CHAPTER 18: CARBOHYDRATE STRUCTURE

\ Carbohydrates are the most abundant organic compounds on earth. The amount of cellulose in plants is estimated at about 10^11 tons, and the amount of chitin in arthropods at about 10^9 tons. However, carbohydrates constitute only about 1% of the mammalian body.

Complex Carbohydrates Complex storage carbohydrates include starch in plants, and glycogen in animals. A normal large dog, for example, possesses about 100 gm of glucose stored as liver glycogen, 250 gm as muscle glycogen, 25 gm as adipose tissue glycogen, and 10 gm as free glucose in the extracellular fluid compartments. At rest, between 160-200 gm of glucose are metabolized per day, with the brain being the major consumer (~ ~120 gm/day). Under normal circumstances, muscle and adipose tissue glycogen are of little use for general metabolism in other organs since these tissues cannot contribute glucose directly to the circulation, and the brain can neither synthesize nor store significant amounts of glycogen. This leaves only 100 gm of liver glycogen, about 3 potatoes in starch equivalents, as the body's main glucose reserve that can be exported to blood. In light of these limited stores of ready-made glucose, it is not surprising that, under conditions other than those immediately after a meal, glucose utilization is reserved mostly for vital functions. In addition, with increasing time after a meal, glucose synthesis from noncarbohydrate sources, predominantly glucogenic amino acids, glycerol, pyruvate, lactate, and propionate (in herbivores), gains steadily in importance.

Carbohydrates are also present in cell membranes as the polysaccharide portion of glycoproteins and glycolipids, and they are present in intercellular materials. They are important for the structure of cartilage and bone, and plasma proteins, for example, are mostly glycoproteins. The carbohydrate moieties of glycoproteins are associated with several physiologic actions.

Monosaccharides: Monosaccharides are simple sugars that may become basic units of more complex molecules. They are single chains of carbon atoms bearing multiple adjacent hydroxyl groups. The general overall structure is (HCOH)n; hence the name carbo- (C) hydrate (H2O). Those with 3-7 carbon atoms are the most important for mammalian metabolism. Glyceraldehyde and dihydroxyacetone are trioses (3-carbon atoms), ribose is a pentose (5-carbon atoms), while glucose, fructose, and galactose are hexoses (6-carbon atoms). Tetroses are 4-carbon sugars, and heptoses 7-carbon. One of the hydroxyl groups is typically oxidized to either an aldehyde or keto group, creating aldose or ketose sugars. Glyceraldehyde is the smallest aldose, and dihydroxyacetone the smallest ketose. The convention used to name D- or L-carbohydrates is based on the orientation (right or left) of the hydroxyl group on the highest-numbered asymmetric carbon. Most all carbohydrates involved in mammalian physiology are of the D series (with the exception of L-rhamnose and L-fucose). L-carbohydrates are poorly utilized because mammalian enzymes involved in carbohydrate metabolism recognize only the D-isomers. Monosaccharides such as galactose, glucose, ribose, and fructose do not usually exist in linear form. Instead, they condense into rings of five or six carbon atoms called furanose or pyranose rings, respectively.

Triose derivatives are formed during catabolism of glucose by glycolysis, while derivatives of trioses, tetroses, pentoses, and the 7-carbon sugar (sedoheptulose), are formed in the breakdown of glucose via the hexose monophosphate shunt.

Pentoses, NAD+ and NADP+ , NADH and NADPH: Pentose polymers such as hemicelluloses, gums, and mucilages constitute part of the simidigestable, “fibrous” material of plants, as exemplified by the xylose and arabinose polysaccharides. Pentose sugars are also an important part of animal metabolism, as seen with some intermediates of the hexose monophosphate shunt, and the uronic acid pathway.

