Fr"O";(Or \9H ,/ OH "Oh OH Cellobiose

16.8 Sucroseand Lactose 5rt

16,7Disocchorides

AIM: To write the structuresond list the sourcesond usesof the disocchoridesmsltose and cellobiose.

Disaccharides are compounds in which two monosaccharidesare bonded together Disaccharidesare glycosidesin which the alcohol is a second Maltose and cellobiose are monosaccharide molecule. Maltose, or malt sugar, is a disaccharide derived from the hydrolysis of derived from the partial hydrolysis of starch. Diastase,an enzT/rnefound in starch and cellulose. germinating barley, catalyzes the hydrolysis of starch to maltose. since starch is composedof many o-glucopyranoseunits connectedby a(l+at) glycosidic bonds and maltose is a disaccharide produced from starch, it is not surprising that maltose is composed of two o-glucopyranose molecules connected by an a (1-4) glycosidic linkage. Maltose is hydrolyzed to o-glucose by the enzyme maltase.

CH,OH CH,OH /-o I-o (;, (;" 'OHOH\ !o" OH OH Maltose Iikewise, cellobioseis a disaccharideproduced from the partial hydrolysis of cellulosebydilute acid.This disaccharideconsists of two l-glucose mol- eculeslike maltose,but the 1-4 glycosidicbond that links the two monosac- charidesis beta. Cellobioseis not hvdrolvzed bv the enzvme maltase. cH'oH l-o cH2oH I/' )o"\ fr"o";(or \9H ,/ OH "oH OH Cellobiose

PRACTICEEXERCISE I6.I4 What is the relationship of maltose to cellobiose?

16.8Sucrose ond loctose

AIM: To write the structuresond list the sourcesond usesof the disocchoridessucrose ond loctose.

Focus The most familiar sugar is sucrose-ordinary table sugar.sucrose is iso- lated from juice Sucroseand lactose are the the or sap of severalplants, including sugarcane,sugar sugars of sap andmilk. beets, and maple trees. The world's production of sucrose from these sources exceeds7 billion kilograms per year. The sucrose molecule is a 512 CHAPTER16 Carbohydrates

disaccharidecomposed of o-o-glucoseand B-D-fructose:

(on o-o-Glucoseunit Ho\!_?

9-D-Fructoseunit

Sucrose Structurally, the linkage between glucose and fructose in sucrose is unusual among sugars,since both sugarsare linked together at the anomeric carbons (carbon I of each sugar).Hydrolysis of sucroseby acid or by enzymes gives invert sugar-a mixture of equal molnr quantities of glucoseand fructose. Honey is a rich natural source of invert sugar. Invert sugar is also produced commercially and used when a noncrystalline sweeteneris desired. The gooey syrup that traditionallybathes chocolate-coveredcherries is one use ofinvert sugar. As a food, suctose has a high caloric value. Many people rely on artificial sweetenerssuch as saccharin,which is much sweeterthan sugar.At one time calcium cyclamate, a chemical 30 times sweeter than sucrose, com- peted with saccharin as an artificial sweetener; then it was removed from the market as a possible cancer-causing agent. The use of saccharin has been reducedfor the samereason. o /#*n t/---\ I I ll I l( FNHSo3-l cu'- \-h-so, L\--l ), Saccharin Calcium cyclamate Lactose,or milk sugar, constitutes SVoof cow's milk artd 7Voof human milk. Pure lactose is obtained from whey, the watery by-product of cheese production. Lactoseis composedof one moleculeof n-galactoseand one of o-glucose.The linkage between the two sugar units is F(l--+4). CH2OH F-O cH2oH(:,. Glucose von unit Ho/-o', A*t / Galactose VV unit \?" '/ oH t-- OH Lactose 16.8 Sucroseand Lactose 515

The glycosidicbond betweenthe t-galactoseand o-glucoseportions of lactose may be cleaved by the enz),.rnelactase. Lactase deficiency is fairly common in people. Lactase deficiency is the reason for the discomfort experiencedbyAlberto, the student describedin the case in Point earlierin this chapter. In the follow-up, below, we learn more of this deflciency and how it can be treated.

PRACTICEEXERCISE I5.I5 Give the names and structures of the compounds produced by the hydrolysisof the glycosidicbond in lactose.

[; r' ' Forlow-upro rHECnsr ln Porxr:Lactose intiilerance

The physician correctly diagnosed Alberto's stom- symptoms by adding readily available lactase ach cramps, gas, and diarrhea to be the result of lac- tablets to their mitk (see figure). Lactose-free milk is tose jntolerance. Llke Alberto, many adults and also avaiiable at some groceries. some children lack the enzyme lactase.As a result, Lactose intolerance j.s fairly common in infants, these people are unable to hydrolyze lactose to j.ts especially among babies of Middle Eastern, Asian, simpler sugar components, o-galactose and o-gIu- and African descent. Europeans are statistically cose. The symptoms of lactose intolerance are Ieast susceptible. Dehydration from diarrhea from caused by undigested lactose that sits in the lactose intolerance or any other cause is always a intestinal tract, where it causes cramps and diar- hazard for babies because, compared with adults, rhea. The diarrhea is caused by an lmbalance in the their bodies contain a relatively small mass of water. osmotic pressure on the walls of the intestine. Lac- Switching a lactase-deficient baby from natural milk tose draws water through the j.ntestinal wall into sources to a commercial milk substitute alieviates the intestines, where the excessive water causes the diarrhea and other symptoms. diarrhea, which can lead to dehydration. Some of the lactose can be oxidlzed by intestinal bacrerra, releasing CO2 into the intestines. The bubbles of gas can cause great discomfort. The effects of lactose lntoierance can be avoided by excluding mitk and miik products from the diet. Some people with lactose lntolerance can eat yognrrt, which is a fer- mented milk product. The fermentation of yogurt by bacteria breaks down the lactose present. In many instances, Iactase production diminishes with age. A reduced level of lactase, combined with hrs con- sumption of a large amount of milk at one sitting, may have triggered Alberto's illness. Alberto later found that he was able to consume small amounts of dairy products without a repeat of the symptoms of lactose intolerance. Some other sufferers of lactose intolerance-estimated to be as many as one-third of all adult Americans-san avoid the unpleasant Lactasetablets.