Carbohydrate Digestion

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Carbohydrate Digestion
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Forms of Carbohydrate

• Simple sugars
• Starch
• Glycogen
• Fiber

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CHO Digestion
Amylase
Sucrase
Lactase
Maltase
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Carbohydrate (CHO) Digestion
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Digestion of carbohydrate in the Small Intestine

• Pancreatic amylase is released
• Intestinal cells release maltase, sucrase, and
  lactase
• Maltose maltase glucose glucose
• Sucrose sucrase glucose fructose
• Lactose lactase glucose galactose
  
• Monosaccharides are absorbed

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Disaccharides digestion
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• Glucose is the most important carbohydrate
• Glucose is the major metabolic fuel of mammals.
• Monosaccharide from diet
• - Glucose
• - Fructose
• - Galactose
• Fructose and Galactose glucose at the
  liver

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Glucose transporters (GLUT)

• GLUT15
• GLUT1 RBC
• GLUT4 adipose tissue, muscle

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The metabolism of glucose

• glycolysis
• aerobic oxidation
• pentose phosphate pathway
• glycogen synthesis and catabolism
• gluconeogenesis

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glycogen
Glycogenolysis
Glycogenesis
starch
lactate
Glycolysis
Digestion absorption
glucose
aerobic oxidation
Lactate, amino acids, glycerol
H2OCO2
Gluconeo-genesis
Pentose phosphate pathway
Ribose, NADPH
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Galactose Metabolism
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Fructose Metabolism
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• Blood glucose carbohydrate metabolism
• 1. Glycolysis
• 2. Glycogenesis
• 3. HMP Shunt
• 4. Oxidation of Pyruvate
• 5. Krebs Cycle
• 6. Change to lipids
• Fasting blood glucose carbohydrate
  metabolism
• 1. Glycogenolysis
• 2. Gluconeogenesis

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• Glycogen is a polymer of glucose residues linked
  by
• ? (1?4) glycosidic bonds, mainly
• ? (1?6) glycosidic bonds, at branch points.

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GLYCOGENESIS

• Synthesis of Glycogen from glucose
• Occurs mainly in muscle and liver cell
• The reaction
• Glucose Glucose-6-P
• (Hexokinase / Glucokinase)
• Glucose-6-P Glucose-1-P
• (Phosphoglucomutase)
• Glucose-1-P UTP UDPG Pyrophosphate
• (UDPG Pyrophosphorylase)

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GLYCOGENESIS

• Glycogen synthase catalyzes the formation of
  a-1,4-glucosidic linkage in glycogen
• Branching enzyme catalyzes the formation of
  a-1,6-glucosidic linkage in glycogen
• Finally the branches grow by further
  additions of 1 ? 4-gucosyl units and further
  branching (like tree!)

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SYNTHESIS OF GLYCOGEN
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GLYCOGENESIS AND GLYCOGENOLYSIS PATHWAY
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GLYCOGENOLYSIS

• The breakdown of glycogen
• Glycogen phosphorylase catalyzes cleavage of the
  1?4 linkages of glycogen to yield
  glucose-1-phosphate
• Debranching enzyme hydrolysis of the 1?6 linkages
• The combined action of these enzymes leads to the
  complete breakdown of glycogen.

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GLYCOGENOLYSIS

• Phosphoglucomutase
• Glucose-1-P Glucose-6-P
• Glucose-6-phosphatase
• Glucose-6-P Glucose
• Glucose-6-phosphatase enzyme a specific
  enzyme in liver and kidney, but not in muscle
• Glycogenolysis in liver yields glucose
  export to blood to increase the blood
  glucose concentration
• In muscle glucose-6-P glycolysis

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Carbohydrate Metabolism Glycogenolysis Activation
Glycogenolysis is activated in response to stress

• Physiologic -- in response to increased blood
  glucose utilization during prolonged exercise.
• Pathologic -- as a result of blood loss.
• Acute stress (regardless of source) activates
  glycogenolysis through the action of
  catecholamine hormone, epinephrine (released by
  the adrenal medulla).
• During prolonged exercise both glucagon and
  epinephrine contribute to stimulation of
  glycogenolysis.

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Insulin
Carbohydrate MetabolismHormonal regulation
Inhibition of Glycogenolysis
Antagonist of glucagon, epinephrine (adrenalin),
cortisol

• Insulin secreted by pancreas b-cells when blood
  glucose is high.
• Synthesized as single peptide chain zymogen
  proinsulin.
• In secretory granules, selective proteolysis
  releases an internal peptide and a 2-chained
  insulin hormone.
• Insulin elicits uptake and intracellular use or
  storage of glucose.
• Hyperglycemia results in elevated blood insulin
  associated with fed state.
• Hyperinsulinemia associated with insulin
  resistance and if chronic can lead to diabetes
  type-2 and related pathologies.

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BLOOD GLUCOSE

• Blood glucose is derived from
• 1. Diet
• 2. Gluconeogenesis
• 3. Glycogenolysis in liver
• Insulin play a central role in regulating blood
  glucose
• Glucagon increase blood glucose
• Growth hormone inhibit insulin activity

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