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
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 - 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)

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 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
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

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 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
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

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
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

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
Glucose-6-P
Glc-1- phosphate
glycolysis
glycogen
Pyruvate

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

22. Glucose No mitochondria Glucose The Full Glycogen Monty Lactate
Lactate transported back to liver for glucose production “Cori Cycle”. Costs energy
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
Glucose-6-P
Glc-1- phosphate
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 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

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