DIGESTION & METABOLISM OF CARBOHYDRATE,FAT AND PROTEIN

IN NON-RUMINANTS

K.GURU MOHAN REDDY

I.D.No. TVM/2016-13

DEPARTMENT OF ANIMAL NUTRITION,

COLLEGE OF VETERINARY SCIENCE,

SRI VENTESWARA VETERINARY SCIENCE,

TIRUPATI.

CARBOHYDRATES• DIGESTION AND ABSORPTION

• Salivary amylase or ptyalin initiates the digestion of polysaccharide, starch and lts derivatives, dextrin and continues the digestion in the stomach ( for only 15-30 mints)

• During this time up to 60-75 % of starches and dextrin's may be converted into maltose.

• The enzyme acts only on alpha-1,4 linkages . So oligo-1,6-glucosidase is needed to act at the 1,6 branch points of amylopectin's.

• Enzymes from pancreas and intestinal glands

• Pancreatic amylase requires inorganic ions for activity.

• Maltase,lactase,sucrose in the intestinal juice hydrolyze the respective

disaccharides mostly during the process of their absorption in the

brush border surrounding each villus.

• Absorption of monosaccharide's may result by either passive diffusion

or active transport.

• Fructose , mannose and other pentose's are absorbed by active

transport which require energy and sodium ions.

• Rate of absorption of hexoses; galactose- fastest, then glucose and

fructose.

• Rate of absorption by birds is rapid.

• Glucose absorption more rapid than xylose, which in turn is absorbed

faster than arabinose.

Metabolism of carbohydrates

• Digestion of carbohydrates yields primarily glucose, fructose and

galactose.

• In the liver, fructose and galactose are converted to glucose.

• There is always a basal requirement of glucose

• A continued supply needed for RBC and NERVOUS SYSTEM.

• Glucose is precursor of lactose in the mammary glands.

• It is needed by adipose tissue to produce glycerol.

• It is needed to continually produce intermediates in the citric acid

cycle.

• So glucose may be used as an immediate source of energy or may be

used to store as glycogen in liver/ muscle or may be used to produce

glycerol for triglyceride synthesis.

Glucose catabolism

• Glucose is phosphorylated to glucose-6-phosphate and it is oxidized

by one of the three pathways:

• 1) Embden-Meyerhof-Parnas scheme of glycolysis

• 2) Hexose-monophosphate shunt or pentose cycle

• 3) Glucuronate, uronate or the carbon-6 pathway

• 1. It is catabolized primarily especially in skeletal muscle, through the Embden-Meyerhof-Parnas scheme of glycolysis to pyruvic acid and

• through citric acid cycle (tricarboxylic acid cycle or Krebs cycle) and oxidative phosphorylation (cytochrome system), pyruvate is oxidized to C02 and H20.

• Glycolytic cycle exists in the cytosol while citric acid cycle and oxidative phosphorylation require oxygen and are thus aerobic.

• The latter take place within the mitochondrion. Under anaerobic conditions, glycolysis terminates in many cases with the formation of lactic acid from pyruvic acid.

• Efficiency of energy conservation of glucose is about 39% with the loss appearing as waste heat

2. Hexose-monophosphate shunt or pentose cycle (pentose shunt or phosphogluconate oxidative shunt):

• It is important in liver, mammary gland and adipose tissue.

• This pathway is needed to synthesize 5- carbon sugar (ribose) and reduced coenzyme NADPH2 .

• 3. Glucuronate, uronate or the carbon-6 pathway:

• The product of this pathway include uridine diphosphoglucuronic acid, which is involved in a number of conjugating reactions that contain glucuronic acid, such as mucopolysaccharides.

• The pathways is also important in xylulose metabolism and ascorbic acid synthesis

• ATP Generated from Complete Oxidation of Glucose

• 1. Glycolysis 2ATP

• 2. Citric acid Cycle 2ATP

• 3. Oxidative Phosphorylation (*)

a. NADH from glycolysis 1x2=2 (0r) 1x3 = 3

b. NADH pyruvate to acetyl CoA 1x3 = 3

c. NADH from citric acid cycle 3x3 = 9

d. FADH2 from citric acid cycle 1x2 = 2

16 x 2 = 32 ATP; 17x2= 34

Total =36 ATP or 38

Glucose to Glycogen

• The excess energy consumed is stored as either liver or muscle glycogen or as fat for

future needs.

