Carbohydrates: The Monosaccharides
The term carbohydrate was originally used to describe compounds that were literally "hydrates of carbon" because they had the empirical formula CH2O. In recent years, carbohydrates have been classified on the basis of their structures, not their formulas. They are now defined as polyhydroxy aldehydes and ketones. Among the compounds that belong to this family are cellulose, starch, glycogen, and most sugars.
There are three classes of carbohydrates: monosaccharides, disaccharides, and polysaccharides. The monosaccharides are white, crystalline solids that contain a single aldehyde or ketone functional group. They are subdivided into two classes aldoses and ketoses on the basis of whether they are aldehydes or ketones. They are also classified as a triose, tetrose, pentose, hexose, or heptose on the basis of whether they contain three, four, five, six, or seven carbon atoms.
With only one exception, the monosaccharides are optically active compounds. Although both D and L isomers are possible, most of the monosaccharides found in nature are in the D configuration. Structures for the D and L isomer of the simplest aldose, glyceraldehyde, are shown below.
The structures of many monosaccharides were first determined by Emil Fischer in the 1880s and 1890s and are still written according to a convention he developed. The Fischer projection represents what the molecule would look like if its three-dimensional structure were projected onto a piece of paper. By convention, Fischer projections are written vertically, with the aldehyde or ketone at the top. The -OH group on the second-to-last carbon atom is written on the right side of the skeleton structure for the D isomer and on the left for the L isomer. Fischer projections for the two isomers of glyceraldehyde are shown below.
These Fischer projections can be obtained from the skeleton structures shown above by imaging what would happen if you placed a model of each isomer on an overhead projector so that the CHO and CH2OH groups rested on the glass and then looked at the images of these models that would be projected on a screen.
Fischer projections for some of the more common monosaccharides are given in the figure below.
|Practice Problem 2:
Glucose and fructose have the same formula: C6H12O6. Glucose is the sugar with the highest concentration in the bloodstream; fructose is found in fruit and honey. Use the Fischer projections in the figure of common monosaccharides to explain the difference between the structures of these compounds. Predict what an enzyme would have to do to convert glucose into fructose, or vice versa.
If the carbon chain is long enough, the alcohol at one end of a monosaccharide can attack the carbonyl group at the other end to form a cyclic compound. When a six-membered ring is formed, the product of this reaction is called a pyranose, shown in the figure below.
When a five-membered ring is formed, it is called a furanose, shown in the figure below.
There are two possible structures for the pyranose and furanose forms of a monosaccharide, which are called the a- and b-anomers.
The reactions that lead to the formation of a pyranose or a furanose are reversible. Thus, it doesn't matter whether we start with a pure sample of a-D-glucopyranose or b-D-glucopyranose. Within minutes, these anomers are interconverted to give an equilibrium mixture that is 63.6% of the b-anomer and 36.4% of the a-anomer. The 2:1 preference for the b-anomer can be understood by comparing the structures of these molecules shown previously. In the b-anomer, all of the bulky -OH or -CH2OH substituents lie more or less within the plane of the six-membered ring. In the a-anomer, one of the -OH groups is perpendicular to the plane of the six-membered ring, in a region where it feels strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As a result, the b-anomer is slightly more stable than the a-anomer.
Carbohydrates: The Disaccharides and Poly-Saccharides
Disaccharides are formed by condensing a pair of monosaccharides. The structures of three important disaccharides with the formula C12H22O11 are shown in the figure below.
Maltose, or malt sugar, which forms when starch breaks down, is an important component of the barley malt used to brew beer. Lactose, or milk sugar, is a disaccharide found in milk. Very young children have a special enzyme known as lactase that helps digest lactose. As they grow older, many people lose the ability to digest lactose and cannot tolerate milk or milk products. Because human milk has twice as much lactose as milk from cows, young children who develop lactose intolerance while they are being breast-fed are switched to cows' milk or a synthetic formula based on sucrose.
The substance most people refer to as "sugar" is the disaccharide sucrose, which is extracted from either sugar cane or beets. Sucrose is the sweetest of the disaccharides. It is roughly three times as sweet as maltose and six times as sweet as lactose. In recent years, sucrose has been replaced in many commercial products by corn syrup, which is obtained when the polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose, which is only about 70% as sweet as sucrose. Fructose, however, is about two and a half times as sweet as glucose. A commercial process has therefore been developed that uses an isomerase enzyme to convert about half of the glucose in corn syrup into fructose (see Practice Problem 4). This high-fructose corn sweetener is just as sweet as sucrose and has found extensive use in soft drinks.
