Overview of Carbohydrate Digestion and Metabolism

1. Overview of Carbohydrate Digestion and Metabolism

2. 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)

3. 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.

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

5. 120 grams of glucose / day = 480 calories

6. Simple Sugars -

7. Disaccharides

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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. 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

16. Carbohydrate absorption Hexose transporter apical basolateral

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

18. 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

19. 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)

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

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

22. Glucose Glucose Glucose No mitochondria Glucose Glycogen Lactate The Full Monty

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

24. Controlling Metabolic Flux 1. Control enzyme levels 2. Control of enzyme activity (activation or inhibition)

25. Control of enzyme activity Rate limiting step

26. insulin IR P Protein Kinase B (active) Protein Kinase B (inactive) OH P Glycogen synthase kinase (active) OH P Glycogen synthase kinase (inactive) P OH Glycogen synthase (inactive) Glycogen synthase (active) Glycogen formation

27. 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

28. Glucose utilization

29. 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

30. 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