6. Of What Value To Plants Is Starch? Of What Value To Animals Is Starch?

Affiliate v. Carbohydrates i/

ane. INTRODUCTION
2. CLASSIFICATION AND Chemistry
3. CARBOHYDRATE METABOLISM IN FISH
4. REFERENCES

Thousand. W. Chow
Food and Agriculture Organization
Rome, Italian republic
J. E. Halver
University of Washington
Seattle, Washington

^1/ Lecture was presented by J. Eastward. Halver

1. INTRODUCTION

Carbohydrates represent a wide group of substances which include the sugars, starches, gums and celluloses. The common attributes of carbohydrates are that they comprise merely the elements carbon, hydrogen and oxygen, and that their combustion will yield carbon dioxide plus ane or more molecules of Water.

The simplest carbohydrates are the three-carbon sugars which figure importantly in intermediary metabolism and the nearly complex are the naturally occurring polysaccharides, primarily of plant, origin. In the diet of animals and fish, two classes of polysaccharides are significant:

(a) structural polysaccharides which are digestible past herbivorous species -cellulose, lignin, dextrans, mannans, inulin, pentosans, pectic acids, algic acids, agar and chitin; and
(b) universally digestible polysaccharides - principally starch.

Carbohydrates brand up three-fourths of the biomass of plants but are present only in small quantities in the animal torso as glycogen, sugars and their derivatives. Glycogen is ofttimes referred to every bit animal starch because it is not present in plants. Derived mono-saccharides such as the sugar acids, amino sugars and the deoxysugars are constituents of all living organisms.

2. Classification AND CHEMISTRY

2.1 Pentoses
2.2 Hexoses
2.3 Disaccharides
2.4 Oligosaccharides
2.5 Polysaccharides

Carbohydrates are classified more often than not according to their degree of complication. Hence, the free sugars such as glucose and fructose are termed monosaccharides; sucrose and maltose, disaccharides; and the starches and celluloses, polysaccharides. Carbohydrates of short chain lengths such as raffinose, stachyose and verbascose, which are three, four and v sugar polymers respectively, are classified as oligosaccharides.

2.1 Pentoses

Pentoses are 5-carbon sugars seldom found in the free country in nature. In plants they occur in polymeric forms and are collectively known every bit pentosans. Thus, xylose and arabinose are the constituents of pentosans nowadays in plant fibres and vegetable gums, respectively. As the saccharide moieties in nucleic acids and riboflavin, ribose and deoxyribose are indispensable constituents of the life procedure. D-ribose has the following chemical structure:

D-Ribose

ii.two Hexoses

The hexoses comprise a big group of sugars. Chief amidst these are: glucose, fructose, galactose and mannose. While glucose and fructose are found free in nature, galactose and mannose occur only in combined form. The hexoses are divided into aldoses and ketoses according to whether they possess aldehydic or ketonic groups. Thus, glucose is an aldo sugar and fructose is a keto sugar. The presence of aymmetric centres in all sugars with three or more carbon atoms gives rise to stereoisomers. Galactose and mannose are stereoisomers of glucose which, theoretically, is just one of 16 stereoisomers. Because the ketohexoses have simply three disproportionate centres, fructose is 1 of eight stereoisomers. The chemic configurations of the four hexoses mentioned are as follows:


D-Glucose	D-Galactose	D-Mannose	D-Fructose

A general miracle, known as mutarotation, is observed in a variety of pentoses and hexoses every bit well as in certain disaccharides. For example, it has been established that two isomers of D-glucose be, hence requiring an additional asymmetric centre in this sugar. It became apparent that D-glucose and well-nigh other sugars have cyclic structures. The position of the hydroxyl group in relation to the ring oxygen characterizes this additional configurations modification. By convention, the positioning of the hydroxyl group on carbon atom 1 on the aforementioned side of the structure as the oxygen ring describes a -modification; and, the positioning of the same hydroxyl grouping on the opposite side of the ring oxygen describes a b -modification.


a -D-Glucose	b -D-Glucose

Carbohydrases, which catalyse the hydrolysis of glycosidic linkages of elementary glycosides, oligosaccharides and polysaccharides often exhibit specificity with regard to substrate configuration. As we shall see later, the specificity for enzyme hydrolysis of certain oligosaccharides helps to explain the poor utilization of this form of carbohydrates in fish nutrition.

Sugars containing the aldo or the keto grouping are capable of reducing copper in alkaline solutions (Fehling's solution) to produce the brick-red colouration of cuprous ions. These sugars are chosen reducing sugars and the reaction, although not specific for reducing sugars, has use for both qualitative and quantitative determinations.

Glucose is widely distributed in small amounts in fruits, constitute juices and honey. Information technology is commercially produced by the acrid or enzyme hydrolysis of grain and root starches. Glucose is of special interest in nutrition because it is the stop-product of carbohydrate digestion in all not-ruminant animals including fish.

