Animal Nutrition- Carbohydrates

In general, carbohydrates are neutral chemical compounds containing the elements carbon, hydrogen and oxygen and have the empirical formula (CH2O)n, where n is 3 or more. However, some compounds with general properties of the carbohydrates also contain phosphorus, nitrogen or sulphur; and others, e.g. deoxyribose (C5H10O4), do not have hydrogen and oxygen in the same ratio as that in water. The carbohydrate group contains polyhydroxy aldehydes, ketones, alcohols and acids, their simple derivatives,and any compound that may be hydrolysed to these.

CLASSIFICATION OF CARBOHYDRATES

The simplest sugars are the monosaccharides, which are divided into subgroups – trioses (C3H6O3), tetroses (C4H8O4), pentoses (C5H10O5), hexoses (C6H12O6) and heptoses (C7H14O7) depending upon the number of carbon atoms present in the molecule.The trioses and tetroses occur as intermediates in the metabolism of other carbohydrates and their importance will be considered in Chapter 9. Monosaccharides may be linked together, with the elimination of one molecule of water at each linkage, to produce di-, tri-, tetra- or polysaccharides, containing, respectively, two, three, four or larger numbers of monosaccharide units.

The term sugar is generally restricted to those carbohydrates containing fewer than ten monosaccharide residues, while the name oligosaccharides (from the Greek oligos,a few) is frequently used to include all sugars other than the monosaccharides. Polysaccharides, also called glycans, are polymers of monosaccharide units. They are classified into two groups, the homoglycans, which contain only a single type of monosaccharide unit, and the heteroglycans, which on hydrolysis yield mixtures of monosaccharides and derived products. The molecular weight of polysaccharides varies from as little as about 8000 in some plant fructans to as high as 100 million inthe amylopectin component of starch. Hydrolysis of these polymers to their constituent sugars can be effected by the action of either specific enzymes or acids.



The complex carbohydrates are an ill-defined group of compounds that contain carbohydrates in combination with non-carbohydrate molecules. They include the glycolipids and glycoproteins.

MONOSACCHARIDES

Structure

The monosaccharide sugars occur in a number of isomeric forms.Thus, glucose and fructose (both hexoses) are structural isomers, glucose having an aldehyde group and fructose having a ketone group. Both of these sugars occur in two mirror image, stereoisomeric forms, dextro and laevo (D- and L-), according to the orientation of the OH group at carbon atom 5. Biologically the D-forms are the more important.

Under physiological conditions, sugars exist mainly in another isomeric form, as ring or cyclic structures, rather than straight chains. Glucose forms a pyranose ring and fructose most commonly forms a furanose ring. Each ring structure can occur in two isomeric forms, designated alpha and beta. Starch and glycogen are polymers of the alfa-form, while cellulose is a polymer of the beta-form.

Properties of the monosaccharides

Because of the presence of an active aldehyde or ketone grouping, the monosaccharides act as reducing substances. The reducing properties of these sugars are usually demonstrated by their ability to reduce certain metal ions, notably copper or silver,in alkaline solution. The aldehyde and ketone groups may also be reduced chemically, or enzymatically, to yield the corresponding sugar alcohols. Examples of oxidation and reduction products are given in the section dealing with monosaccharide derivatives.

Pentoses

The most important members of this group of simple sugars are the aldoses L-arabinose,
D-xylose and D-ribose, and the ketoses D-xylulose and D-ribulose.

diagram..........................

L-Arabinose occurs as pentosans in arabinans. It is a component of hemicelluloses and it is found in silage as a result of their hydrolysis. It is also a component of gum arabic and other gums. D-Xylose also occurs as pentosans in xylans. These compounds form the main chain in grass hemicelluloses. Xylose, along with arabinose,is produced in considerable quantities when herbage is hydrolysed with normal sulphuric acid. D-Ribose is present in all living cells as a constituent of ribonucleic acid (RNA), and it is also a component of several vitamins and coenzymes.

diagram..............

The phosphate derivatives of D-xylulose and D-ribulose occur as intermediates in the pentose phosphate metabolic pathway.

