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2012-02-14 17:03:11
Carb Digestion Metabolic Pathways

Midterm Week 4
Show Answers:

  1. What enzymes are secreted to break down starches and where? What do they do?
    • Mouth: alpha amylase hydrolyzes alpha-1,4 bond of starches
    • Stomach: alpha-amyalse to continue breaking down starch - mostly short chain polysaccharides
    • Small intestine: pancreatic alpha-amylase and alpha dextrinase - secreted into the lumen
  2. What is the enzyme activity in the brush border?
    • Digestion must occur before absorption can
    • Lactase: breaks down lactose into galactose and glucose - hydrolyze the galactose beta (1-4) glucose bond
    • Maltase: hydrolyzes alpha-1,4 bond in maltose
    • Isomaltase: hydrolyzes alpha-1,6 bond in dextrines to form 2 glucoses
    • Alpha dextrinase: hydrolyzes alpha-1,4 of dextrines
    • Sucrase and Invertase: hydrolyze the glucose alpha (1-2) fructose bond
    • There are no enxymes that cleave bonds in dietary fibre (beta-1,4 between glucoses)
  3. How are monosaccharides absorbed?
    • Very efficient
    • Nearly all absorbed by the end of the jejunum
    • Active transport: used for glucose and galactose - uses SGLT1 (sodium glucose transporter)
    • Facilited transport: for frutose as maintains [] gradient therefore uses downward gradient to head to the liver
  4. What are the 3 functions of carbs?
    • 1. Glucose = primary source of energy for cells: especially brain and RBC
    • 2. carbs spare proteins: prevents breakdown of proteins for energy and allows protein to concentrate on building, repairing, and mainting body tissues
    • 3. Carbs prevent ketosis: when carbs are limited, fat can be broken down for energy whic creates ketone bodies and amkes the body acidic - partial breakdown can of fat can be used short term by the body
  5. What are the key metabolic pathways of CHO use and storage?
    • Glycogenesis
    • Glyconeogenesis
    • Glycogenolysis
    • Glycolysis
    • TCA cycle
    • Hexose monophosphate shunt
  6. What is glycogenesis? How and where does it occur?
    • Synthesis of glycogen from glucose to maintain homeostasis
    • alpha-1,4 with alpha-1,6 branches
    • Use enzymes glycogen synthase and a branching enzyme (makes alpha-1,6 bonds)
    • Requires energy
    • Major sites: liver, skeletal muscle
    • Hormonal control: insulin - released to stimulate glycogen synthesis
  7. What is glucogenolysis? What does it involve? Where does it occur?
    • Breakdown od glycogen into glucose units
    • Uses the enzymes glycogen phosphorylase and and debranching enzyme
    • Hormonal control (eg. glucagon - high during fasting therefore signals glucogenolysis)
    • Muscle does not contribute to blood glucose - it breaks down its own glycogen and uses it locally
    • Liver contributes free glucose and has blood glucose control
  8. What is glycolysis? What is produced? How does this affect RBC? What are the 2 different kinds?
    • Initial reactions for compelte oxidation of glucose to O2 and H2O
    • Glycolytic enzymes in cytoplasm
    • Substrate level phosphorylation yeilds minor amount of ATP
    • RBC don't have mitochondria therefore this is important
    • Endpoints depend on available O2
    • Aerobic: pyruvate goes to mitochondira for complete oxidation
    • Anaerobic: lactate is formed
  9. What is the hexose monophosphate shunt? Where does it have low and high activity?
    • Important for NADPH production and ribose synthesis
    • high acitivty in liver, adipose tissues, lactating mammary gland
    • Low acitivty in skeletal muscles
    • Leads to production of pentose phosphates
    • Different steps lead to NADPH synthesis (important in FA synthesis)
  10. What is pyruvate dehydrogenation?
    • Deydrogenation of pyruvate to form acetyl CoA which is a meeting point for many metabolic processes
    • Uses pyruvate dehydrogenase complex
    • Negative regulation by ATP and NADH
    • Micronutrients drive the reaction: thiamine, niacin, riboflavin, pantothenic acid
  11. What is th Krebs cycle?
    • Over 90% of the energy in food released here
    • A common and final catabolic pathway b/c products of CHO, fat and protein feed into the cycle
    • Occurs in mitochondiral matric
    • Centre of energy metabolism
    • Provides reduceed cosubstrates NADH and FADH2 that yield ATP through their oxidation via ETC
    • Not all substrates entering cycle are fully oxidized
    • Produces: 3NADH, 1FADH2, 2CO2, 1ATP and 1 Acetyl CoA
  12. What is gluconeogenesis and waht can be used to do this?
    • Glucose synthesis from non-CHO sources
    • Pyruvate, some a.a., lactate, glycerol
    • Only uses glucogenic a.a.
    • All get converted to pyruvate which then needs a bypass step
  13. When is gluconeogenesis needed? What is done with muscle lactate?
    • When blood glucose drops - need hormonal control
    • occurs in the liver and also in kidneys (starvation)
    • Muscle adipose tissue lack needed bypass anzymes
    • Cori cycle: muscle lactate transported to liver - glucose synthesis - back to muscle
  14. What is a key step in gluconeogenesis?
    • the bypass of the pyruvate kinase reaction involves formation of oxaloacetate
    • moving oxaloacetate out of the mitochondira (conversion to malate first) and into the cytoplasm
    • Oxaloacetate to PEP by PEP carboxykinase (a cytoplasmic enzyme)
  15. What can acetyl CoA be used for? What about oxaloacetate-malate?
    • Can be used for fatty acid synthesis or in krebs cycle. BUT can not be used to make glucose, as it provides no net increase in cycle intermediates
    • Allows carbon skeletons of various amino acids to enter gluneogenesis