Carbohydrate Digestion and Metabolism

1. Carbohydrate Digestionand Metabolism

2. Overview of Carbohydrate Digestion and Metabolism

3. Carbohydrates • Carbohydrates are composed of carbon and water and have a composition of (CH2O)n. • The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram. energy Carbon dioxide Water Chlorophyll GLUCOSE 6 CO2 + 6 H20 + energy (sun) C6H12O6 + 6 O2

4. Simple Sugars -

5. Disaccharides

6. Complex carbohydrates • Oligosaccharides • Polysaccharides • Starch • Glycogen • Dietary fiber (Dr. Firkins)

7. Starch • Major storage carbohydrate in higher plants • Amylose – long straight glucose chains (α1-4) • Amylopectin – branched every 24-30 glc residues (α 1-6) • Provides 80% of dietary calories in humans worldwide

8. Glycogen G • Major storage carbohydrate in animals • Long straight glucose chains (α 1-4) • Branched every 4-8 glc residues (α 1-6) • More branched than starch • Less osmotic pressure • Easily mobilized G G G G G G G G G G a 1-6 link G G G G a 1-4 link G G G G

9. Digestion • Pre-stomach – Salivary amylase : a 1-4 endoglycosidase G G G a Limit dextrins G G G G G G G G G G G G amylase G G G G G a 1-6 link G G G G maltotriose G G a 1-4 link G G G G G G maltose G G isomaltose

10. Stomach • Not much carbohydrate digestion • Acid and pepsin to unfold proteins • Ruminants have forestomachs with extensive microbial populations to breakdown and anaerobically ferment feed

11. Small Intestine • Pancreatic enzymes a-amylase maltotriose maltose + G G G G G G G G G G a amylase amylose G G G G G G G G G G G G G G G G G amylopectin a Limit dextrins

12. Oligosaccharide digestion..cont a Limit dextrins G G G G sucrase G G G G G G maltase G Glucoamylase (maltase) or a-dextrinase G G G a-dextrinase G G G G G G G G G G G

13. Small intestine Portal for transport of virtually all nutrients Water and electrolyte balance • Enzymes associated with • intestinal surface membranes • Sucrase • a dextrinase • Glucoamylase (maltase) • Lactase • peptidases

15. Carbohydrate absorption Hexose transporter apical basolateral

16. Carbohydrate Comparative Ruminant vs. Non-Ruminant Animal

17. Digestion and Absorption Non-ruminant Ruminant CHO in feed microbial fermentation digestive enzymes Glucose in small intestine Volatile fatty acidsin rumen Absorption into blood circulation

18. Digestion of Carbohydrates • Monosaccharides • Do not need hydrolysis before absorption • Very little (if any) in most feeds • Di- and poly-saccharides • Relatively large molecules • Must be hydrolyzed prior to absorption • Hydrolyzed to monosaccharides Only monosaccharides can be absorbed

19. Non-Ruminant Carbohydrate Digestion • Mouth • Salivary amylase • Breaks starches down to maltose • Plays only a small role in breakdown because of the short time food is in the mouth • Ruminants do not have this enzyme • Not all monogastrics secrete it in saliva

20. Carbohydrate Digestion • Pancreas • Pancreatic amylase • Hydrolyzes alpha 1-4 linkages • Produces monosaccharides, disaccharides, and polysaccharides • Major importance in hydrolyzing starch and glycogen to maltose Amylase Polysaccharides Disaccharides

21. Digestion in Small Intestine • Digestion mediated by enzymes synthesized by cells lining the small intestine (brush border) Brush Border Enzymes Monosaccharides Disaccharides * Exception is β-1,4 bonds in cellulose

22. Digestion in Small Intestine Sucrase Sucrose Glucose + Fructose * Ruminants do not have sucrase Maltase Maltose Glucose + Glucose Lactase Lactose Glucose + Galactose * Poultry do not have lactase

23. Miller et al. (eds.), 1991 Digestion of Disaccharides • Newborns have a full complement of brush-border enzymes

24. Digestion in Large Intestine • Carnivores and omnivores • Limited anaerobic fermentation • Bacteria produce small quantities of cellulase • SOME volatile fatty acids (VFA) produced by microbial digestion of fibers • Propionate • Butyrate • Acetate

25. Digestion in Large Intestine • Post-gastric fermenters (horse and rabbit) • Can utilize large quantities of cellulose • Cecum and colon contain high numbers of bacteria which produce cellulase • Cellulase is capable of hydrolyzing the beta 1,4- linkage

26. Overview Monogastric Carbohydrate Digestion Location EnzymesForm of Dietary CHO Mouth Salivary Amylase Starch Maltose Sucrose Lactose Stomach (amylase from saliva) Dextrin→Maltose Small Intestine Pancreatic Amylase Maltose Brush Border Enzymes Glucose Fructose Galactose + + + Glucose Glucose Glucose Large Intestine None Bacterial Microflora Ferment Cellulose

27. Carbohydrate Absorption in Monogastrics • With exception of newborn animal (first 24 hours), no di-, tri-, or polysaccharides are absorbed • Monosaccharides absorbed primarily in duodenum and jejunum • Little absorption in stomach and large intestine

28. Small Intestine Carbohydrates Monosaccharides Portal Vein Active Transport Liver Distributed to tissue through circulation