Fructose is the simply important ketohexose and is plant in the free state alongside glucose in ripening fruits and dearest. Combined with glucose information technology forms sucrose. Fructose is somewhat sweeter than sucrose and is produced in increasing quantities commercially as a sweetener.

Galactose occurs in milk in combination with glucose. Information technology is also present in oligo-saccharides of plant origin, in combination with both glucose and fructose.

Mannose is nowadays in some plant polysaccharides collectively termed mannans.

2.3 Disaccharides

Disaccharides are condensation products of 2 molecules of monosaccharides. Sucrose is the predominant disaccharide occurring in the gratis form and is the master substance of sugar cane and saccharide beet. It is too formed during germination of legume seeds. Other common disaccharides are maltose and lactose. Maltose is a dimer of glucose, and lactose is a copolymer of galactose and glucose. The two molecules of glucose in maltose are held together in an a -1,four glycosidic linkage whereas the ii hexose entities of galactose are linked at the b -1,four position. Glucose and fructose are combined in an a -1,2 linkage in sucrose. The abbreviated name of sucrose is D-Glu-(a, one® 2)-D-Fru.

a -Maltose

b -Lactose

Sucrose

2.4 Oligosaccharides

The Oligosaccharides raffinose, stachyose and verbascose are present in significant quantities in legume seeds. Raffinose, which is the nearly widespread among the three, consists of one molecule of glucose linked to a molecule of sucrose at the a -ane,6 position. Its abbreviated chemical name is a -D-Gal (1® 6) -a - D -Glu - (1® 2) - b -D-Fru. Further chain elongation at the galactose terminate with another galactose molecule will yield stachyose. These galactose-galactose linkages are all at the a-l,6 position and digestion of these Oligosaccharides past animals requires a highly specific enzyme non elaborated by the animals themselves but by certain leaner present in the animals guts. The gradual disappearance of oligosaccharides from the cotelydons of legume seeds during germination is part of an intricate process offset with uptake of water by the seed. This uptake of wet releases gibberellic acrid which in plow activates the Deoxyribonucleic acid in the seed, thereby triggering the life wheel of the institute. The DNA directs the production of a -galactosidase which is required for the hydrolysis of these Oligosaccharides. Any interference of the Deoxyribonucleic acid transcription process blocks enzyme production and volition be evidenced past continued senescence of the seed and persistence of oligosaccharides in the seed cotelydons.

2.5 Polysaccharides

The polysaccharides correspond a large group of complex carbohydrates which are condensation products of undetermined numbers of sugar molecules. The diverse subgroups are rather ill-divers and there is a lack of agreement on their classification. Most polysaccharides are insoluble in water. Upon hydrolysis with acids or enzymes they eventually yield their elective monosaccharides.

Starch is a high molecular weight polymer of D-glucose and is the principal reserve carbohydrate in plants. Well-nigh starches consist of a mixture of two types of polymers, namely; amylose and amylopectin. The proportion of amylose and amylopectin is generally ane part of amylose and three parts of amylopectin. Enzymes capable of catalyzing the hydrolysis of starch are nowadays in the digestive secretions of animals and fish within their cells. The a-amylases which are found well-nigh in all living cells cleave the a -D-(i® 4) linkages at random and bring about an eventual total conversion of the starch molecule into the reducing sugars. The chief a -amylases of fauna origin are those produced in the salivary gland and the pancreas. Starch is insoluble in water and is stained blue by iodine.

Glycogen is the only complex carbohydrate of animal origin. It exists in express quantities in liver and muscle tissues and acts as a readily available energy source.

Dextrins are intermediate compounds resulting from incomplete hydrolysis or digestion of starch. The presence of a -D-(i® vi) linkages in amylopectin and the inability of a -amylase to carve these bonds give rise to low molecular weight carbohydrate segments called limit dextrins. These residues are acted upon primarily past acidophilic bacteria in the digestive tract.

Cellulose is made up of long bondage of glucose units held together by b -D-(1® 4) linkages. The enzymes which cleave these linkages are not ordinarily nowadays in the digestive secretions of animals and fish although some species of shellfish are believed to elaborate cellulase, the enzyme which catalyzes the hydrolysis of cellulose. Cellulase producing micro-organisms present in the gut of herbivorous animals and fish impart to their host animals the ability to utilize as food the otherwise boxy cellulose.

Other complex polysaccharides in common occurrence are the hemicelluloses and pentosans. Hemicellulose represents a grouping of carbohydrates including araban, xylan, sure hexosans and polyuronides. These substances are generally less resistant to chemical treatment and undergo some degree of enzymatic hydrolysis during normal digestive processes. Pentosans are polymers of either xylose or arabinose as constituents of constitute structural material and vegetable gums, respectively.

