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

Oxidation of Monosaccharides Recall from Ch.15 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

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)

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

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:

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

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

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

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

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)

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

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

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

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

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