Chapter 4 Carbohydrates

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Chapter 4 Carbohydrates

• Objectives
• Learn the major sources of dietary carbohydrates.
  
• What are these digested to and absorbed as?
• What are the major types of glucose transporters
  and where are they found? How are these
  transporters regulated?
• Understand how the major carbohydrates are
  utilized metabolically by your cells
• Understand the basic processes involved in
  glycolysis, glycogenesis, glycogenolysis,
  gluconeogenesis, and the hexosemonophosphate
  shunt
• Understand how fructose and galactose enter these
  pathways
• Learn how these pathways are regulated by insulin
  and glucagon
• What are the major ways that regulation takes
  place (mechanisms of regulation)?
• Learn how ethanol consumption impacts the
  metabolic pathways for carbohydrates
• Acetaldehyde toxicity
• High NADHNAD ratio
• Substrate competition
• Induced metabolic tolerance
• Learn how some diseases are associated with
  carbohydrate metabolism
• Diabetes
• Hypoglycemia

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Enzymes involved in carbohydrate digestion and
metabolism are stereospecific for D sugars
Reference molecule
Chiral carbon? Anomeric carbon?
Fig. 4-2, p. 74
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Pentoses? Reducing sugars? Oligosaccharides?
Table 4-1, p. 75
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Fig. 4-5a, p. 77
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r
Fig. 4-6, p. 78
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Table 4-2, p. 81
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Golgi Apparatus
Glucose
Transport
Vesicle-bound transporters
Synthesized transporters from ribosomes
Translocation
Signal?
Insulin
Insulin receptor
Fig. 4-8, p. 82
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(a) Glucose
(reference food)
(b) Low glycemic index
(c) Fasting baseline
Blood glucose (mg/dL)
Hours
Glucose meal
Fig. 4-9, p. 83
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Cereals Cereals Snacks Snacks Pasta Pasta Beans Beans
All Bran 51 chocolate bar 49 fettucini 32 baked 44
Bran Buds psyll 45 corn chips 72 linguini 50 black beans, boiled 30
Bran Flakes 74 croissant 67 macaroni 46 butter, boiled 33
Cheerios 74 doughnut 76 spagh, 5 min boiled 33 cannellini beans 31
Corn Chex 83 graham crakers 74 spagh, 15 min boiled 44 garbanzo, boiled 34
Cream of Wheat 66 jelly beans 80 Soups/Vegetables Soups/Vegetables kidney, boiled 29
Frosted Flakes 55 pizza, cheese tom 60 carrots, fresh, boil 49 kidney, canned 52
Grapenuts 67 Pizza Hut, supreme 33 corn, sweet 56 lentils, green, brown 30
muesli, natural 54 popcorn, light micro 55 parsnips 97 lima, boiled 32
Fruit Fruit potato chips 56 peas, fresh, boil 48 navy beans 38
apple 38 Power bars 58 Drinks Drinks pinto, boiled 39
apricots 57 pretzels 83 apple juice 40 red lentils, boiled 27
banana 56 saltine crakers 74 colas 65 soy, boiled 16
cantalope 65 Cereal Grains Cereal Grains Gatorade 78 Breads Breads
cherries 22 barley 25 orange juice 46 bagel, plain 72
dates 103 basmati white rice 58 Milk Products Milk Products baquette, Frnch 95
grapefruit 25 bulgar 48 ice cream, van 60 pita 57
grapes 46 couscous 65 ice milk, van 50 pizza, cheese 60
kiwi 52 skim milk 32
 
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Fig. 4-10, p. 84
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Fig. 4-11a, p. 85
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Glycogen synthase active when dephosphorylated,
inactive when phosphorylated insulin vs.
glucagon
Fig. 4-11b, p. 85
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Glycogenolysis activated by glucagon and
epinephrine through action on glycogen
phosphorylase (phosphorylase a, phosphorylated
active phosphorylase b, dephosphorylated -
inactive
Fig. 4-12, p. 86
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Fig. 4-13, p. 87
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A




                                       










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Fig. 4-14, p. 88
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1. Citrate synthase
2. Aconitase
3. Isocitrate dehydrogenase
4. a ketoglutarate dehydrogenase
5. Succinyl thiokinase
6. Succinate dehydrogenase
7. Fumarase
8. Malate dehydrogenase

Fig. 4-15, p. 91
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Fig. 4-16, p. 91
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Fig. 4-17, p. 93
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Used when nucleic acids are needed
Used when NADPH is needed
Fig. 4-19, p. 95
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Brain, neurons, and RBCs are dependent on glucose
as a nutrient. When dietary intake of glucose is
decreased and glycogen stores are depleted, we
can make new glucose from alternative fuel
sources in a process called gluconeogenesis

• Fuels used to make new glucose include
• amino acids,
• lactate,
• Glycerol
• Organ which most often performs gluconeogenesis
  is the liver

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Irreversible rxn
Fig. 4-21, p. 97
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Four Mechanisms for Regulating Blood Glucose

• Allosteric modulation by compounds within the
  pathways
• Hormonal activation of covalent modification of
  specific enzymes
• Directional shifts in reversible reactions by
  changes in reactant or product concentrations
• Translocation of enzymes within the cell

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• Citrate synthase
• Aconitase
• Isocitrate dehydrogenase
• a ketoglutarate dehydrogenase
• Succinyl thiokinase
• Succinate dehydrogenase
• Fumarase
• Malate dehydrogenase

Regulated by NADH/NAD
Regulated by ATP/ADP
Fig. 4-15, p. 91
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Fig. 4-22, p. 101
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Ethyl Alcohol Metabolic Impact

• Ethyl alcohol most closely resembles a
  carbohydrate
• It has caloric value, and is a common dietary
  component of alcoholic beverages
• Each gram of alcohol yields 7kcal of energy
• May account for 10 of total energy intake in
  moderate consumers
• May account for 50 of total energy intake in
  alcoholics.
• Ethanol is absorbed throughout the digestive
  tract and is transported in the blood.
• It is then oxidatively degraded, mostly in the
  liver, to acetaldehyde and then to acetate.
  Acetate eventually is converted to acetylcoA
• Alcohol dehydrogenase
• MEOS or cytochrome P-450
• catalase

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ethanol NAD ----gt acetaldehyde NADH
Acetaldehyde is toxic reactive with amino groups
and may interact with proteins competes for the
plasma carrier of pyridoxal (vitamin B6)
acetaldehyde NAD ----gt acetic acid NADH
Acetic acid can lead to acidosis
Because the two reactions require NAD, NADH can
build up. What problems might this
cause? anaerobic metabolism to regenerate
NAD lack of pyruvate for gluconeogenesis -
hypoglycemia
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Fig. 4-23, p. 102
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Metabolic consequences

• Acetaldehyde toxicity
• Elevated NADHNAD ratio
• Metabolic competition
• Induced metabolic tolerance

• Elevates HDL in serum and lowers serum
  lipoproteins
• Slowing development of smooth muscles in
  atherosclerosis

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• Diseases of Carbohydrate Metabolism
• Diabetes
• Hypoglycemia