Transcript Chapter 11 (part 2)

Chapter 11 (part 2)
Regulation of Glycolysis
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.
Need to recycle NAD+ from NADH if
gylcolysis is to continue under
anaerobic conditions
Lactate formation
NADH
NAD
OH
O
O
O
H3C
C
H3C
C
O
NADH
C
H
C
O
NAD
•In animals under anaerobic conditions pyruvate
is converted to lactate by the enzyme lactate
dehydrogenase
•Impt for the regeneration of NAD+ under
anaerobic conditions.
• 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
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.
Free Energy Change in Glycolysis
Hexokinase
Phosphofructokinase-1
Pyruvate kinase
Control Points
in Glycolysis
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.
Insulin Induced Exocytosis of
Glucose Transporter
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)
• PKF-1 has quaternary structure
• Inhibited by ATP and Citrate
• Activated by AMP and Fructose-2,6bisphosphate
• Regulation related to energy status of cell.
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.
Effect of ATP on PFK-1
Activity
Effect of ADP and AMP on PFK-1 Activity
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 2,6bisphosphatase.
•In mammals the 2 activities are on the same enzyme
•PFK-2 inhibited by Pi and stimulated by citrate
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)
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.
Pyruvate Kinase Regulation
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)
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
Other
Sugars can
enter
glycolysis
How other sugars enter glycolysis
• Mannose can be phosphorylated to mannose-6phosphate 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-1phosphate 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.
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)