Cho metab

• Glycolysis


  Anaerobic process that converts hexose to two pyruvates, generating 2 ATP and 2 NADH
• For certain cells in the brain and eye, glycolysis is the only ATP generating pathway
• Glycolysis


  Glucose+2ADP+2NAD++2Pi -> 2pyruvate+2ATP+2NADH+2H++2H20

  
• Glycolysis


  • Essentially all cells carry out glycolysis
  • Ten reactions - same in all cells - but rates differ
  • Two phases: First phase converts glucose to two G-3-P, Second phase produces two pyruvates
  • Products are pyruvate, ATP and NADH
  • Three possible fates for pyruvate

  
• Hexokinase


  1st step in glycolysis; ATP makes the phosphorylation of glucose spontaneous

  
• Hexokinase


  • Two major forms: hexokinase (all cells) & glucokinase (liver)
  • Km for hexokinase is 10-6 to 10-4 M; cell has 4 X 10-3 M glucose
  • Km for glucokinase is 10-2 M only turns on when cell is rich in glucose
  • Glucokinase functions when glucose levels are high to sequester glucose in the liver
  • Hexokinase is regulated - allosterically inhibited by (product) glucose-6-P

  
• Phosphoglucoisomerase


  Uses open chain structure as substrate; Near-equilibrium reaction (reversible); Highly stereospecific

  
• Phosphoglucoisomerase reaction occurs to activate C-3 for cleavage in aldolase reaction

  
• Phosphofructokinase (PFK)


  The committed step in glycolysis; Second priming reaction of glycolysis; Highly regulated

  
• Phosphofructokinase (PFK)


  • ATP inhibits, AMP reverses inhibition
  • Citrate is also an allosteric inhibitor
  • Fructose-2,6-bisphosphate is allosteric activator
  • PFK increases activity when energy status is low
  • PFK decreases activity when energy status is high
• Aldolase


  Hexose cleaved to form two trioses; Near-equilibrium reaction
• Triose Phosphate Isomerase (TPI)


  Near equilibrium reaction; Conversion of DHAP to G-3-P maintains steady state [G-3-P]

  
• Triose phosphate isomerase is a near-perfect enzyme (Kcat/Km near diffusion limit)

  
• Glycolysis - Second Phase


  Metabolic energy produces 4 ATP; Net ATP yield for glycolysis is two ATP; Second phase involves two very high energy phosphate intermediates (1,3 BPG and Phosphoenolpyruvate)

  
• Glyceraldehyde-3P-Dehydrogenase


  G3P is oxidized and phosphorylated to 1,3-BPG; Near equilibrium reaction; Pi is used as phosphate donor; NADH generated is reoxidized by respiratory electron transport chain

  
• Phosphoglycerate Kinase (PGK)


  ATP synthesis from a high-energy phosphate; Referred to as "substrate-level phosphorylation"; Near-equilibrium reaction

  
• 2,3-BPG (for hemoglobin) is made by circumventing the PGK reaction
• 2,3-BPG


  Acts to maintain Hb in low oxygen affinity form; RBC contain high levels of 2,3 BPG (4 to 5 mM)
• Phosphoglycerate Mutase


  Phosphoryl group moves from C-3 to C-2; Mutases are isomerases that transfer phosphates from one hydroxyl to another

  
• Enolase


  Near equilibrium reaction; "Energy content" of 2-PG and PEP are similar; Requires Mg2+ for activity

  
• Pyruvate Kinase


  Substrate level phosphorylation generates second ATP; Large, negative ΔG - regulation!; Allosterically activated by AMP, F-1,6-bisP; Allosterically inhibited by ATP and acetyl-CoA

  
• Possible fates for pyruvate after glycolysis


  • Oxidized to Acetyl-CoA which can enter Citric acid (TCA) cycle under aerobic conditions
  • Reduced to ethanol (fermentation) or lactate under anaerobic conditions

  
• Lactate formation


  In animals under anaerobic conditions pyruvate is converted to lactate by the enzyme lactate dehydrogenase; Important for the regeneration of NAD+ under anaerobic conditions

  
• The circulatory systems of large animals are not efficient enough in O2 transport to sustain long periods of muscular activity; Anaerobic conditions lead to lactate accumulation and depletion of glycogen stores; Short period of intense activity must be followed by recovery period; Lactic acidosis causes blood pH to drop
• Alcohol Fermentation