The monosaccharides and disaccharides represent only a small fraction of the total amount of carbohydrates in the natural world. The great bulk of the carbohydrates in nature are present as polysaccharides, which have relatively large molecular weights. The polysaccharides serve two principal functions. They are used by both plants and animals to store glucose as a source of future food energy and they provide some of the mechanical structure of cells.
Very few forms of life receive a constant supply of energy from their environment. In order to survive, plant and animal cells have had to develop a way of storing energy during times of plenty in order to survive the times of shortage that follow. Plants store food energy as polysaccharides known as starch. There are two basic kinds of starch: amylose and amylopectin. Amylose is found in algae and other lower forms of plants. It is a linear polymer of approximately 600 glucose residues whose structure can be predicted by adding a-D-glucopyranose rings to the structure of maltose. Amylopectin is the dominant form of starch in the higher plants. It is a branched polymer of about 6000 glucose residues with branches on 1 in every 24 glucose rings. A small portion of the structure of amylopectin is shown in the figure below.
n = 1000 - 6000
The polysaccharide that animals use for the short-term storage of food energy is known as glycogen. Glycogen has almost the same structure as amylopectin, with two minor differences. The glycogen molecule is roughly twice as large as amylopectin, and it has roughly twice as many branches.
There is an advantage to branched polysaccharides such as amylopectin and glycogen. During times of shortage, enzymes attack one end of the polymer chain and cut off glucose molecules, one at a time. The more branches, the more points at which the enzyme attacks the polysaccharide. Thus, a highly branched polysaccharide is better suited for the rapid release of glucose than a linear polymer.
Polysaccharides are also used to form the walls of plant and bacterial cells. Cells that do not have a cell wall often break open in solutions whose salt concentrations are either too low (hypotonic) or too high (hypertonic). If the ionic strength of the solution is much smaller than the cell, osmotic pressure forces water into the cell to bring the system into balance, which causes the cell to burst. If the ionic strength of the solution is too high, osmotic pressure forces water out of the cell, and the cell breaks open as it shrinks. The cell wall provides the mechanical strength that helps protect plant cells that live in fresh-water ponds (too little salt) or seawater (too much salt) from osmotic shock. The cell wall also provides the mechanical strength that allows plant cells to support the weight of other cells.
The most abundant structural polysaccharide is cellulose. There is so much cellulose in the cell walls of plants that it is the most abundant of all biological molecules. Cellulose is a linear polymer of glucose residues, with a structure that resembles amylose more closely than amylopectin, as shown in the figure below. The difference between cellulose and amylose can be seen by comparing the figures of amylose and cellulose. Cellulose is formed by linking b-glucopyranose rings, instead of the a-glucopyranose rings in starch and glycogen.
n = 5000 - 10,000
The -OH substituent that serves as the primary link between -glucopyranose rings in starch and glycogen is perpendicular to the plane of the six-membered ring. As a result, the glucopyranose rings in these carbohydrates form a structure that resembles the stairs of a staircase. The -OH substituent that links the b-glucopyranose rings in cellulose lies in the plane of the six-membered ring. This molecule therefore stretches out in a linear fashion. This makes it easier for strong hydrogen bonds to form between the -OH groups of adjacent molecules. This, in turn gives cellulose the rigidity required for it to serve as a source of the mechanical structure of plant cells.
Cellulose and starch provide an excellent example of the link between the structure and function of biomolecules. At the turn of the century, Emil Fischer suggested that the structure of an enzyme is matched to the substance on which it acts, in much the same way that a lock and key are matched. Thus, the amylase enzymes in saliva that break down the a-linkages between glucose molecules in starch cannot act on the b-linkages in cellulose.
Most animals cannot digest cellulose because they don't have an enzyme that can cleave b-linkages between glucose molecules. Cellulose in their diet therefore serves only as fiber, or roughage. The digestive tracts of some animals, such as cows, horses, sheep, and goats contain bacteria that have enzymes that cleave these b-linkages, so these animals can digest cellulose.
|Practice Problem 3:
Termites provide an example of the symbiotic relationship between bacteria and higher organisms. Termites cannot digest the cellulose in the wood they eat, but their digestive tracts are infested with bacteria that can. Propose a simple way of ridding a house from termites, without killing other insects that might be beneficial.
For many years, biochemists considered carbohydrates to be dull, inert compounds that filled the space between the exciting molecules in the cell the proteins. Carbohydrates were impurities to be removed when "purifying" a protein. Biochemists now recognize that most proteins are actually glycoproteins, in which carbohydrates are covalently linked to the protein chain. Glycoproteins play a particularly important role in the formation of the rigid cell walls that surround bacterial cells.