13 - Carbohydrates

Galactosemia

In a condition called galactosemia, an enzyme needed to convert galactose to glucose is missing. The accumulation of galactose in the blood and tissues can lead to cataracts, mental retardation, failure to thrive, and liver disease. The treatment for galactosemia is the removal of all galactose-containing foods, mainly milk and milk products, from the diet. If this is done for an infant immediately after birth, the damaging effects of galactose accumulation can be avoided.

Aldose

In an aldose, the carbonyl group is on the first carbon as an aldehyde (─ CHO).

Ketose

In an ketose contains the carbonyl group on the second carbon atom as a ketone (C═O).

Fructose

In contrast to glucose and galactose, fructose, C6H12O6, is a ketohexose. The structure of fructose differs from glucose at carbons 1 and 2 by the location of the carbonyl group. Fructose is the sweetest of the carbohydrates, almost twice as sweet as sucrose (table sugar). This characteristic makes fructose popular with dieters because less fructose, and therefore fewer calories, is needed to provide a pleasant taste. Fructose, also called levulose and fruit sugar, is found in fruit juices and honey.

Stereoisomers

In stereoisomers, the atoms are bonded in the same sequence but differ in the way they are arranged in space.

Glycosidic Bond

In the Haworth structure of a disaccharide, a glycosidic bond is an ether bond that connects two monosaccharides. In maltose, a glycosidic bond forms between the ─ OH groups of carbons 1 and 4 of two α-D-glucose molecules with a loss of a water molecule. The glycosidic bond in maltose is designated as an α-1,4 linkage to show that an alpha ─ OH group on carbon 1 is joined to carbon 4 of the second glucose molecule. Because the second glucose molecule still has a free ─ OH group on carbon 1, it can form an open chain, which allows maltose to form both a and b isomers. The open chain provides an aldehyde group that can be oxidized, making.

Respiration

In the body, glucose is oxidized in a series of metabolic reactions known as respiration, which releases chemical energy to do work in the cells. Carbon dioxide and water are produced and returned to the atmosphere. The combination of photosynthesis and respiration is called the carbon cycle, in which energy from the Sun is stored in plants by photosynthesis and made available to us when the carbohydrates in our diets are metabolized.

Chiral Carbon Atoms

A carbon compound is chiral if it has at least one carbon atom bonded to four different atoms or groups. This type of carbon atom is called a chiral carbon because there are two different ways that it can bond to four atoms or groups of atoms. The resulting structures are nonsuperimposable mirror images.

Oxidation of Monosaccharides

Although monosaccharides exist mostly in cyclic forms, we have seen that a small amount of the open-chain form is always present, which provides an aldehyde group. An aldehyde group with an adjacent hydroxyl can be oxidized to a carboxylic acid by an oxidizing agent such as Benedict's reagent. The sugar acids are named by replacing the ose ending of the monosaccharide with onic acid. Then the Cu2+ is reduced to Cu+, which forms a brick-red precipitate of Cu2O. A carbohydrate that reduces another substance is called a reducing sugar.

Amylose

Amylose, which makes up about 20% of starch, consists of 250 to 4000 α-D-glucose molecules connected by α-1,4-glycosidic bonds in a continuous chain. Sometimes called a straight-chain polymer, polymers of amylose are actually coiled in helical fashion.

Enantiomers

Stereoisomers that are mirror images that cannot be superimposed.

Sucrose

Sucrose consists of an α-D-glucose and a β-D-fructose molecule joined by an α,β1,2-glycosidic bond (see Figure 13.10). Unlike maltose and lactose, the glycosidic bond in sucrose is between carbon 1 of glucose and carbon 2 of fructose. Thus, sucrose cannot form an open chain and cannot be oxidized. Sucrose cannot react with Benedict's reagent and is not a reducing sugar.

Reducing Sugar

A carbohydrate with an aldehyde group capable of reducing the Cu2+ in Benedict's reagent.

Disaccharide

A disaccharide consists of two monosaccharide units joined together, which can be split into two monosaccharide units. For example, ordinary table sugar, sucrose, C12H22O11, is a disaccharide that can be split by water (hydrolysis) in the presence of an acid or an enzyme to give one molecule of glucose and one molecule of another monosaccharide, fructose.

Polysaccharide

A polysaccharide is a carbohydrate that contains many monosaccharide units, which is called a polymer. In the presence of an acid or an enzyme, a polysaccharide can be completely hydrolyzed to yield many monosaccharide molecules. A polysaccharide is a polymer of many monosaccharides joined together. Four important polysaccharides—amylose, amylopectin, cellulose, and glycogen—are all polymers of D-glucose that differ only in the type of glycosidic bonds and the amount of branching in the molecule.

Carbohydrates

A simple or complex sugar composed of carbon, hydrogen, and oxygen. Simple sugars, which have formulas of Cn(H2O)n, were once thought to be hydrates of carbon, thus the name carbohydrate.

