Overview of Carbohydrate Digestion and Metabolism

FST/AN/HN 761 FST 761 Dr. Jeff Firkins – Carbohydrates Dr. Josh Bomser – Lipids TA- Amy Long, MS Reading / Writing Assignments Text - Biochemical and Physiological Aspects of Human Nutrition- Martha H. Stipanuk. Today – Overview of carbohydrates (Jan 7)

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. No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.

Photosynthesis: Sun’s energy becomes part of glucose molecule Carbon dioxide Water Chlorophyll GLUCOSE 6 CO2 + 6 H20 + energy (sun) C6H12O6 + 6 O2

120 grams of glucose / day = 480 calories

Simple Sugars - Glucose – basic unit of polysaccharides Galactose – found as part of lactose in milk Fructose – found in fruits, vegetables and honey (consumption on the rise….High fructose corn syrup)

Disaccharides

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

Starch Major storage carbohydrate in higher plants Amylose – long straight glucose chains (a1-4) Amylopectin – branched every 24-30 glc residues (a 1-6) Provides 80% of dietary calories in humans worldwide

Glycogen Major storage carbohydrate in animals Long straight glucose chains (a1-4) Branched every 4-8 glc residues (a 1-6) More branched than starch Less osmotic pressure Easily mobilized G 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

Digestion Pre-stomach – Salivary amylase : a 1-4 endoglycosidase a Limit dextrins G G G G G G G G G amylase G G G G Key- We must breakdown these very large oligosaccharides into monosaccharides in order to absorb them. Alpha amylase. – Cannot attack a1-4 linkase close to 1-6 branch points. 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

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 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

Oligosaccharide digestion..cont a Limit dextrins G G G G sucrase G G G G G maltase G G Glucoamylase (maltase) or a-dextrinase G G G Maltase – specifically removes a single glucose from the nonreducing end of a linear a1-4 glucose chain…breaking down maltose into glucose. (exosaccharidases) Alpha dextinase – cleaves 1,6-alpha glucosidic linkages a-dextrinase G G G G G G G G G G G

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

Carbohydrate absorption Hexose transporter Monosaccharides are still too large for passive diffusion across brush border membrane. We use facilitated diffusion to absorb these molecules. Glucose and galactose use a sodium-glucose symport (SGLUT1) while fructose uses the glut5 We must transport Na out of the cell to maintain proper electrochemical gradient (sodium potassium pump) Water will also follow sodium into enterocyte. This is critical to maintain proper water balance. apical basolateral

Glucose and galactose absorption Read Chapter 5 and answer the questions on page 102 of Stipanuk. Be prepared to discuss them on Friday

Carbohydrate malabsorption Lactose intolerance (hypolactasia), page 100. Decline lactase with age Lactose fermented in LI – Gas and volatile FA Water retention – diarrhea/bloating Not all populations Northern European – low incidence Asian/African Americans – High b 1-4 linkage

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 Glc-1- phosphate glycolysis glycogen Pyruvate

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

Glucose No mitochondria Glucose The Full Glycogen Monty Lactate Lactate transported back to liver for glucose production “Cori Cycle”. Costs energy The Full Monty

Fasted State Glucose Need 13.8 kJ/mol ATP = -30 kJ/mol -16.7 kJ/mol G-6-Pase Hexokinase Pentose Phosphate Shunt Glucose-6-P Glc-1- phosphate glycolysis GNG glycogen Pyruvate

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 OH P OH P P OH Glycogen formation Protein Kinase B (inactive) Protein Kinase B (active) OH P Glycogen synthase kinase (active) OH P Glycogen synthase kinase (inactive) P OH Glycogen synthase (inactive) 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

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