Hexoses

Glucose and fructose are the most important naturally occurring hexose sugars,while mannose and galactose occur in plants in a polymerised form as mannans and galactans.

D-Glucose, grape sugar or dextrose, exists in the free state as well as in combined form. The sugar occurs free in plants, fruits, honey, blood, lymph and cerebrospinal fluid, and it is the sole or major component of many oligosaccharides, polysaccharides and glucosides. In the pure state, glucose is a white crystalline solid and, like all sugars, is soluble in water.

D-Fructose, fruit sugar or laevulose, occurs free in green leaves, fruits and honey. It also occurs in the disaccharide sucrose and in fructans. Green leafy crops usually contain appreciable amounts of this sugar, both free and in polymerised form. The free sugar is a white crystalline solid and has a sweeter taste than sucrose.The exceptionally sweet taste of honey is due to this sugar.

D-Mannose does not occur free in nature but exists in polymerised form as mannan and also as a component of glycoproteins. Mannans are found widely distributed in yeasts, moulds and bacteria.

D-Galactose does not occur free in nature except as a breakdown product during fermentation. It is present as a constituent of the disaccharide lactose, which occurs in milk. Galactose also occurs as a component of the anthocyanin pigments, galactolipids, gums and mucilages.

Heptoses

D-Sedoheptulose is an important example of a monosaccharide containing seven
carbon atoms and occurs, as the phosphate, as an intermediate in the pentose phos-
phate metabolic pathway.

diagram...................

MONOSACCHARIDE DERIVATIVES

Phosphoric acid esters

The phosphoric acid esters of sugars play an important role in a wide variety of metabolic reactions in living organisms. The most commonly occurring derivatives are those formed from glucose, the esterification occurring at either carbon atoms 1 or 6 or both.

diagram............

Amino sugars

If the hydroxyl group on carbon atom 2 of an aldohexose is replaced by an amino group (–NH2), the resulting compound is an amino sugar. Two such naturally occurring important compounds are D-glucosamine, a major component of chitin, and D-galactosamine, a component of the olysaccharide of cartilage.

diagram.............

Deoxy sugars

Replacement of a hydroxyl group by hydrogen yields a deoxy sugar. The derivative of ribose, deoxyribose, is a component of deoxyribonucleic acid (DNA). Similarly,deoxy derivatives of the two hexoses, galactose and mannose, occur as fucose and rhamnose, respectively, these being components of certain heteropolysaccharides.

diagram................

Sugar acids

The aldoses can be oxidised to produce a number of acids, of which the most important are:

diagram...........

In the case of glucose, the derivatives corresponding to these formulae are gluconic, glucaric and glucuronic acids, respectively. Of these compounds, the uronic acids, particularly those derived from glucose and galactose, are important components of a number of heteropolysaccharides.

Sugar alcohols

Simple sugars can be reduced to polyhydric alcohols; for example, glucose yields sorbitol, galactose yields dulcitol, and both mannose and fructose yield mannitol. Mannitol occurs in grass silage and is formed by the action of certain anaerobic bacteria on the fructose present in the grass.

diagram..............

Glycosides

If the hydrogen of the hydroxyl group attached to the carbon 1 atom of glucose is replaced by esterification, or by condensation, with an alcohol (including a sugar molecule) or a phenol, the derivative so produced is termed a glucoside. Similarly galactose forms galactosides and fructose forms fructosides.The general term glycoside is used collectively to describe these derivatives and the linkage is described as a glycosidic bond.

Oligosaccharides and polysaccharides are classed as glycosides, and these compounds yield sugars or sugar derivatives on hydrolysis. Certain naturally occurring glycosides contain non-sugar residues. For example, the nucleosides contain a sugar combined with a heterocyclic nitrogenous base.

table..........