29. Nutrient Absorption - Carbohydrate • Active transport for glucose and galactose • Sodium-glucose transporter 1 (SGLT1) • Dependent on Na/K ATPase pump • Facilitated transport for fructose

31. Carbohydrate Digestion in Ruminants • Ingested carbohydrates are exposed to extensive pregastric fermentation • Rumen fermentation is highly efficient considering the feedstuffs ingested • Most carbohydrates fermented by microbes

32. Reticulorumen • Almost all carbohydrate is fermented in the rumen • Some ‘bypass’ starch may escape to the small intestine • No salivary amylase, but have plenty of pancreatic amylase to digest starch

33. Microbial Populations • Cellulolytic bacteria (fiber digesters) • Produce cellulase - cleaves β1→4 linkages • Primary substrates are cellulose and hemicellulose • Prefer pH 6-7 • Produce acetate, propionate, little butyrate, CO2 • Predominate in animals fed roughage diets

34. Microbial Populations • Amylolytic bacteria (starch, sugar digesters) • Digest starches and sugars • Prefer pH 5-6 • Produce propionate, butyrate and sometimes lactate • Predominate in animals fed grain diets • Rapid change to grain diet causes lactic acidosis (rapidly decreases pH) • Streptococcus bovis

35. Microbial Metabolism Sugars ADP ATP NADP+ NADPH Biosynthesis Catabolism in rumen: VFA CO2 CH4 Heat Growth Maintenance Replication

36. Bacterial Digestion of Carbohydrates Rumen: • Microbes attach to (colonize) fiber components and secrete enzymes • Cellulose, hemicellulose digested by cellulases and hemicellulases • Complex polysaccharides are digested to yield sugars that are fermented to produce VFA • Starches and simple sugars are more rapidly fermented to VFA • Protozoa engulf starch particles prior to digesting them

37. Ruminant Carbohydrate Digestion • Small Intestine • Cecum and Large Intestine • Secretion of digestive enzymes • Digestive secretions from pancreas and liver • Further digestion of carbohydrates • Absorption of H2O, minerals, amino acids, glucose, fatty acids • Bacterial population ferments the unabsorbed products of digestion • Absorption of H2O, VFA and formation of feces

38. Summary of Carbohydrate in Monogastrics • Polysaccharides broken down to monosaccharides • Monosaccharides taken up by active transport or facilitated diffusion and carried to liver • Glucose is transported to cells requiring energy • Insulin influences rate of cellular uptake

40. Carbohydrates Metabolism in Monogastrics Glucose • Serve as primary source of energy in the cell • Central to all metabolic processes Cytosol - anaerobic Hexokinase Pentose Phosphate Shunt Glc-1- phosphate Glucose-6-P glycolysis glycogen Pyruvate

41. cytosol Pyruvate mitochondria (aerobic) Aceytl CoA FATTY ACIDS Krebs cycle Reducing equivalents AMINO ACIDS Oxidative Phosphorylation (ATP)

42. Control of enzyme activity Rate limiting step

43. Glucose utilization

44. Stage 1 – postparandial All tissues utilize glucose Stage 2 – postabsorptive KEY – Maintain blood glucose Glycogenolysis Glucogneogenesis Lactate Pyruvate Glycerol AA Propionate Spare glucose by metabolizing fat Stage 3- Early starvation Gluconeogenesis Stave 4 – Intermediate starvation gluconeogenesis Ketone bodies Stage 5 – Starvation

45. Carbohydrate Metabolism/ Utilization- Tissue Specificity • Muscle – cardiac and skeletal • Oxidize glucose/produce and store glycogen (fed) • Breakdown glycogen (fasted state) • Shift to other fuels in fasting state (fatty acids) • Adipose and liver • Glucose  acetyl CoA • Glucose to glycerol for triglyceride synthesis • Liver releases glucose for other tissues • Nervous system • Always use glucose except during extreme fasts • Reproductive tract/mammary • Glucose required by fetus • Lactose  major milk carbohydrate • Red blood cells • No mitochondria • Oxidize glucose to lactate • Lactate returned to liver for Gluconeogenesis

46. Carbohydrate Digestion Rate Composition and Digestion of Carbohydrate Fractions ___________________________________________________________ Composition Rumen Digestion (%/h) _____________________________________________________ Sugars 200-350 Fermentation and Organic Acids 1-2 Starch 10-40 Soluble Available Fiber 40-60 Pectins B glucans Insoluble Available Fiber 2-10 Cellulose Hemicellulose Unavailable Fiber (lignin) 0 ___________________________________________________________ ___________________________________________________________ ___________________________________________________________

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48. Carbohydrate Metabolism in Ruminants • Ingested carbohydrates are exposed to extensive pregastric fermentation • Rumen fermentation is highly efficient considering the feedstuffs ingested • Most carbohydrates fermented by microbes

49. Volatile Fatty Acids Carbohydrates VFA’s Microbial Fermentation Glucose • Short-chain fatty acids produced by microbes • 3 basic types: - Rumen, cecum, colon Butyric acid (4c) Acetic acid (2c) Propionic acid (3c)

50. VFA Formation 2 acetate + CO2 + CH4 + heat 1 Glucose 2 propionate + water 1 butyrate + CO2 + CH4 VFAs absorbed passively from rumen to portal blood Provide 70-80% of ruminant’s energy needs