3. Sugar METABOLISM IN FISH

3.ane Digestion, Absorption and Storage
3.2 Other Factors Affecting Metabolism
3.iii Energy Transformation

Much of the carbohydrates that enter the diets of animals, including fish, is of constitute origin. Carnivorous fish similar the Atlantic salmon and the Japanese yellowtail, therefore, deal with fiddling carbohydrate,. Indeed, experiments have shown that these species are ill-equipped to handle pregnant quantities of raw carbohydrate, in their diets. On the other hand, omnivores such as the common bother and the channel catfish are able to digest fair amounts of carbohydrates in their diets. The grass carp, a plant eater, subsists primarily on a vegetarian diet.

3.1 Digestion, Absorption and Storage

The ability of animals to assimilate starch depends on their ability to elaborate amylase. All species of fish take been shown to secrete a -amylase. It has also been demonstrated that action of this enzyme was greatest in herbivores. In carnivores such as the rainbow trout and body of water perch, amylase is primarily of pancreatic origin whereas in herbivores the enzyme is widespread throughout the entire digestive tract. In Tilapia mossambica the pancreas has been shown to be the site of greatest amylase action followed by the upper intestine. Although the digestion of starch and dextrin by the carnivorous rainbow trout was shown to decrease progressively as levels of the carbohydrates were increased beyond the 20 pct level, the fish could effectively utilize up to 60 percent glucose, sucrose or lactose in the diet. This demonstrates that, contrary to earlier belief, carnivorous fish are capable of efficiently utilizing elementary carbohydrate as a primary energy source.

The crystalline structure of starch appears also to influence its set on by amylase as evidenced past the two-fold increase in metabolizable energy content of fully cooked (gelatinized) maize in feeding trials with channel catfish. Rainbow trout have also been shown to have a higher tolerance for carbohydrate (present every bit wheat starch) in the nutrition when it was cooked. The process of gelatinization involves both oestrus and water. If an aqueous suspension of starch is heated, the granules do not change in advent until a sure critical temperature is reached. At this signal some of the starch granules cracking and simultaneously lose their crystallinity. The critical temperature is that at which hydrogen bonds of the starch molecule loosen to permit complete hydration, leading to a phenomenon known as "swelling".

Alpha-amylase, promotes a more or less random fragmentation of the starch molecule by hydrolyzing at the a -D-(l® 4) glucosidic bonds in the inner and outer chains of the compound. The outcome of complete hydrolysis of the amylose component are maltose and D-glucose, while the amylopectin component is reduced to maltose, D-glucose and branched limit dextrins. Every bit a issue of these action patterns by a -amylase on starch, other enzymes are needed for consummate hydrolysis of starch to D-glucose in fish. In this regard, it has been demonstrated that fifty-fifty the cannibal bounding main bream possess the power to assimilate maltose. On the other hand, cellulase and a -galactosidase accept non been shown to be secreted past fish although cellulase of bacterial origin is present in the gut of most species of carps. The lack of a -galactosidase may partly explain the poor response by fish to dietary soybean meal which contains meaning levels of the galactosidic oligosaccharides raffinose, and stachyose. Every bit has been pointed out earlier, these oligosaccharides do undergo enzymatic hydrolysis during the germination process to yield galactose and sucrose. It would, therefore, appear that the nutritive value of soybean meal volition exist enhanced if the majority of this indigestible starch is offset transformed. This can be achieved by soaking the beans for 48 hours prior to processing for meal production. It should as well be pointed out that the nutritive value of pulses and other legume seeds can besides be improved for fish since oligosaccharides constitute a large portion of the carbohydrates in legume seeds.

Information on glucose absorption past fish are scanty. Piece of work with goldfish has shown that active transport of glucose is coupled with Na⁺ transport every bit in most mammals. It is generally believed that absorption takes place on the mucosal surface of intestinal cells. The mono-saccharides which result from carbohydrate digestion consist primarily of glucose, fructose, galactose, mannose, xylose and arabinose. Although the rates of absorption of these sugars have been determined for many country mammals, like information for fish is not bachelor.

Glucose does not appear to exist a superior energy source for fish over poly peptide or fat although digestible carbohydrates practise spare protein for tissue building. Also, dissimilar in mammals, glycogen is non a meaning storage depot of energy despite evidence of an agile and reversible Emden - Meyerhoff pathway in fish. The more efficient metabolism of amino acids over glucose for energy could be due to the ability of fish to excrete nitrogenous waste as ammonia from their gills without the high toll of energy in converting the waste to urea.

