Glycolysis
Gluconeogenesis
Glycolysis - Overview
One of best characterized pathways
Characterized in the first half of 20th century
Glucose --> 2 pyruvates + energy
Strategy
add phosphoryl groups to glucose
convert phosphorylated intermediates into compounds with
high phosphate group-transfer potentials
couple the subsequent hydrolysis of reactive substances to
ATP synthesis
Glucose + 2NAD+ + 2 ADP + 2Pi -->
2NADH + 2 pyruvates + 2ATP + 2H2O + 4H+
Overview of Glycolysis
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The Embden-Meyerhof (Warburg) Pathway
Essentially all cells carry out glycolysis
Ten reactions - similar in most 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
NADH must be recycled
Three possible fates for pyruvate
Glycolysis
Glycolysis
Fate of pyruvate
Mitochondrial
oxidation
1 NADH --> ~3 ATP
Reduction to
lactate
Decarboxylation to
acetaldehyde
Reduction to
ethanol
Enzymes of glycolysis
Catalyzed reactions
and properties
Enzymes of glycolysis
Catalyzed reactions
and properties
Glucose
Hexokinase, glucokinase
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6phosphate
Phosphofructokinase
Fructose-1,6biphosphate
Dihydroxyacetone
phosphate
Triose phosphate isomerase
Aldolase
Glyceraldehyde-3phosphate
First Phase of Glycolysis
The first reaction - phosphorylation of glucose
• Hexokinase or glucokinase
• This is a priming reaction - ATP is consumed here
in order to get more later
• ATP makes the phosphorylation of glucose
spontaneous
Hexokinase
1st step in glycolysis; G large, negative
• Hexokinase (and glucokinase) act to phosphorylate
glucose and keep it in the cell
• Km for glucose is 0.1 mM; cell has 4 mM glucose
• So hexokinase is normally active!
• Glucokinase (Kmglucose = 10 mM) only turns on when
cell is rich in glucose
• Hexokinase is regulated - allosterically inhibited by
(product) glucose-6-P -
Hexokinase
• First step in glycolysis
• Large negative deltaG
• Hexokinase is regulated - allosterically inhibited by
(product) glucose-6-P
• Corresponding reverse reaction (Gluconeogenesis) is
catalyzed by a different enzyme (glucose-6phosphatase)
• Is it the committed step in glycolysis ?
Glucose
Glucose-6-P
dehydrogenase
Glycogen
Glucose-6-P
Ribose-5-P + NADPH
Fructose-6-P
Glyceraldehyde-3-P
Pyruvate
ATP
Nucleic acid
synthesis
Reducing
power
Rx 2: Phosphoglucoisomerase
Glucose-6-P to
Fructose-6-P
Rx 3: Phosphofructokinase
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PFK is the committed step in glycolysis!
The second priming reaction of glycolysis
Committed step and large, neg delta G - means PFK is
highly regulated
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
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
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– 1,3 BPG
– Phosphoenolpyruvate
Glyceraldehyde-3phosphate
Glyceraldehyde-3-phosphate
dehydrogenase
1,3-biphosphoglycerate
Phosphoglycerate
kinase
3-phosphoglycerate
Phosphoglycerate
mutase
2-phosphoglycerate
Enolase
phosphoenolpyruvate
Pyruvate kinase
pyruvate
Rx 10: Pyruvate Kinase
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PEP to Pyruvate makes ATP
These two ATP (from one glucose) can be
viewed as the "payoff" of glycolysis
Large, negative G - regulation!
Allosterically activated by AMP, F-1,6-bisP
Allosterically inhibited by ATP and acetyl-CoA
The Fate of NADH and Pyruvate
Aerobic or anaerobic??
• NADH is energy - two possible fates:
– If O2 is available, NADH is re-oxidized in the
electron transport pathway, making ATP in
oxidative phosphorylation
– In anaerobic conditions, NADH is re-oxidized
by lactate dehydrogenase (LDH), providing
additional NAD+ for more glycolysis
The Fate of NADH and Py
Aerobic or anaerobic??
• Pyruvate is also energy - two possible fates:
– aerobic: citric acid cycle
– anaerobic: LDH makes lactate
Energetics of
Glycolysis
The elegant evidence of
regulation!
• Standard state G values
are scattered: + and  G in cells is revealing:
• Most values near zero
• 3 of 10 reactions have
large, negative  G
• Large negative  G
reactions are sites of
regulation!
Gluconeogenesis
Synthesis of "new glucose" from common metabolites
• Humans consume 160 g of glucose per day
• 75% of that is in the brain
• Body fluids contain only 20 g of glucose
• Glycogen stores yield 180-200 g of glucose
• So the body must be able to make its own glucose
Comparison of
glycolysis and
gluconeogenesis
pathways
Substrates for Gluconeogenesis
Pyruvate, lactate, glycerol, amino acids and all
TCA intermediates can be utilized
• Fatty acids cannot!
• Most fatty acids yield only acetyl-CoA
• Acetyl-CoA (through TCA cycle) cannot
provide for net synthesis of sugars
Gluconeogenesis I
• Occurs mainly in liver and kidneys
• Not the mere reversal of glycolysis for 2
reasons:
– Energetics must change to make
gluconeogenesis favorable (delta G of
glycolysis = -74 kJ/mol
– Reciprocal regulation must turn one on and the
other off - this requires something new!
Energetics of
Glycolysis
The elegant evidence of
regulation!
 G in cells is revealing:
• Most values near zero
• 3 of 10 reactions have
large, negative  G
• Large negative  G
reactions are sites of
regulation!
• Reactions 1, 3 and 10
should be different to go
into opposite direction
Gluconeogenesis II
Something Borrowed, Something New
• Seven steps of glycolysis are retained:
– Steps 2 and 4-9
• Three steps are replaced:
– Steps 1, 3, and 10 (the regulated steps!)
• The new reactions provide for a spontaneous
pathway (G negative in the direction of sugar
synthesis), and they provide new mechanisms
of regulation