• When blood glucose is elevated following a meal containing carbohydrate, insulin is

secreted which triggers the formation of glycogen. UTP is used for synthesis.

• Blood glucose levels: Ruminants 40 to 60mg/100ml

• Non-ruminants 80 to 120 mg %.

• Birds have higher blood-sugar values than do mammals.

• Cold-blooded animals (frog) have 20 mg %.

Glycogen to Glucose

• 1. In the muscles, under the influence of epinephrine, glycogen is

converted to glucose-6-p and enters the glycolytic cycle to provide

ATP.

• As muscle has no glucose-6-phosphatase and the glucose-6-

phosphate can't diffuse out of the cells, this metabolite can only be

used at the cell for energy.

• 2.In the liver, however, glycogenolysis occurs due to the hormone

glucagon secreted by the pancreas in response to the low blood

glucose level .

• The resulting glucose-6-phosphate is hydrolyzed by glucose-6-

phosphatase present in the liver cells releasing glucose into the

circulation where it can be utilized by the brain and muscle for energy.

• Using glycogen as a storage element is 97% as efficient as using

glucose directly.

• Glycogen reserve is short lived. A 24-hour fast will reduce the levels

to nearly zero. Glycogen stores have to be constantly replenished.

Glucose to Fat• The ability of the liver and other tissues to store sugar as glycogen is

limited.

• When the carbohydrate intake regularly exceeds the current need of the

body for energy purposes, sugar is transformed into fat .

• The formation of body fat from carbohydrate food was first demonstrated

by Lawes and Gilbert in 1859 by means of slaughter experiments.

• Formation of fat from glucose involves the synthesis of two components,

fatty acids and glycerol.

• Oxygen Debt During Prolonged Strenuous Exercise

• In case of extreme exertion, need for ATP exceeds the ability of the animal

to provide oxygen to the tissues to keep the oxidative phosphorylation and

TCA cycle operative.

• As a result glucose metabolism stops at pyruvate and pyruvate concentration

increases in muscle.

• However, in the presence of lactate dehydrogenase, pyruvate is promptly

reduced to lactate and NADH2 (which was produced in the reaction

glyceraldehyde-3-P to 1-3-diphosphoglycerate) is oxidized back to NAD+.

• This oxidized cofactor can then be returned to react with

glyceraldehyde-3-P and allow glycolysis to continue.

Glucose pyruvate + 2 ATP

NADH2

Lactate dehydrogenase

NAD+

Lactate

• Lactate diffuses from the muscle cell into the blood, carried to the

liver. It is converted back to pyruvate by lactate dehydrogenase.

• The pyruvate is then converted to glucose by gluconeogenesis and the

released glucose then diffuses back into the circulation to return to the

skeletal muscle (and brain) to be reduced again to lactate via the

glycolytic cycle.

• This cycle of reactions is called the Cori cycle.

• In this way ATP continues to be generated to a limited degree in spite of a dearth of oxygen.

• The organism has thus been able to function in spite of a limited tissue oxygen supply.

DIGESTION AND METABOLISM OF PROTEIN

• Dietary proteins are hydrolyzed to their constituent amino acids,

absorbed and transported to the liver- via the hepatic portal vein. Some

amino acids appear in the lymph but the amounts are small.

• Protein digestion begins in the stomach with significant denaturation

by hydrochloric acid followed by peptic digestion (Rennin in young

calves; pepsin).

• Pepsin cleaves bonds adjacent to aromatic amino acids and splits the

polypeptide into large peptides and relatively few amino acids.

• In duodenum the ingesta come in contact with the pancreatic and bile

secretion.