Ribose is an important constituent of nucleotides and nucleic acids. It is also part of the structure of two important coenzymes, NAD+ and NADP+. Nicotinamide adenine dinucleotide (NAD+ ) is made up of the base nicotinamide, ribose, and ADP. An additional phosphate may be attached to NAD+ , thus creating NADP+ . These coenzymes serve as mobile electron acceptors for a number of oxidation-reduction reactions in both the cytosol and in mitochondria, and in their reduced forms, NADH and NADPH, the nicotinamide rings of these dinucleotides have the structures. Although NAD+ and NADP+ are involved in two-electron transfers, only one hydrogen atom (the hydride ion (H- )) is accepted by the dinucleotide, while the other appears as a proton (H+ ), having effectively donated its electron to neutralize the positive charge on the nitrogen atom in the nicotinamide ring. For this reason, the designations NAD(P)+ and NAD(P)H for the oxidized and reduced coenzymes, respectively, are preferred over NAD(P) and NAD(P)H2.

Reduction of NAD+ and NADP+ by dehydrogenases changes the character of the nicotinamide ring from aromatic to quinonoid, with resultant changes in absorption spectra. Since many enzymatic assays rely on differences in spectrophotometric light absorption as substrates are converted to products, clinical chemistry labs make substantial use of differences in light absorption between the oxidized (NAD+ and NADP+ ) and reduced pyridine nucleotides (NADH and NADPH), 260 nm and 340 nm, respectively.

Hexoses: Of the hexoses, glucose, fructose, and galactose are physiologically the most important. Glucose is a major mammalian fuel, found widely in fruits and vegetables as a monosaccharide, in disaccharides such as sucrose, maltose, and lactose, and in polysaccharides such as glycogen and starch. It is converted to other carbohydrates having highly specific functions (e.g., glycogen for storage, ribose in nucleic acids, galactose in lactose of milk, in certain complex lipids, and in combination with protein in glycoproteins). Diseases associated with glucose metabolism include (among others), diabetes mellitus, galactosemia, and glycogen storage diseases.

Fructose is a 6-carbon ketose found in fruit and honey as a monosaccharide, and in sucrose (a disaccharide of fructose and glucose). Diets high in sucrose can lead to large amounts of fructose entering the hepatic portal vein. Although fructose is not absorbed as rapidly as glucose in the jejunum, it is more rapidly glycolyzed by the liver since it bypasses the step in glycolysis catalyzed by phosphofructokinase, the point where metabolic control is exerted on the rate of glucose catabolism. In some animals a significant amount of fructose resulting from the digestion of sucrose is converted to glucose in the intestinal wall prior to passage into the portal circulation. Free fructose is found in seminal plasma and is secreted in quantity into the fetal circulation of ungulates and whales, where it accumulates in amniotic and allantoic fluids. In all of these situations, free fructose represents a potential source of fuel.

Galactose is a component of lactose (milk sugar), which is synthesized in mammary glands. Galactose is found in some vegetable polysaccharides, and is convertible to glucose in the body (mainly in the liver where it is almost entirely removed from the portal circulation following absorption from the intestine). Galactose also forms part of the polysaccharide chain of many glycoproteins.

Disaccharides and Trisaccharides: Disaccharides consist of two monosaccharides joined by a glycosidic bond.

Maltose (or malt sugar) is an intermediate in the intestinal digestion (i.e., hydrolysis) of glycogen and starch, and is found in germinating grains (and other plants and vegetables). It consists of two molecules of glucose in an a-(1,4) glycosidic linkage. Trehalose, which contains two molecules of glucose linked together somewhat differently from maltose, is a major carbohydrate found in the hemolymph of many insects. It is also found in young mushrooms, where it accounts for about 1.5% of their weight. Cellobiose, the repeating disaccharide unit of cellulose, has b-(1,4) glycosidic linkages which are broken by bacterial cellulases, but not by mammalian constitutive digestive enzymes.

Lactose is found in milk, but otherwise does not occur in nature. It consists of galactose and glucose in a b-(1,4) glycosidic linkage.

Sucrose, or cane sugar, consists of glucose and fructose linked in an a-(1,2) glycosidic bond. It is abundant in the plant world and is familiar as table sugar. Sucrose and maltose are readily hydrolyzed by disaccharidases found in the brush border of the small intestine. Hydrolysis of sucrose to glucose and fructose is sometimes called inversion, since it is accompanied by a net change in optical rotation from dextro to levo as the equimolar mixture of glucose and fructose is formed on the mucosal surface. Therefore, the intestinal brush border enzyme that hydrolyzes sucrose (i.e., sucrase), is sometimes called invertase.