The cyanogenetic glycosides liberate hydrogen cyanide (HCN) on hydrolysis; because of the toxic nature of this compound, plants containing this type of glycoside are potentially dangerous to animals. The glycoside itself is not toxic and must be hydrolysed before poisoning occurs. However, the glycoside is easily broken down to its components by means of an enzyme that is usually present in the plant. An example of a cyanogenetic glycoside is linamarin (also called phaseolunatin), which occurs in linseed, Java beans and cassava. If wet mashes or gruels containing these foods are given to animals, it is advisable to boil them when mixing in order to inactivate any enzyme present. On hydrolysis, linamarin yields glucose, acetone and hydrogen cyanide.

OLIGOSACCHARIDES

Disaccharides

A large number of disaccharide compounds are theoretically possible, depending upon the monosaccharides present and the manner in which they are linked.The most nutritionally important disaccharides are sucrose, maltose, lactose and cellobiose,which on hydrolysis yield two molecules of hexoses:

chain.............

Sucrose is formed from one molecule of alfa-D-glucose and one molecule of beta-D-fructose joined together through an oxygen bridge between their respective carbonatoms 1 and 2. As a consequence, sucrose has no active reducing group.

diagram..........

Sucrose is the most ubiquitous and abundantly occurring disaccharide in plants, where it is the main transport form of carbon.This disaccharide is found in high concentration in sugar cane (200 g/kg) and in sugar beet (150–200 g/kg); it is also present in other roots such as mangels and carrots, and it occurs in many fruits. Sucrose is easily hydrolysed by the enzyme sucrase or by dilute acids.When heated to a temperature of 160 °C it forms barley sugar and at a emperature of 200 °C it forms caramel.

Lactose, or milk sugar, is a product of the mammary gland. Cow’s milk contains 43–48 g/kg lactose. It is not as soluble as sucrose and is less sweet, imparting only a faint sweet taste to milk. Lactose is formed from one molecule of beta-D-glucose joined to one of beta-D-galactose in a beta-(1:4)-linkage and has one active reducing group.

diagram..........

Lactose readily undergoes fermentation by a number of organisms, including Streptococcus lactis. This organism is responsible for souring milk by converting the lactose into lactic acid (CH3.CHOH.COOH). If lactose is heated to 150 °C it turns yellow; at a temperature of 175 °C the sugar is changed into a brown compound, lactocaramel. On hydrolysis lactose produces one molecule of glucose and one molecule of galactose.

Maltose, or malt sugar, is produced during the hydrolysis of starch and glycogen by dilute acids or enzymes. It is produced from starch during the germination of barley by the action of the enzyme amylase. The barley, after controlled germination and drying, is known as malt and is used in the manufacture of beer and Scotch malt whisky. Maltose is water-soluble, but it is not as sweet as sucrose. Structurally it consists of two alfa-D-glucose residues linked in the alfa-1,4 positions; it has one active reducing group.

diagram...........

Cellobiose does not exist naturally as a free sugar, but it is the basic repeating unit of cellulose. It is composed of two beta-D-glucose residues linked through a beta-(1:4)-bond. This linkage cannot be split by mammalian digestive enzymes. It can, however, be split by microbial enzymes. Like maltose, cellobiose has one active reducing group.

diagram..............

Trisaccharides

Raffinose and kestose are two important naturally occurring trisaccharides.They are both non-reducing and on hydrolysis produce three molecules of hexose sugars:

chain..............

Raffinose is the commonest member of the group, occurring almost as widely as sucrose in plants. It exists in small amounts in sugar beet and accumulates in molasses during the commercial preparation of sucrose. Cotton seed contains about 80 g/kg of raffinose. On hydrolysis, this sugar produces glucose, fructose and galactose.

Kestose and its isomer isokestose occur in the vegetative parts and seeds of grasses. These two trisaccharides consist of a fructose residue attached to a sucrose molecule.

Tetrasaccharides

Tetrasaccharides are made up of four monosaccharide residues. Stachyose, a member of this group, is almost as ubiquitous as raffinose in higher plants and has been isolated from about 165 species. It is a non-reducing sugar and on hydrolysis produces two molecules of galactose, one molecule of glucose and one of fructose:

chain..........

