Transcript Glucose
Overview of Carbohydrate Digestion and Metabolism Carbohydrates •Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they 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. Photosynthesis: Sun’s energy becomes part of glucose molecule energy Carbon dioxide Water Chlorophyll GLUCOSE 6 CO2 + 6 H20 + energy (sun) C6H12O6 + 6 O2 120 grams of glucose / day = 480 calories Simple Sugars - Disaccharides Complex carbohydrates Oligosaccharides Polysaccharides Starch Glycogen Dietary fiber Starch Major storage carbohydrate in higher plants Amylose – long straight glucose chains (a1-4) Amylopectin – branched every 24-30 glc residues (a 1-6) Glycogen Major storage carbohydrate in animals Long straight glucose G chains (a1-4) G Branched every 4-8 glc residues (a 1-6) a 1-4 link More branched than starch Easily mobilized G G G G G G G G G G G G G a 1-6 link G G G G To ensure a readily available supply, liver and muscle cells store carbohydrate as glycogen . Glycogen is stored hydrated with water; thus the water makes glycogen large, cumbersome, and unsuitable for long-term energy storage. The 70-kg "average" man stores only an IS-hour fuel supply as glycogen, compared with a 2-month supply stored as fat. If all human energy stores were glycogen, humans would need to weigh 60 additional pounds Approximately 150 g of glycogen is stored in muscle, which can be increased fivefold with physical training but is not available to maintain blood glucose directly. It is the glycogen store in the human liver (approximately 90 g) that is involved in the hormonal control of blood sugar. Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidase G a 1-4 link G G G G G G G G G G G G G G amylase a 1-6 link G G G G G G a Limit G G G G G G G G dextrins maltotriose G G G maltose G G isomaltose Stomach Not much carbohydrate digestion Acid and pepsin to unfold proteins Ruminants have fore stomachs with extensive microbial populations to breakdown and anaerobically ferment feed Small Intestine Pancreatic enzymes a-amylase maltotriose G G G G G G G G amylose G G a amylase G G G G G G G G G G G amylopectin + maltose G G G G G G a Limit dextrins 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 malabsorption Lactose intolerance (hypolactasia), Decline lactase with age Lactose fermented in LI – Not all populations b 1-4 linkage Gas and volatile FA Water retention – diarrhea/bloating Northern European – low incidence Asian/African Americans – High Metabolism – the chemical changes that take place in a cell that produce energy and basic materials needed for important life processes -millions of cells -Multiple organs (liver, adipose, heart, brain) -Thousands of enzymes -Various conditions (fed, fasted, exercise, stress) Carbohydrates 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 mitochondria (aerobic) Pyruvate Aceytl CoA FATTY ACIDS Krebs cycle AMINO ACIDS Reducing equivalents Oxidative Phosphorylation (ATP) No mitochondria Glucose Glycogen Lactate Glucose Glucose Glucose The Full Monty Fasted State Glucose G-6-Pase Pentose Phosphate Shunt Hexokinase Glucose-6-P GNG glycolysis Pyruvate Need 13.8 kJ/mol ATP = -30 kJ/mol -16.7 kJ/mol Glc-1- phosphate glycogen Controlling Metabolic Flux 1. Control enzyme levels 2. Control of enzyme activity (activation or inhibition) Control of enzyme activity Rate limiting step insulin IR P Protein Kinase B (inactive) Glycogen synthase kinase (active) P Glycogen synthase (inactive) OH Protein Kinase B (active) P OH synthase kinase P Glycogen(inactive) OH Glycogen synthase (active) Glycogen formation Controlling Metabolic Flux 1. Control enzyme levels 2. Control of enzyme activity (activation or inhibition) 3. Compartamentalization Fatty acid oxidation occurs in mitochondrial matrix Fatty acid synthesis occurs in endoplasmic reticulum membrane exposed to the cytoplasm of the cell. 4. Hormonal control 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 Glycemic Index The glycemic index is used to rank carbohydrates by their ability to raise blood glucose levels as compared with a reference food foods with a low glycemic index have consistently shown beneficial effects on blood glucose control in both the short and long term in diabetic patients. The glycemic index of a diet has a predictable effect on blood glucose levels The glycemic load of a food is the glycemic index of the carbohydrate divided by 100 and multiplied by its amount of available carbohydrate content (i.e. carbohydrates minus fiber) in grams. Carbohydrate Regulation of Blood Lipids Carbohydrate-induced hypertriglyceridemia can result from consuming a high-carbohydrate diet. The body regulates macro nutrient levels to provide adequate fuel for body tissues. The brain uses the most of the approximately 200 g of glucose required per day. "When blood glucose level falls to less than 40 mg/ dL, counterregulatory hormones release macronutrients from stores. "When blood glucose level rises to more than 180 mg/dL, glucose is spilled into the urine High intakes of carbohydrate trigger large releases of insulin for compensatory responses, including insulindependent glucose uptake by muscle or fat and active synthesis of glycogen and fat. Blood glucose then drops to a normal range. Approximately 2 hours after a meal, intestinal absorption is complete, but insulin effects persist and blood glucose falls. The body interprets this hypoglycemic state as starvation and secretes counterregulatory hormones to release free fatty acids from fat cells. Fatty acids are packed into transport lipoproteins (very-low-density lipoproteins [VLDLs) in the liver, thereby elevating serum triglycerides. 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