Carbohydrates CHAPTER SEVENTEEN

Carbohydrates CHAPTER SEVENTEEN
Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.
Monosaccharides Carbohydrate: A polyhydroxyaldehyde, polyhydroxyketone, or a substance that gives these compounds on hydrolysis. Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. Monosaccharides have the general formula CnH2nOn. The most common have from 3 to 9 carbons. Aldose: a monosaccharide containing an aldehyde group. Ketose: a monosaccharide containing a ketone group. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Monosaccharides There are only two trioses. Often aldo- and keto- are omitted and these compounds are referred to simply as trioses, tetroses and so forth. Although these modified designations do not tell the nature of the carbonyl group, they at least tell the number of carbons in the carbohydrate. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Fischer Projection Formulas
Fischer projection: a two dimensional representation for showing the configuration of a tetrahedral stereocenter. Horizontal lines represent bonds projecting forward. Vertical lines represent bonds projecting to the rear. The first and last carbons in the chain are written in full; others are indicated by the crossing of bonds. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

D- and L-Monosaccharides
In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

According to the conventions proposed by Fischer D-monosaccharide: A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the right. L-monosaccharide: A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the left. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Amino sugars N-acetyl-D-glucosamine is a component of many polysaccharides, including connective tissue such as cartilage. It is also a component of chitin, the hard shell-like exoskeleton of lobsters, crabs, and shrimp. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Cyclic Structure Monosaccharides have hydroxyl and carbonyl groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals. Anomeric carbon: The hemiacetal carbon of a cyclic form of a monosaccharide. Anomers: Monosaccharides that differ in configuration only at their anomeric carbons. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Haworth Projections 5- and 6-membered hemiacetals are represented as planar pentagons or hexagons viewed through the edge. Most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right α- means that the -OH on the anomeric carbon is cis to the terminal -CH2OH; β- means it is trans. A 6-membered hemiacetal is shown by the infix pyran-. A 5-membered hemiacetal is shown by the infix furan-. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Cyclic Structures Aldopentoses also form cyclic hemiacetals. The most prevalent forms of D-ribose and other pentoses in the biological world are furanoses. The prefix deoxy- means “without oxygen”. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Conformational Formulas
Five-membered rings are close to planar so that Haworth projections are adequate to represent furanoses. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Compare the orientations of groups on carbons 1-5 in the Haworth and chair representations of β-D-glucopyranose. In each case, beginning at carbon 1, they are up-down-up-down-up. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Mutarotation Mutarotation: The change in specific rotation that occurs when an α or β form of a carbohydrate is converted to an equilibrium mixture of the two. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Glycosides Glycosidic bond: The bond from the anomeric carbon of the glycoside to an -OR group. To name a glycoside, name the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate; replace the ending -e by -ide. Methyl b-D-glucopyranoside Methyl a-D-ribofuranoside Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

N-Glycosides The anomeric carbon of a cyclic hemiacetal also reacts with an N-H of an amine to form an N‑glycoside. Especially important in the biological world are the N-glycosides of D-ribose and 2-deoxy-D- ribose with the following heterocyclic aromatic amines. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Reduction to Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Reduction to Alditols Name alditols by replacing the -ose of the monosaccharide name by -itol. Sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, and seaweed; it is about 60% as sweet as sugar. Other common alditols include Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Oxidation to Aldonic Acids
The –CHO group can be oxidized to –COOH. Reducing sugar: Any carbohydrate that reduces an oxidizing agent to form an aldonic acid. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Oxidation to Uronic Acids
The body uses glucuronic acid to detoxify foreign alcohols and phenols. These compounds are converted in the liver to glycosides of glucuronic acid and then excreted in the urine. The intravenous anesthetic Propofol is converted to the following water-soluble glucuronide and excreted. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Ascorbic Acid (Vitamin C)
L-ascorbic acid is very easily oxidized to L-dehydroascorbic acid. Both compounds are physiologically active and are found in most body fluids. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Blood Group Substances
Membranes of animal plasma cells have large numbers of relatively small carbohydrates. These membrane-bound carbohydrates are part of the mechanism by which cell types recognize each other; they act as antigenic determinants. Among the first discovered of these antigenic determinants were the blood group substances. In the ABO system, individuals are classified according to four blood types: A, B, AB, and O At the cellular level, the biochemical basis for this classification is a group of relatively small membrane- bound carbohydrates. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Starch Starch is used for energy storage in plants. It can be separated into two fractions; amylose and amylopectin Amylose is composed of unbranched chains of up to 4000 D‑glucose units joined by a‑1,4- glycosidic bonds. Amylopectin is a highly branched polymer of D‑glucose; chains consist of units of D-glucose joined by a‑1,4-glycosidic bonds and branches created by a‑1,6-glycosidic bonds. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Glycogen Glycogen is the reserve carbohydrate for animals. A nonlinear polymer of D-glucose units joined by a-1,4- and a-1,6-glycosidic bonds. The total amount of glycogen in the body of a well-nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Cellulose Figure 17.5 Cellulose is a linear polymer of D‑glucose units joined by b-1,4-glycosidic bonds. It has an average molecular weight of 400,000 corresponding to approximately 2800 D-glucose units per molecule. Both rayon and acetate rayon are made from chemically modified cellulose. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

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