Regulation of Glycoysis

1

• Regulation of Glycoysis

2
Pyruvate can go in three major directions after
glycolysis

• Under aerobic conditions pyruvate is oxidized to
  Acetyl-CoA which can enter Citric acid (TCA)
  cycle.
• Under anaerobic conditions pyruvate can be
  reduced to ethanol (fermentation) or lactate
• Under anaerobic conditions formation of ethanol
  and lactate is important in the oxidization NADH
  back to NAD
• Under aerobic conditions NADH is oxidized to NAD
  by the respiratory electron transport chain.

3
Need to recycle NAD from NADH if gylcolysis is
to continue under anaerobic conditions
4
Lactate formation
  

• In animals under anaerobic conditions pyruvate is
  converted to lactate by the enzyme lactate
  dehydrogenase
• Impt for the regeneration of NAD under anaerobic
  conditions.

5

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

Cori Cycle
6
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 impt in baking where it makes dough
  rise and brewing where it carbonates beer.

7
Free Energy Change in Glycolysis
Hexokinase
Phosphofructokinase-1
Pyruvate kinase
8
Control Points in Glycolysis
9
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.

10
Insulin Induced Exocytosis of Glucose Transporter
11
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.

12
Regulation of PhosphoFructokinase (PFK-1)

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

13
PFK-1 regulation by adenosine nucleotides

• ATP is substrate and inhibitor. Binds to active
  site and allosteric site on PFK. Binding of ATP
  to allosteric site increase Km for ATP
• AMP and ADP are allosteric activators of PFK.
• AMP relieves inhibition by ATP.
• ADP decreases Km for ATP
• Glucagon (a pancreatic hormone) produced in
  response to low blood glucose triggers cAMP
  signaling pathway that ultimately results in
  decreased glycolysis.

14
Effect of ATP on PFK-1 Activity
15
Effect of ADP and AMP on PFK-1 Activity
16
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 inhibited by Pi and stimulated by citrate

17
Glucagon Regulation of PFK-1 in Liver

• G-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)

18
Regulation of Pyruvate Kinase

• Allosteric enzyme
• Activated by Fructose-1,6-bisphosphate (example
  of feed-forward regulation)
• Inhibited by ATP
• When high fructose 1,6-bisphosphate present plot
  of S vs Vo goes from sigmoidal to hyperbolic.
• Increasing ATP concentration increases Km for
  PEP.
• In liver, PK also regulated by glucagon. Protein
  kinase A phosphorylates PK and decreases PK
  acitivty.

19
Pyruvate Kinase Regulation
20
Deregulation of Glycolysis in Cancer Cells

• Glucose uptake and glycolysis is ten times faster
  in solid tumors than in non-cancerous tissues.
• Tumor cells initally 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)

21
Pasteur Effect

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

22
Other Sugars can enter glycolysis
23
How other sugars 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.

24
(No Transcript)
25
Galactosemia

• Deficiency of galactose-1- phosphate
  uridylytransferase.
• galactose-1-phosphate accumulates
• Leads to liver damage
• Untreated infants fail to trive often have mental
  reatrdation.
• Can be treated with galactose free diet.

26
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)