• The pancreatic enzymes - trypsin, chymotrypsin & carboxypeptidase

And enzymes of the intestinal mucosa - amino peptidase and

dipeptidase -complete the digestion of proteins to free amino acids.

• Pancreatic nucleases -ribonuclease, deoxvribonuclease - perform

digestion of dietary nucleic acids present in every plant and animal

cell.

• Amino acids are absorbed into blood and reached the portal

circulation.

• No peptide appear in the portal blood.

• Rate of absorption of amino acids is maximum in the proximal two-

thirds of the small intestine.

• It is an active type, similar to glucose, in which the transport of

sodium is involved.

• Tripeptides are absorbed more rapidly than dipeptides, which are in

turn faster than free amino acids and these peptides are hydrolyzed by

peptidases.

• There appears to be a competition for absorption of free amino acids

within groups i.e., acidic, basic, neutral and imino acids.

• Natural L-forms are absorbed more rapidly than D-forms.

• Neutral amino acids are absorbed rapidly than basic amino acids.

• Vitamin B6 also appears to greatly enhance intestinal transport of

amino acids.

• This competition disappears, when amino acids are absorbed as

oligopeptides.

• All essential amino acids are not absorbed with equal efficiency,

54% for isoleucine, 80% for histidine in the pig consuming fish meal.

• Generally, amino acid concentration in tissues is 5 to 10 times of that

in plasma

• In most cases, the contribution of stomach to the total digestive process is

about 20%.

• Removal of peptic digestion is rapidly accommodated for in the small

intestine, but removal of pancreatic digestion seriously reduces total protein

digestibility.

• For the young animals receiving milk, the stomach is substantially more

important

• Protein reserves are not distinct like glycogen for carbohydrates and depot

fat for dietary fat.

• But protein reserves are available from practically all body tissues for the

purpose of meeting emergent situations.

• Liver is the key organ- that synthesizes proteins, supplies amino acids to the

circulation when needed and process nitrogen for excretion when in excess.

• The nitrogenous products are digested and enter the blood stream

mostly as amino acids in all ruminants and non-ruminants.

• Body tissue proteins continually undergo catabolism to amino acids.

The amino acids in the blood constitute the amino acid pool.

Metabolism of amino acids

• Amino acids undergo - transamination,

- oxidative and nonoxidative deamination,

- decarboxylation.

• These reactions provide for inter conversion, synthesis of dispensable

amino acids, utilization of amino acids for energy and the utilization or

excretion of excess ammonia.

• Transamination

• It refers to the process whereby amino groups are transferred from an amino acid to a keto acid without the intermediate formation of ammonia.

• It requires enzymes called transaminases or aminotransferases.

• Transaminases are involved directly in the biosynthesis of a number of dispensable amino acids.

• They link protein and carbohydrate metabolism.

• They are also involved in amino acid degradation and in the synthesis of such compounds as urea and butyric acid.

• The functional coenzyme of transaminases is pyridoxal phosphate, and all reactions are freely reversible.

• The two most widespread transaminases are

-alanine transaminase (formerly glutamic, pyruvic transaminase)

- aspartic transaminase (formerly glutamic, oxaloacetic transaminase)

• Liver contains transaminases that are specific for the formation of each

amino acid in natural proteins except glycine, lysine and threonine.

• Glutamine and asparagine, the two common amides, participate in the

transamination of more than 30 ∝-keto acids.

• The rate of transamination differs among the amino acids.

• Transamination of lysine takes place with difficulty, if at all.

• Transamination of histidine and arginine are difficult because of the

imidazole ring and the imino group, respectively.

Deamination

• Deamination is the removal of amino group from an amino acid Which may be oxidative or nonoxidative.

• Oxidative deamination:

• A deamination reaction proceeds with a simultaneous oxidation as in the conversion of an ∝-amino acid to an ∝-keto acid and the amino group to ammonia.

• The reaction is catalyzed by amino acid oxidase, an enzyme which contains the oxidizing coenzyme, FAD.

• ALANINE + FAD + H2O PYRUVIC ACID + NH3 + FADH2

• The reduced coenzyme (FADH2) is oxidized by molecular oxygen to form hydrogen peroxide which is decomposed to water by catalase.