  Important for the regeneration of NAD+ under anaerobic conditions; Process common to microorganisms like yeast; Yields neutral end products (CO2 and ethanol); CO2 generated important in baking and brewing
• Control Points in Glycolysis


  • Regulation of Hexose Transporters
  • Regulation of Hexokinase
  • Regulation of PhosphoFructokinase (PFK-1)
  • Regulation of PFK by Fructose-2,6-bisphosphate
  • Glucagon Regulation of PFK-1 in Liver
  • Regulation of Pyruvate Kinase

  
• Regulation of Hexose Transporters


  Intra-cellular [glucose] are much lower than blood [glucose]; Glucose imported into cells through a passive glucose transporter; Elevated blood glucose and insulin levels leads to increased number of glucose transporters in muscle and adipose cell plasma membranes

  
• Regulation of Hexokinase


  Glucose-6-phosphate is an allosteric inhibitor of hexokinase; Levels of glucose-6-phosphate increase when down stream steps are inhibited; This coordinates the regulation of hexokinase with other regulatory enzymes in glycolysis; Hexokinase is not necessary the first regulatory step inhibited

  
• Regulation of PhosphoFructokinase (PFK-1)


  • PFK-1 has quaternary structure
  • Inhibited by ATP and Citrate
  • Activated by AMP and Fructose-2,6-bisphosphate
  • Regulation related to energy status of cell

  
• PFK-1 regulation by adenosine nucleotides


  ATP is substrate and inhibitor; AMP and ADP are allosteric activators of PFK; Glucagon triggers cAMP signaling pathway that ultimately results in decreased glycolysis

  
• Regulation of PFK by Fructose-2,6-bisphosphate


  Fructose-2,6-bisphosphate is an allosteric activator of PFK in eukaryotes, but not prokaryotes; Formed from fructose-6-phosphate by PFK-2; Degraded to fructose-6-phosphate by fructrose 2,6-bisphosphatase; In mammals the 2 activities are on the same enzyme; PFK-2 is inhibited by Pi and stimulated by citrate

  
• Glucagon Regulation of PFK-1 in Liver


  1. Protein mediated cAMP signaling pathway; Induces protein kinase A that activates phosphatase activity and inhibits kinase activity; Results in lower F-2,6-P levels decrease PFK-1 activity (less glycolysis)
• Regulation of Pyruvate Kinase

  Allosteric enzyme; Activated by Fructose-1,6-bisphosphate; Inhibited by ATP; Increasing ATP concentration increases Km for PEP; In liver, PK is also regulated by glucagon - protein kinase A phosphorylates PK and decreases PK activity

  
• Glucose uptake and glycolysis is ten times faster in solid tumors than in non-cancerous tissues; Tumor cells initially lack connection to blood supply so limited oxygen supply; Tumor cells have fewer mitochondrial, depend more on glycolysis for ATP; Increase levels of glycolytic enzymes in tumors (oncogene Ras and tumor suppressor gene p53 involved)

  
• Pasteur Effect


  Under anaerobic conditions glycolysis proceeds at higher rates than during aerobic conditions; Slowing of glycolysis in the presence of oxygen is the Pasteur Effect; Cells sense changes in ATP supply and demand and modulate glycolysis

  
• How other sugars can enter glycolysis


  • Mannose can be phosphorylated to mannose-6-phosphate by hexokinase and then converted to fructose-6-phosphate by phosphomannose isomerase
  • Fructose can be phosphorylated by fructokinase to form fructose-1 phosphate (F-1-P); F-1-P can then be converted to glyceraldehyde and DHAP by F-1-P aldolase; Triose kinase then converts glyceraldehyde to G-3-P

  
• Galactosemia


  Deficiency of galactose-1-phosphate uridylytransferase; galactose-1-phosphate accumulates; Leads to liver damage; Untreated infants fail to thrive often have mental retardation; Can be treated with galactose free diet
• Lactose Intolerance


  Humans undergo reduction in lactase at 5 to 7 years of age; In lactase deficient individuals, lactose is metabolized by bacteria in the large intestine; Produce CO2, H2 and short chain acids; Short chain acids cause ionic imbalance in intestine (diarrhea)
• Citric Acid Cycle


  Also known as TCA Cycle or Krebs Cycle

  
• Glycolysis


  Metabolic pathway that converts glucose into pyruvate