3.2 Other Factors Affecting Metabolism

Apart from genetic adaptation, climatic factors also play an of import part in carbohydrate metabolism in fish. Acclimation in fish, in essence, reflects enzyme acclimation, since the creature's ability to survive depends largely upon its ability to carry out normal metabolic functions. Some enzymes for metabolic acclimation evidence good compensation while others do not. The enzymes associated with free energy liberation (enzymes of glycolysis, pentose shunt, tricarboxylic acid wheel, electron transport and fat acrid oxidation) exhibit temperature compensation whereas, those enzymes dealing largely with the degradation of metabolic products prove poor or reverse compensation (see Table 1).

Tabular array 1 Enzymes Field of study to Metabolic Acclimation ^1/

Enzymes exhibiting compensation	Enzymes exhibiting reverse or no compensation
phosphofructokinase	catalase
aldolase	peroxidase
lactic dehydrogenase	acid phosphatase
six-phosphogluconate dehydrogenase	D-amino acid oxidase
succinic dehydrogenase	Mg-ATP ase
malic dehydrogenase	choline acetyl transferase
cytochrome oxidase	acetylcholine esterase
succinate-cytochrome C reductase	alkaline phosphatase
NAD-cytochrome C reductase	allantoinase
aminoacyl transferase	uricase
Na-One thousand-ATPase	amylase
protease	lipase
	malic enzyme
	glucose-six-phosphate dehydrogenase

^1/Adapted from: Comparative Fauna Physiology, edited past C.L. Prosser, 1973

It is interesting to note that two key enzymes involved in sugar metabolism, amylase and glucose-6-phosphate dehydrogenase, together with an enzyme involved in fatty digestion, lipase, testify no temperature compensation. It is not certain if this is in any way connected with the cessation of feeding by fish at depression temperatures. The molecular mechanism of thermal acclimation are not well understood and may consist of changes in synthesis or amounts of a given enzyme. Differences in kinetics, changes in the proportion of isoenzymes suitable for particular temperatures, and changes in co-factors such every bit lipids, co-enzymes, or other factors such equally pH and ions may be of import in the animate being'south aligning to temperature changes.

3.3 Energy Transformation

Despite species differences in the tolerance of dietary carbohydrates it is by and large believed that the primary end-product of carbohydrate digestion, glucose, is metabolized in a mode prevailing in all cells, i.east., via the reversible Emden-Meyerhoff pathway. In this pathway, glucose has merely 1 principal fate: phosphorylation to glucose-half dozen-phosphate. The major metabolic transformations are depicted as follows:

Reversible arrows show reaction step or steps catalyzed by aforementioned enzymes in both directions.

Broken arrows show reactions over many intermediate steps.

Paired solid arrows show different enzymes involved in the two directions of the reaction.

(Adapted from: Principles of Biochemistry, past A. White, et al., 1978)

All transformations keep with a loss of free energy. Thus, the formation of two moles of lactate from glucose-6-phosphate occurs with gratuitous free energy change of D G^o = -22000 cal/mole. The internet result is the formation of 4 molecules of ATP. A functional reversal of this transformation can merely occur via a different sequence requiring the input of vi ATP molecules per mole of glucose-6-phosphate recovered.

Cells practise not store glucose or glucose-6-phosphate. The readily bachelor storage form is glycogen which is made from glucose-i-phosphate by one pathway and returned by another. Although in mammalian cells glucose-half dozen-phosphate is transformed into fat acids, such transformation does not announced to take place in fish. Studies with the common carp indicate that the forerunner for lipogenesis is citrate formed when amino acids are actively metabolized through the tricarboxylic acid cycle.

The major form of utilizable energy in all cells is ATP. In most cells this free energy currency is generated by the oxidation of NADH past the mitochondrial electron-transport systems. The reductants of NAD⁺ for this procedure are intermediates derived from the TCA cycle and fatty acids. The energy yield from glucose in a respiring system may be summarized in the following sequence of reactions:

Reaction		ATP Yield
1. glucose® fructose-1,six-diphosphate		-2
2. 2 triose phosphate® 2,3-phosphoglyceric acid		+2
3. 2 NAD⁺ ® 2 NADH® 2 NAD⁺		+vi
4. 2 phosphoenol pyruvate® two pyruvic acid		+two
5. 2 pyruvic acrid® 2 acetyl CoA + 2 CO₂
	2 NAD⁺ ® 2 NADH® ii NAD²	+6
6. two Acetyl CoA® four CO₂		+24
Overall:
	C_sixH₁₂O₆ + 6O₂ ® half dozen CO₂ + half dozen H₂O	+38

four. REFERENCES

Prosser, C.L. (ed.),1973 Comparative animal physiology. Philadelphia, West.B. Saunders Company, 1011 p. 3rd ed.

White, A., et al., 1978 Principles of biochemistry. New York, McGraw-Colina Book Visitor, 1492 p. 6th ed.

Source: https://www.fao.org/3/x5738e/x5738e06.htm

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