• A specific enzyme, glutamic dehydrogenase deaminates L-glutamic acid.

• Glutamate+ NAD+ ∝ - keto glutaric acid+NH3 +NADH

• The NADH is oxidized by the mitochondrial electron transport system and 3 moles of ATP are formed by oxidative phosphorylation

• Non oxidative deamination:

• These reactions are catalyzed by amino acid dehydratase and also require vitamin B6.

• Serine Pyruvate + NH4+

• Threonine ∝- keto glutaric acid + NH4+

• Aspartic acid Fumaric acid +NH4+

Decarboxylation of amino acids

• It involves enzymes found especially in liver, kidney and brain, that

decarboxylate amino acids.

• The reaction produces carbon dioxide and primary amines called

biogenic amines.

• A general aromatic L-amino acid decarboxylase can decarboxylate

histidine to histamine, tyrosine to tyramine, dopa (3, 4-dihydroxy

phenylalanine) to dopamine (hydrodxytyramine),5-hydroxy

tryptophan to 5-hydroxy- tryptamine (serotonin), although there

appear to be specific decarboxylases also.

Synthesis of Dispensable Amino Acids

• The most abundant free amino acids in cells are alanine, aspartic acidand glutamic acid.

• Glutamic acid makes an effective source of nonspecific nitrogen.

• Alanine is chiefly formed by transamination of pyruvate anddecarboxylation of aspartate.

• Aspartic acid is formed by transamination of oxaloacetate andhydrolysis of asparagine.

• Transamination of ∝- Ketoglutarate yields glutamic acid. It is aprecursor of proline , hydroxyproline and ornithine.

• Serine is obtained from glycine.

• Serine + methionine yields cysteine.

• Glutamate + 3-phosphoglycerate yields serine.

• Tyrosine and cysteine are derived from phenylalanine and methionine, respectively.

• With an adequate supply of amino nitrogen and a source of carbon and energy, the dispensable amino acids (which make up almost 40% of tissue protein) in short supply are synthesized to make up the deficit

Disposal of Excess Amino Acids

• The extra amino acids are promptly metabolized often within hours.

• The ammonia is converted to urea in the liver and excreted.

• Most terrestrial vertebrates excrete ammonia as urea in the urine,

• fowl and land-based reptiles excrete it as uric acid,

• most fishes excrete ammonia directly through the gill tissue into their

water environment.

• In fowl, uric acid is synthesized, from glutamine, glycine and aspartic acid

with the help of xanthine oxidase.

• Urea is not synthesized in birds. The only urea in chicken urine comes from

the breakdown of dietary arginine .

• Urea is formed in the liver through Krebs-Henseleit cycle. This is an energy

requiring process.

• Note that both the amino groups found in the urea molecule came from

glutamate one from oxidative deamination and the other from

transamination

The following steps are involved in the urea cycle.

1) Formation of carbamoyl phosphate (CP). Enzyme required is carbamoyl phosphate synthetase.

2) Formation of citrulline from CP and ornithine. Enzyme is ornithine— f carbamoyl transferase.

3) Formation of argininosuccinate and arginine. A second amino group is transferred from aspartic acid to carbamoyl keto group of citrulline, to form the amino acid arginine. Enzymes needed are argininosuccinate synthetase and lyase.

4) In the presence of an enzyme, arginase and magnesium, arginine yields one molecule of urea and of ornithine.

• The regenerated ornithine can participate in the next turn of the cycle. There is a net loss of 1 ATP per mole of urea synthesized from glutamic acid

• Metabolic precursors of the nitrogen in the purine ring of uric acid-two of these

arise from the amide N of glutamine and the other two from the amine groups of

glycine and aspartic acid.

• Glutamine is the carrier of ammonia from deamination of amino acids.

• The purines and pyrimidine's absorbed from the intestines can be used for the

synthesis of nucleotides and nucleic acids.

