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GLYCOLYSIS
Glycolysis: oxidative breakdown of glucose to
pyruvate with capture of some energy as ATP, NADH
(first step in respiration)
Glycolysis
glucose
pyruvate
Subsequent reactions
lactate (or)
CO2+ethanol (or)
O2 absent
malate
CO2 + H2O
-- O2 present
Glycolysis can be divided into two phases: priming and payoff
Phase 1: five priming reactions
1. glucose + ATP
G-6-P + ADP
2. G-6-P
F-6-P
3. F-6-P + ATP
F-1,6-bisP + ADP
4. F-1,6,bisP
DHAP + G-3-P
5. DHAP
G-3-P
Glycolysis can be divided into two phases: priming and payoff
Phase 2: five payoff reactions
6. G-3-P + PI + NAD+
1,3-bisPGA + NADH
7. 1,3-bisPGA + ADP
3-PGA + ATP
8. 3-PGA
2-PGA
9. 2-PGA
PEP
10. PEP + ADP
pyruvate + ATP
Reaction 6
DGo’ =+6.3 kJ/mol
DG’ = -1.29 kJ/mol
Near equilibrium:
not regulated
Note that the C in
-C-O-PO3 has been
oxidized (from aldehyde
to acid)
The coupling of acyl-phosphate
formation to oxidation avoids
an energy hump that would
drastically slow the reaction.
Summary of glycolysis:
glucose + 2 ADP + 2 Pi + 2 NAD +
2 pyruvate + 2 ATP + 2 H2O + 2 NADH + 2 H+
What happens next? It depends on the environment:
aerobic or anaerobic. Regeneration of NAD + is
essential to keep the process going.
including oxidation
of NADH
In fungi and plants in the absence of O2, pyruvate is decarboxylated to
acetaldehyde; acetaldehyde is reduced to ethanol; NADH is oxidized to NAD+.
The NAD+ is recycled to oxidize more glyceraldehyde-3-P.
In animals and some bacteria, pyruvate is reduced to lactate as
NADH is oxidized to NAD+.
Control: activity of glycolysis depends on allosteric enzymes and
responds to energy requirement.
In muscle:
Control: activity of glycolysis depends on allosteric enzymes and
responds to energy requirement.
In liver, fructose-2,6-bisP is a important stimulator of PFK:
enhances substrate activation
relieves allosteric inhibition
Cancer cells produce most of their ATP by
glycolysis (Warburg effect)
Why do proliferating cells switch to a less efficient
metabolism?
Probable answer: growth requires more Ccompounds and reduction power (NADPH), than ATP
energy.
(see Science 324:1029 May 22, 2009)
Summary
•Glucose is metabolized to pyruvate in a series of 10 reactions.
•Glycolysis provides the cell (cytoplasm) with 2 mol ATP/glucose.
•Glycolysis also provides cytoplasm with 2 mol NADH/glucose.
•In the absence of O2, NADH is oxidized by reduction of pyruvate.
•In the presence of O2, NADH is oxidized in the mitochondria.
•Rate of glycolysis is controlled at 3 key allosteric enzymes.