A number of trisaccharides also occur free in nature and are consumed by animals. Raffinose contains glucose, fructose, and galactose held together by both a- and b-glycosidic bonds. This trisaccharide is found in abundance in sugar beets, and several other higher plants. Melezitose contains two molecules of glucose and one of fructose and is found in the sap of some coniferous trees.

\ SUMMARY

Chapter 18: Carbohydrate Structure

Carbohydrates are abundant organic compounds on earth, but only make up about 1% of the mammalian body. Complex storage carbohydrates include starch in plants and glycogen in animals. Glucose is the main glucose reserve in the body. Carbohydrates are also present in cell membranes and are important for the structure of cartilage, bone, and plasma proteins. Monosaccharides are simple sugars that can become basic units of more complex molecules. The most important monosaccharides for mammalian metabolism are trioses, pentoses, and hexoses. Pentose sugars are important in animal metabolism and are part of the structure of nucleotides and coenzymes. Hexoses like glucose, fructose, and galactose are physiologically important. Diseases associated with glucose metabolism include diabetes mellitus, galactosemia, and glycogen storage diseases. Disaccharides consist of two monosaccharides joined by a glycosidic bond. Maltose, trehalose, cellobiose, lactose, and sucrose are examples of disaccharides. Trisaccharides like raffinose and melezitose also occur in nature.

\ OUTLINE

I. Introduction

Carbohydrates are the most abundant organic compounds on earth
Carbohydrates constitute only about 1% of the mammalian body

II. Complex Carbohydrates

Storage carbohydrates include starch in plants and glycogen in animals
Glucose utilization is reserved mostly for vital functions
Glucose synthesis from noncarbohydrate sources gains importance over time

III. Carbohydrates in Biological Systems

Carbohydrates are present in cell membranes as the polysaccharide portion of glycoproteins and glycolipids
Carbohydrates are important for the structure of cartilage, bone, and plasma proteins

IV. Monosaccharides

Monosaccharides are simple sugars that may become basic units of more complex molecules
Important monosaccharides for mammalian metabolism include trioses, pentoses, and hexoses
Monosaccharides condense into rings of five or six carbon atoms called furanose or pyranose rings

V. Pentoses, NAD+ and NADP+, NADH and NADPH

Pentose polymers are part of the fibrous material of plants and important in animal metabolism
Ribose is an important constituent of nucleotides, nucleic acids, and coenzymes NAD+ and NADP+
Reduction of NAD+ and NADP+ by dehydrogenases changes the character of the nicotinamide ring

VI. Hexoses

Glucose, fructose, and galactose are physiologically important hexoses
Glucose is a major mammalian fuel and found in various forms
Fructose is a ketose found in fruit and honey, and rapidly glycolyzed by the liver
Galactose is a component of lactose and forms part of the polysaccharide chain of many glycoproteins

VII. Disaccharides and Trisaccharides

Disaccharides consist of two monosaccharides joined by a glycosidic bond
Maltose, trehalose, cellobiose, lactose, and sucrose are common disaccharides
Raffinose and melezitose

\ QUESTIONS

Qcard 1:

Question: What are the most abundant organic compounds on earth?

Answer: Carbohydrates.

Qcard 2:

Question: What are the complex storage carbohydrates in plants and animals?

Answer: Starch in plants and glycogen in animals.

Qcard 3:

Question: How much glucose is stored as liver glycogen in a normal large dog?

Answer: About 100 gm.

Qcard 4:

Question: What are the main glucose reserves in the body that can be exported to blood?

Answer: Liver glycogen.

Qcard 5:

Question: What are some examples of carbohydrates present in cell membranes?

Answer: Polysaccharide portion of glycoproteins and glycolipids.

Qcard 6:

Question: What is the general overall structure of monosaccharides?

Answer: (HCOH)n.

Qcard 7:

Question: What is the convention used to name D- or L-carbohydrates based on?

Answer: The orientation (right or left) of the hydroxyl group on the highest-numbered asymmetric carbon.