• Purine compounds which are degraded in the tissue are readily reused. When

excreted they are converted to uric acid in primates, to allontoin in most mammals,

and to urea in fish. Pyrimidine's are excreted as urea and ammonia

Fate of Carbon Skeletons of Amino Acids

• The carbon skeletons of the amino acids which have lost their amino groups enter the citric acid cycle.

• All the amino acid residues which do not enter at either acetyl CoA or

acetoacetate have the potential of being converted to glucose in the

process of gluconeogenesis. These are called glucogenic.

• Those entering at acetyl CoA or acetoacetate could provide ketones. And are referred to as ketogenic.

• Some are referred to as both glucogenic and ketogenic

Catabolism of Tissue Protein and Amino Acids

• The protein mass of the body, like the adipose mass, is in a continuousstate of flux, with tissues constantly being catabolized andresynthesized.

• As amino acids are released they become available to the generalamino acid pool and can either be reused for protein synthesis orutilized as a source of energy.

• Two types of catabolism are observed-one a normal function of tissuemaintenance and renewal, and the other that follows periods of undernutrition/starvation.

Why Should there be a Continuous Turnover or Protein renewal

• As protein synthesis requires energy, it would appear that the protein renewal to be wasteful.

• Yet this continuous turnover of tissue protein represents changing environment.

• With a continuing protein turnover capability, the animal has the means, flexibility and speed to adapt to certain subtle or gross changes in its environment which, in the long run, may affect its ability to survive.

• The rate of turnover is not similar for all tissues. Renewal of intestinal mucosa is extremely rapid.

• During periods of under nutrition/starvation, tissues lose their protein and release amino acids.

• About one fourth of the body protein especially liver, muscle are depleted and repleted.

• In general the integrity of brain and kidney are maintained.

• Thus, due to this property, the vital functions may be protected up to 30-50 days of total starvation.

• During starvation or fasting, the primary need is energy, particularly glucose. Met through alanine & glutamine

FAT-DIGESTION AND METABOLISM

• Dietary lipids consumed include triglycerides primarily, phospholipids and cholesterol

• Although gastric juice contains a lipase, it is essentially inactive because of low pH of the stomach.

• This lipase may be much more important in the young ones where the gastric pH is higher and the fat of milk is highly emulsified.

• The principal site of lipid digestion is the small intestine

• Under the influence of the peristaltic action of the stomach, duodenum and presence of bile, fat exists in the duodenum as a coarse emulsion of triglycerides.

• Pancreatic lipase (Ca++ ions and bile salts increase its activity) and colipase hydrolyze the triglycerides into fatty acids and monoglycerides and reduce the lipid to a finer and finer emulsion.

ABSORPTION

• After digestion fats are present in the small intestine in the solubilized form

of mixed micelles.

• Efficient absorption requires a rapid movement of the highly hydrophobic

molecule through the unstirred water layer adjacent to the mucosa.

• This is the rate limiting stage of absorption.

• The mixed bile salt micelles, with their hydrophilic groups, aid this process.

• Absorption across the brush border membrane of the intestinal cells is by

passive diffusion and is at its maximum in the jejunum.

• The bile salts are absorbed by an active process in the distal ileum.

• Following absorption there is a resynthesize of triacylglycerol's,

• a process that requires energy, and they are formed into chylomicrons(minute fat droplets), which then pass into the lacteals of the villi,enter the thoracic duct and join the general circulation.

• Medium- and short-chain fatty acids, such as those occurring inbutterfat, require neither bile salts nor micelle formation

• as they can be absorbed very rapidly from the lumen of the intestinedirectly into the portal bloodstream.

• The entry of these fatty acids is sodium-dependent and takes place against a concentration gradient by active transport.

• In fowls, the lymphatic system is negligible and most of the fat is transported in the portal blood as low-density lipoproteins.

• Depot fats are formed from ingested fats and carbohydrates.

• That is why the nature of the fat deposited can be markedly affected by the character of its food source.

• The influence of the kind of the food fat upon the character of the body fat is striking-iodine number of food fat is proportional to that of body fat

• When the ration contains much unsaturated fatty acids in the form of oils, the body fat is also soft, i.e. of low melting point (higher iodine number)

• Adipose tissue is not static and there is a constant exchange of fatty acids between the body fat and fat in the blood. Hence deposits of soft fat can be modified by a change in diet.