POLYSACCHARIDES

Homoglycans

These carbohydrates are very different from the sugars. The majority are of high molecular weight, being composed of large numbers of pentose or hexose residues.Homoglycans do not give the various sugar reactions characteristic of the aldoses and ketoses. Many of them occur in plants either as reserve food materials such as starch or as structural materials such as cellulose.

Arabinans and xylans

These are polymers of arabinose and xylose, respectively. Although homoglycans based on these two pentoses are known, they are more commonly found in combination with other sugars as constituents of heteroglycans.

Glucans

Starch is a glucan and is present in many plants as a reserve carbohydrate. It is most abundant in seeds, fruits, tubers and roots. Starch occurs naturally in the form of granules, whose size and shape vary in different plants. The granules are built up in concentric layers, and although glucan is the main component of the granules they also contain minor constituents such as protein, fatty acids and phosphorus compounds, which may influence their properties.

Starches differ in their chemical composition and, except in rare instances, are mixtures of two structurally different polysaccharides, amylose and amylopectin. The proportions of these present in natural starches depend upon the source,although in most starches amylopectin is the main component, amounting to about 70–80 per cent of the total. An important qualitative test for starch is its reaction with iodine: amylose produces a deep blue colour and amylopectin solutions produce a blue–violet or purple colour.

Amylose is mainly linear in structure, the alfa-D-glucose residues being linked between carbon atom 1 of one molecule and carbon atom 4 of the adjacent molecule. A small proportion of alfa-(1:6) linkages may also be present.Amylopectin has a bush like structure containing primarily alfa-(1:4) linkages, but it also has an appreciable number of alfa-(1:6) linkages.

diagram.............

Starch granules are insoluble in cold water, but when a suspension in water is heated the granules swell and eventually gelatinise. On gelatinisation, potato starch granules swell greatly and then burst open; cereal starches swell but tend not to burst.

Animals consume large quantities of starch in cereal grains, cereal by-products and tubers.

Glycogen is a term used to describe a group of highly branched polysaccharides isolated from animals or microorganisms. The molecules can be hydrolysed rapidly in conditions requiring the mobilisation of glucose, such as exercise and stress. Glycogens occur in liver, muscle and other animal tissues. They are glucans, analogous to amylopectin in structure, and have been referred to as ‘animal starches’. Glycogen is the main carbohydrate storage product in the animal body and plays an essential role in energy metabolism.

Dextrins are intermediate products of the hydrolysis of starch and glycogen:

chain..........

Dextrins are soluble in water and produce gum-like solutions. The higher members of these transitional products produce a red colour with iodine, while the lower members do not give a colour. The presence of dextrins gives a characteristic flavour to bread crust, toast and partly charred cereal foods.

Cellulose is the most abundant single polymer in the plant kingdom, forming the fundamental structure of plant cell walls. It is also found in a nearly pure form in cotton. Pure cellulose is a homoglycan of high molecular weight in which the repeating unit is cellobiose. Here the beta-glucose residues are 1,4-linked.

diagram............

In the plant, cellulose chains are formed in an ordered manner to produce compact aggregates (microfibrils), which are held together by both inter- and intramolecular hydrogen bonding. In the plant cell wall, cellulose is closely associated, physically and chemically, with other components, especially hemicelluloses and lignin.

Callose is a collective term for a group of polysaccharides consisting of beta-(1,3)-and frequently beta-(1,4)-linked glucose residues.These beta-glucans occur in higher plants as components of special walls appearing at particular stages of development.A large part of the endosperm cell wall of cereal grains is composed of beta-glucans of this type. They are also deposited by higher plants in response to wounding and infection.

Fructans

These occur as reserve material in roots, stems, leaves and seeds of a variety of plants, but particularly in the Compositae and Gramineae. In the Gramineae, fructans are found only in temperate species. These polysaccharides are soluble in cold water and are of a relatively low molecular weight. All known fructans contain beta-D-fructose residues joined by 2,6 or 2,1 linkages. They can be divided into three groups:

(1) the levan group, characterised by 2,6 linkages;

(2) the inulin group, con-
taining 2,1 linkages; and

(3) a group of highly branched fructans found,

for example,in couch grass (Agropyron repens) and in wheat endosperm. This group contains
both types of linkage.