Qcard 8:

Question: What are the most important hexoses in mammalian metabolism?

Answer: Glucose, fructose, and galactose.

Qcard 9:

Question: What is the major mammalian fuel?

Answer: Glucose.

Qcard 10:

Question: What are some diseases associated with glucose metabolism?

Answer: Diabetes mellitus, galactosemia, and glycogen storage diseases.

Qcard 11:

Question: What is the disaccharide found in milk?

Answer: Lactose.

Qcard 12:

Question: What is the disaccharide found in cane sugar?

Answer: Sucrose.

MIND MAP

Central Idea: Carbohydrates are important organic compounds that play a crucial role in various biological processes.

Main Branches:

Complex Carbohydrates * Starch in plants * Glycogen in animals
Carbohydrates in Biological Systems * Cell membranes * Intercellular materials * Cartilage and bone * Plasma proteins
Monosaccharides * Trioses (3-carbon atoms) * Pentoses (5-carbon atoms) * Hexoses (6-carbon atoms) * Tetroses (4-carbon sugars) * Heptoses (7-carbon sugars) * Aldose and ketose sugars * D- and L-carbohydrates * Furanose and pyranose rings
Pentoses, NAD+ and NADP+, NADH and NADPH * Pentose polymers * Role of pentose sugars in animal metabolism * Ribose in nucleotides and nucleic acids * NAD+ and NADP+ as coenzymes * Absorption spectra of NADH and NADPH
Hexoses * Glucose as a major fuel * Fructose and its rapid glycolysis by the liver * Galactose and its conversion to glucose * Physiological importance of glucose, fructose, and galactose
Disaccharides and Trisaccharides * Maltose and its role in glycogen and starch digestion * Trehalose in insects and mushrooms * Cellobiose as the repeating unit of cellulose * Lactose in milk * Sucrose as table sugar * Hydrolysis of sucrose and optical rotation change * Trisaccharides like raffinose and melezitose

Sub-branches:

Complex Carbohydrates * Starch in plants * Storage function * Glycogen in animals * Storage function in liver, muscle, and adipose tissue
Carbohydrates in Biological Systems * Cell membranes * Polysaccharide portion of glycoproteins and glycolipids * Intercellular materials

Study Plan: Chapter 18: Carbohydrate Structure

Day 1:

Read and understand the introduction to carbohydrates.
Focus on the abundance of carbohydrates in plants and animals.
Take notes on the different types of complex carbohydrates and their storage locations in the body.
Memorize the amount of glucose stored in different tissues.
Understand the importance of glucose utilization for vital functions.

Day 2:

Study the role of carbohydrates in cell membranes and intercellular materials.
Learn about the significance of carbohydrates in the structure of cartilage, bone, and plasma proteins.
Take note of the physiological actions associated with the carbohydrate moieties of glycoproteins.
Review the concept of monosaccharides as basic units of complex molecules.
Memorize the names and structures of important monosaccharides.

Day 3:

Focus on the classification of monosaccharides based on the number of carbon atoms.
Understand the difference between aldose and ketose sugars.
Memorize the names and structures of trioses, tetroses, pentoses, and hexoses.
Study the naming convention for D- and L-carbohydrates.
Learn about the formation of furanose and pyranose rings in monosaccharides.

Day 4:

Review the formation of triose derivatives during glucose catabolism.
Understand the breakdown of glucose via the hexose monophosphate shunt.
Study the role of pentose polymers in plant material and animal metabolism.
Focus on the importance of ribose in nucleotides, nucleic acids, and coenzymes.
Memorize the structures of NAD+, NADP+, NADH, and NADPH.

Day 5:

Learn about the reduction of NAD+ and NADP+ by dehydrogenases.
Understand the changes in absorption spectra during the reduction process.
Review the physiological significance of hexoses, particularly glucose, fructose, and galactose.
Study the sources and functions of glucose, fructose, and galactose in the body.
Memorize the structures and characteristics of disaccharides and trisaccharides.

Note: Throughout the study plan, make sure to take breaks, review previous topics, and test your understanding through practice questions or quizzes.