• When a ration which will produce hard fat is given after a period on

feeds rich in unsaturated fat, the deposited fat gradually becomes

harder. This process is called "hardening off”,

• It is noted that rations containing cottonseed oil produce lard (pig fat)

graded as hard lard, and those rich in highly unsaturated fat e.g. maize

oil or soybean oil produce soft lard.

• The "hardening off" process is taken advantage of in feeding practice

in finishing pigs for market.

• Anderson and Mendel (1928) showed that the process takes place

more rapidly where the animal was fasted for a period before the

hardening ration was given.

• Catabolism of Fat and Fatty Acids

• The end result of catabolism of fats and fatty acids is the production of ATP, CO2 and H2O with the liberation of excess heat.

• The initial degradation of fat leads to the formation of glycerol and acetyl CoA.

• In the ruminant absorbed acetate, butyrate and ketone bodies are also available tor immediate catabolism.

• Mobilization and Oxidation of Fat

• It is well known that between meals the blood free fatty acid levels are elevated as a result of the mobilization of fat.

• Upon eating, they drop promptly.

• Triglycerides are released from the adipose tissue under hormonal control which activates adenylyl cyclase which in turn causes the synthesis of cyclic AMP .

• Triglyceride gives glycerol and fatty acid.

• β-Oxidation of Fatty Acids

• Fatty acids are combined with an albumin and circulate as an albumin fatty acid complex.

• In the cell, fatty acid oxidation begins in the extra mitochondrial cytoplasm with the formation of fatty acyl CoA (fatty acid + coenzyme A -> fatty acyl CoA).

• Fatty acyl CoA needs a special carrier mechanism in the form of carnitine to pass into the mitochondrion.

• Knoop proposed that fatty acids were oxidized physiologically by β-oxidation.

• In the mitochondria, fatty acyl CoA is successively dehydrogenated,

hydrated, dehydrogenated again, and cleaved to acetyl CoA and a fatty

acid shorter by two carbon atoms.

• This process is continued stepwise each sequence producing a

molecule of acetyl CoA.

• Acetyl CoA enter the TCA cycle and oxidized to C02 + H20.

• The acetyl CoA that is formed here may also follow the below

mentioned fates.

- It may condense to form acetoacetate and ketone bodies.

- It may be converted to malonyl CoA as in fatty acid synthesis, or

- It may react with acetoacetyl units in sterol synthesis.

Conversion of Fat into Glucose• Animals cannot convert fatty acids into glucose.

• Specifically; acetyl CoA cannot be converted into pyruvate or

oxaloacetate in animals.

• The two carbon atoms of the acetyl group of acetyl CoA enter the

citric acid cycle but two carbon atoms leave the cycle in the

decarboxylation's catalyzed by isocitrate dehydrogenase and a-

ketoglutarate dehydrogenase .

• Consequently, oxaloacetate is regenerated but it is not formed de-novo

when the acetyl unit of acetyl CoA is oxidized by the citric acid cycle.

• In contrast, plants have two additional enzymes enabling them to convert

the carbon atoms of acetyl CoA into oxaloacetate via glyoxylate cycle.

• Anyway the glycerol moiety of fat can yield glucose and thus 'fat' can

contribute somewhat to the glucose pool.

Interrelations with Proteins and Carbohydrates

• The metabolism of fatty acids and that of carbohydrates and proteins are

intimately related.

• The constituents are in a constant state of flux. Even the structural proteins,

carbohydrates and storage lipids are constantly broken down and rebuilt.

• The status of a particular animal is the net result between the rates of

synthesis and of breakdown of its body constituents.

• Glycerol is the only component of lipids that is involved in the synthesis of

carbohydrates.

• On the other hand, lipids can be formed from carbohydrate in many ways.

• Indeed, the metabolism of lipids, carbohydrates and proteins is

metabolically dynamic.

Thank you