Most fructans on hydrolysis yield, in addition to D-fructose, a small amount of D-glucose, which is derived from the terminal sucrose unit in the fructan molecule.The structure of a typical grass fructan is depicted here:

diagram...........

Galactans and mannans

These are polymers of galactose and mannose, respectively, and occur in the cell walls of plants. A mannan is the main component of the cell walls of palm seeds, where it occurs as a food reserve and disappears during germination. A rich source of mannan is the endosperm of nuts from the South American tagua palm tree (Phytelephas macrocarpa); the hard endosperm of this nut is known as ‘vegetable ivory’.The seeds of many legumes, including clovers, trefoil and lucerne, contain galactans.

Glucosaminans

Chitin is the only known example of a homoglycan containing glucosamine, being a linear polymer of acetyl-D-glucosamine. Chitin is of widespread occurrence in lower animals and is particularly abundant in Crustacea, in fungi and in some green algae.After cellulose, it is probably the most abundant polysaccharide of nature.

Heteroglycans

Pectic substances

Pectic substances are a group of closely associated polysaccharides that are soluble in hot water and occur as constituents of primary cell walls and intercellular regions of higher plants.They are particularly abundant in soft tissues such as the peel of citrus fruits and sugar beet pulp. Pectin, the main member of this group, consists of a linear chain of D-galacturonic acid units in which varying proportions of the acid groups are present as methyl esters.The chains are interrupted at intervals by the insertion of L-rhamnose residues. Other constituent sugars, e.g. D-galactose, L-arabinose and D-xylose, are attached as side chains. Pectic acid is another member of this class of compounds; it is similar in structure to pectin but is devoid of ester groups.Pectic substances possess considerable gelling properties and are used commercially in jam making.

Hemicelluloses

Hemicelluloses are defined as alkali-soluble cell wall polysaccharides that are closely associated with cellulose. The name hemicellulose is misleading and implies erroneously that the material is destined for conversion to cellulose. Structurally,hemicelluloses are composed mainly of D-glucose, D-galactose, D-mannose, D-xylose and L-arabinose units joined together in different combinations and by various glycosidic linkages. They may also contain uronic acids.
Hemicelluloses from grasses contain a main chain of xylan made up of beta-(1:4)-linked D-xylose units with side chains containing methylglucuronic acid and frequently glucose, galactose and arabinose.

Exudate gums and acid mucilages

Exudate gums are often produced from wounds in plants, although they may arise as natural exudations from bark and leaves. The gums occur naturally as salts, especially of calcium and magnesium, and in some cases a proportion of the hydroxyl groups are esterified, usually as acetates. Gum arabic (acacia gum) has long been a familiar substance; on hydrolysis it yields arabinose, galactose, rhamnose and glucuronic acid. Acidic mucilages are obtained from the bark, roots, leaves and seeds of a variety of plants. Linseed mucilage is a well-known example that produces arabinose, galactose, rhamnose and galacturonic acid on hydrolysis.

Hyaluronic acid and chondroitin

These two polysaccharides have a repeating unit consisting of an amino sugar and D-glucuronic acid. Hyaluronic acid, which contains acetyl-D-glucosamine, is present in the skin, the synovial fluid and the umbilical cord. Solutions of this acid are viscous and play an important part in the lubrication of joints. Chondroitin is chemically similar to hyaluronic acid but contains galactosamine in place of glucosamine. Sulphate esters of chondroitin are major structural components of cartilage, tendons and bones.

LIGNIN

Lignin, which is not a carbohydrate but is closely associated with this group of compounds, confers chemical and biological resistance to the cell wall, and mechanical strength to the plant. Strictly speaking the term ‘lignin’ does not refer to a single, well-defined compound but is a collective term that embraces a whole series of closely related compounds. Lignin is a polymer that originates from three derivatives of phenylpropane: coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The lignin molecule is made up of many phenylpropanoid units associated in a complex cross-linked structure: