -helix of amylose, forming a dark blue complex • Because amylopectin, glycogen and cellulose do not form -helices, they do not complex well with iodine, so do not show the blue color (they show a purple or brown color) • Monosaccharides do not interact with the iodine, so no color is produced