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2010-10-08 09:38:13
glycolysis TCA cycle pyruvate dehydrogenase

Glycolysis & the TCA Cycle
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  1. three major pathways for glucose metabolism
    oxidation to pyruvate through glycolysis; oxidation to ribose 5-phosphate through the pentose phosphate pathway; storage in the form of glycogen (animals) or starch or sucrose (plants)
  2. average concentration of glucose in a healthy human
    around 5 mM (4 - 8 mM)
  3. GLUT-1
    unregulated glucose transporter; widespread; high-affinity (Km = 1 mM); present in blood & cornea & placenta & brain & transformed (cancerous) cells
  4. GLUT-2
    unregulated glucose transporter; found in liver and beta cells of pancreas; low-affinity (Km = 1 mM); allows glucose to enter liver quickly when blood glucose is high and allows glucose to exit the liver after gluconeogenesis when blood glucose is low
  5. GLUT-3
    unregulated glucose transporter; found in neurons; high-affinity (Km = 1 mM); insulin-dependent
  6. GLUT-4
    surface expression of this transporter is dependent on insulin; normally sequestered in microvesicles in the cytosol; found in skeletal muscle & adipose tissue & the heart; moderate affinity (Km = 5 mM)
  7. phosphofructokinase
    converts fructose-6-phosphate to fructose 1 6-bisphosphate; requires 1 molecule of ATP; is the committed step of glycolysis
  8. hexokinase
    enzyme that converts glucose to glucose-6-phosphate; reaction requires ATP; reaction is irreversible and results in trapping of glucose molecule in cell
  9. phosphoglycerate kinase
    converts 1 3-bisphosphoglycerate to 3-phosphoglycerate; requires 1 molecule of ATP
  10. pyruvate kinase
    converts PEP into pyruvate; produces ATP
  11. sulfhdryl reagents
    inhibit conversion of G3P to 1 3 bisphosphoglycerate
  12. fluoride
    inhibits step between intermediary of G3P -> 1 3 bisphosphoglycerate
  13. mitochondrial shuttles
    transfer reducing equivalents from NADH in the cytosol across the mitochondrial membrane -> regeneration of NAD+; important shuttles include glycerol phosphate shuttle (NADH -> FADH2 in mitochondria) and malate-aspartate shuttle (NADH -> NADH in mitochondria)
  14. regulation of hexokinase
    negative feedback inhibition by G6P (competitive inhibition)
  15. regulation of PFK-1
    highly regulated; inhibited by ATP & citrate & H+ - allosteric regulation; also inhibited by glucagon
  16. regulation of pyruvate kinase
    inhibited by ATP and alanine
  17. difference between glucokinase and hexokinase
    glucokinase = only in liver; not inhibited by product; is positively regulated by glucose or insulin; is low affinity & high capacity while hexokinase is high affinity & low capacity
  18. regulation of glycolysis by glucagon
    major controller of fructose 2 6 bisphosphate; secreted from pancreas; slows glycolysis by binding to receptors that activate adenylyl cyclase -> cAMP activation -> decreased F 2 6 BP
  19. regulation of glycolysis by epinephrine
    in liver - both epinephrine and glucagon increase adenylyl cyclase -> cAMP activation -> PKA -> phosphorylation of PFK-2 (deactivates) -> down-regulation of fructose 2 6 bisphosphate -> decreased rate of glycolysis; in heart - same pathway but opposite results - phosphorylation activates PFK-2 -> up-regulation of fructose 2 6 bisphosphate
  20. regulation of glycolysis by insulin
    opposes actions of glucagon in liver; affects metabolism of glucose by stimulating cAMP phosphodiesterase -> conversion of cAMP to AMP -> prevents down-regulation of glycolysis; can also indirectly upregulate fructose 2 6 bisphosphate through PFK-2
  21. methods for recovery of NAD+
    aerobic oxidation; anaerobic homolactic fermentation; anaerobic alcoholic fermentation
  22. pyruvate dehydrogenase reaction
    overall: pyruvate + fatty acids + amino acids -> acetyl coA + NADH + H+ + CO2; step 1 - pyruvate decarboxylated by E1 segment of PDH; acetyl -> enzyme-bound cofactor linked to TPP (CO2 released); step 2 - acetyl transferred to E2 cofactor (lipoic acid); step 3 - acetyl -> coenzyme A -> acetyl coA; 2 electrons transferred to E3 -> NADH
  23. arsenic poisoning
    arsenic affects E2 unit of pyruvate dehydrogenase
  24. vitamin derivatives in pyruvate dehydrogenase reaction
    TPP = vitamin B1 derivative; FAD = vitamin B2/riboflavin derivative; NAD = niacin derivative; B-complex insufficiency -> cannot make acetyl coA from pyruvate -> problems with energy metabolism
  25. regulation of pyruvate dehydrogenase reaction
    regulated by allosteric inhibition and competitive inhibition of end-products; acetyl coA & NADH can activate kinase that deactivates PDH (surplus of products -> PDH inactivation); substrates of reaction have opposite effect - deactivate kinase that deactivates PDH; upregulation of small ions -> activation of PDH
  26. regulation of TCA cycle
    energetically favorable; regulation occurs via end products
  27. enzymes targeted by metabolic poisons
    fluorcitrate -> aconitase; malonate -> succinate dehydrogenase; arsenite -> alpha-ketoglutinate dehydrogenase