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Carbohydrate Digestion and Metabolism Overview of Carbohydrate Digestion and Metabolism 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 Simple Sugars - Disaccharides Complex carbohydrates Oligosaccharides Polysaccharides Starch Glycogen Dietary fiber (Dr. Firkins) 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 Glycogen G G G Major storage G G G G G carbohydrate in G G animals G a 1-6 link G GG Long straight glucose a 1-4 link G G chains (α 1-4) G G Branched every 4-8 glc G residues (α 1-6) More branched than starch Less osmotic pressure Easily mobilized Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidase G G G G GG a Limit dextrins GG G G G G G G G amylase G G G GGG G a 1-6 link G GG maltotriose GG a 1-4 link G GG G G maltose G G isomaltose Stomach Not much carbohydrate digestion Acid and pepsin to unfold proteins Ruminants have forestomachs with extensive microbial populations to breakdown and anaerobically ferment feed Small Intestine Pancreatic enzymes a-amylase maltotriose maltose G G G G G G G G amylose G G a amylase G G G G G GG GG G G amylopectin + G G G GG G a Limit dextrins Oligosaccharide digestion..cont a Limit dextrins G G G GG G G sucrase G G G maltase Glucoamylase (maltase) or G G G a-dextrinase a-dextrinase G G GG G G G GG G G G Small intestine Portal for transport of virtually all nutrients Water and electrolyte balance Enzymes associated with intestinal surface membranes i. Sucrase ii. a dextrinase iii.Glucoamylase (maltase) iv.Lactase v. peptidases Carbohydrate absorption Hexose transporter apical basolateral Carbohydrate Comparative Ruminant vs. Non-Ruminant Animal Digestion and Absorption Non-ruminant Ruminant CHO in feed microbial fermentation digestive enzymes Glucose in small intestine Volatile fatty acids in rumen Absorption into blood circulation 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 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 Carbohydrate Digestion Pancreas Pancreatic amylase Hydrolyzes alpha 1-4 linkages Produces monosaccharides, disaccharides, and polysaccharides Major importance in hydrolyzing starch and glycogen to maltose Polysaccharides Amylase Disaccharides Digestion in Small Intestine Digestion mediated by enzymes synthesized by cells lining the small intestine (brush border) Disaccharides Brush Border Enzymes Monosaccharides * Exception is β-1,4 bonds in cellulose Digestion in Small Intestine Sucrose Sucrase Glucose + Fructose * Ruminants do not have sucrase Maltose Lactose Maltase Lactase Glucose + Glucose Glucose + Galactose * Poultry do not have lactase Digestion of Disaccharides Newborns have a full complement of brush-border enzymes Miller et al. (eds.), 1991 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 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 Overview Monogastric Carbohydrate Digestion Location Enzymes Form of Dietary CHO Mouth Salivary Amylase Starch Maltose Sucrose Lactose Stomach (amylase from saliva) Small Intestine Pancreatic Amylase Brush Border Enzymes Large Intestine None Dextrin→Maltose Maltose Glucose Fructose Galactose + + + Glucose Glucose Glucose Bacterial Microflora Ferment Cellulose 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 Small Intestine Carbohydrates Monosaccharides Portal Vein Liver Active Transport Distributed to tissue through circulation Nutrient Absorption - Carbohydrate Active transport for glucose and galactose Sodium-glucose transporter 1 (SGLT1) Dependent on Na/K ATPase pump Facilitated transport for fructose Carbohydrate Digestion in Ruminants Ingested carbohydrates are exposed to extensive pregastric fermentation Most carbohydrates fermented by microbes Rumen fermentation is highly efficient considering the feedstuffs ingested 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 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 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 Microbial Metabolism Sugars ADP Catabolism in rumen: VFA CO2 CH4 Heat ATP NADP+ NADPH Growth Maintenance Replication 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 Ruminant Carbohydrate Digestion Small Intestine Secretion of digestive enzymes Digestive secretions from pancreas and liver Further digestion of carbohydrates Absorption of H2O, minerals, amino acids, glucose, fatty acids Cecum and Large Intestine Bacterial population ferments the unabsorbed products of digestion Absorption of H2O, VFA and formation of feces 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 Carbohydrates Metabolism in Monogastrics Serve as primary source of energy in the cell Central to all metabolic processes Glucose Cytosol - anaerobic Hexokinase Pentose Phosphate Shunt Glucose-6-P glycolysis Pyruvate Glc-1- phosphate glycogen cytosol Pyruvate mitochondria (aerobic) Aceytl CoA FATTY ACIDS Krebs cycle Reducing equivalents AMINO ACIDS Oxidative Phosphorylation (ATP) Control of enzyme activity Rate limiting step Glucose utilization 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 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 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 a Carbohydrate Metabolism in Ruminants Ingested carbohydrates are exposed to extensive pregastric fermentation Most carbohydrates fermented by microbes Rumen fermentation is highly efficient considering the feedstuffs ingested Volatile Fatty Acids Carbohydrates Microbial Fermentation VFA’s Glucose Short-chain fatty acids produced by microbes - Rumen, cecum, colon 3 basic types: CH3 C O– Acetic acid (2c) O O O CH3 CH2 CH3 C O– Propionic acid (3c) CH2 CH2 C O– Butyric acid (4c) 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 Rumen Fermentation Gases (carbon dioxide and methane) are primary byproducts of rumen fermentation Usually these gases are eructated or belched out - if not, bloat occurs Bloat results in a severe distension of the rumen typically on the left side of the ruminant and can result in death Uses of VFA Acetate Propionate Energy Fatty acid synthesis Energy Gluconeogenic – glucose synthesis Butyrate Energy Rumen epithelial cells convert to ketone (beta hydroxybytyrate) Proportions produced depends on diet VFA Production – Molar Ratios Forage:Grain Acetate Propionate Butyrate 100:0 71.4 16.0 7.9 75:25 68.2 18.1 8.0 50:50 65.3 18.4 10.4 40:60 59.8 25.9 10.2 20:80 53.6 30.6 10.7 Rumen VFA Profiles Metabolism of VFA Overview Acetate and butyrate are the major energy sources (through oxidation) Propionate is reserved for gluconeogenesis Acetate is the major substrate for lipogenesis Propionate is also lipogenic (though glucose) Glucose Requirements There is less fluctuation in blood glucose in ruminants and blood glucose is lower at 40-60 mg/dl Reduced fluctuation due to: Eat more constantly than monogastrics Continuous VFA production Continuous digesta flow Continuous gluconeogenesis Overview of Carbohydrates and Ruminants Diet Protein Carbohydrate Fat _____________________________________________ Bacterial Protein Rumen Fatty Acids Starch VFA Propionate Acetate Butyrate _____________________________________________ Blood Fatty Acids Amino Acids Glucose _____________________________________________ Tissue Lactose Fat Protein Carbohydrate Digestion and Absorption Ruminant vs. Monogastrics Digestive Feature Ruminant Salivary amylase Zero Non ruminant High – primates Moderate – pig Low - carnivores Pregastric fermentation High+ Zero in MOST cases Gastric Very low Very low Pancreatic amylase in SI Moderate High Glucose absorption from SI Zero to low High Low Low to High Post SI