Fischer Projections

A system for drawing stereoisomers with horizontal lines representing bonds that project forward, vertical lines that represent bonds that project backward, and with a carbon atom at each intersection.

Glycogen

Glycogen, or animal starch, is a polymer of glucose that is stored in the liver and muscle of animals. It is hydrolyzed in our cells at a rate that maintains the blood level of glucose and provides energy between meals. The structure of glycogen is very similar to that of amylopectin found in plants, except that glycogen is more highly branched. In glycogen, the glucose units are joined by α-1,4-glycosidic bonds, and branches occurring about every 10 to 15 glucose units are attached by α-1,6-glycosidic bonds.

Photosynthesis

In a series of reactions called photosynthesis, energy from the Sun is used to combine the carbon atoms from carbon dioxide (CO2) and the hydrogen and oxygen atoms of water (H2O) into the carbohydrate glucose.

Lactose

Lactose, milk sugar, is a disaccharide found in milk and milk products (see Figure 13.9). The bond in lactose is a β-1, 4-glycosidic bond because the ─ OH group on carbon 1 of β-D-galactose forms a glycosidic bond with the ─ OH group on carbon 4 of a D-glucose molecule. Because D-glucose still has a free ─ OH group on carbon 1, it can form an open chain, which allows lactose to form both α and b isomers. The open chain provides an aldehyde group that can be oxidized, making lactose a reducing sugar.

Maltose

Maltose, or malt sugar, is obtained from starch and is found in germinating grains. When maltose in barley and other grains is hydrolyzed by yeast enzymes, glucose is obtained, which can undergo fermentation to give ethanol. Maltose is used in cereals, candies, and the brewing of beverages.

Chiral

Objects such as hands that have nonsuperimposable mirror images are chiral (pronunciation 'kai-rel). Left and right shoes are chiral; left- and right-handed golf clubs are chiral. When we think of how difficult it is to put a left-hand glove on our right hand, put a right shoe on our left foot, or use left-handed scissors if we are right-handed, we begin to realize that certain properties of mirror images are very different.

Cellulose

Cellulose is the major structural material of wood and plants. Cotton is almost pure cellulose. In cellulose, glucose molecules form a long unbranched chain similar to that of amylose. However, the glucose units in cellulose are linked by β-1,4-glycosidic bonds.

Glucose

The most common hexose, glucose, C6H12O6, also known as dextrose and blood sugar, is found in fruits, vegetables, corn syrup, and honey. D-glucose is a building block of the disaccharides sucrose, lactose, and maltose, and polysaccharides such as amylose, cellulose, and glycogen.

Reduction of Monosaccharides

The reduction of the carbonyl group in monosaccharides produces sugar alcohols, which are also called alditols. D-glucose is reduced to D-glucitol, better known as D-sorbitol. The sugar alcohols are named by replacing the ose ending of the monosaccharide with itol. Sugar alcohols such as D-sorbitol, D-xylitol from D-xylose, and D-mannitol from D-mannose are used as sweeteners in many sugar-free products such as diet drinks and sugarless gum as well as products for people with diabetes. However, there are some side effects of these sugar substitutes. Some people experience some discomfort such as gas and diarrhea from the ingestion of sugar alcohols. The development of cataracts in diabetics is attributed to the accumulation of D-sorbitol in the lens of the eye.

Monosaccharides

The simplest carbohydrates are the monosaccharides. A monosaccharide cannot be split or hydrolyzed into smaller carbohydrates. One of the most common carbohydrates, glucose, C6H12O6, is a monosaccharide. A monosaccharide with three carbon atoms is a triose, one with four carbon atoms is a tetrose; a pentose has five carbons, and a hexose contains six carbons. We can use both classification systems to indicate the type of carbonyl group and the number of carbon atoms.

Haworth Structures

These rings, known as Haworth structures, are produced from the reaction of a carbonyl group and a hydroxyl group in the same molecule. While the carbonyl group in the open chain could react with several of the ─OH groups, let's look at how we draw the Haworth structures for some d stereoisomers, starting with the open-chain structure of D-glucose.

Amylopectin

Amylopectin, which makes up as much as 80% of starch, is a branched-chain polysaccharide. Like amylose, the glucose molecules are connected by α-1,4-glycosidic bonds. However, at about every 25 glucose units, there is a branch of glucose molecules attached by an α-1,6-glycosidic bond between carbon 1 of the branch and carbon 6 in the main chain.

Galactose

Galactose, C6H12O6, is an aldohexose that is obtained from the disaccharide lactose, which is found in milk and milk products. Galactose is important in the cellular membranes of the brain and nervous system. The only difference in the Fischer projections of D-glucose and D-galactose is the arrangement of the ─ OH group on carbon 4.