Metabolism: Glycolysis
Glycolysis
1897: Hans and Eduard Buchner
(Sucrose cell-free experiments;
fermentation can take place outside of
living cells) METABOLISM became simple
chemistry
Glycolysis: “Embden-Meyerhof pathway”
The all-important Glucose
The only fuel the brain uses in nonstarvation conditions
The only fuel red blood cells can use
WHY?
 Evolutionary: probably available for
primitive systems (from formaldehyde)
 Low tendency to glycosylate proteins,
strong tendency to exist in ring form
(recall: all equatorial!)
The products and their fates
Sidebar: Fermentation
WHY? (lower energy yield)
 Oxygen is not required!
 OBLIGATE ANAEROBES (Cannot live with
oxygen)
 FACULTATIVE ANAEROBES (Can live with
or without oxygen)
Glycolysis: a 10-step pathway
Three major stages
1. Glucose (G) to Fructose-1,6-bisphosphate (F1,6-BP)
 Three steps, INVESTMENT of 2 ATP
2. F-1,6-BP to two three-carbon fragments
 Two component steps to glyceraldehde-3phosphate (G3P)
3. G3P to Pyruvate
 Five steps, YIELD of 4 ATP
RXN 1: Phosphorylation of G
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 (and glucokinase) act to
phosphorylate glucose and keep it in the cell
 ATP is used, thus FIRST PRIMING REACTION G large,
negative
RXN 2: G6P to F6P
 Isomerization of glucose-6-phosphate to
fructose-6-phosphate
 By phosphoglucose isomerase
 Why does this reaction occur??
 next step (phosphorylation at C-1) would be tough
for hemiacetal -OH, but easy for primary -OH
 isomerization activates C-3 for cleavage in aldolase
reaction
RXN 3: F6P to F-1,6-BP
Fructose-6-P to Fructose-1,6-bisphosphate
= the “COMMITTED STEP”
By phosphofructokinase (PFK)
 ATP is used, thus SECOND PRIMING REACTION
G large, negative
On PFK
 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
Recall Stage 1
RXN 4: C6 cleaves to 2 C3s (DHAP,
Gly-3-P)
Done by Aldolase
RXN 5: DHAP converted to Gly-3-P
Triose phosphate isomerase is used
Important: Glu and His in active site;
mechanism through ene-diol
intermediate
Recall stage 2
So far…
We’ve USED UP 2 ATP molecules to
process 1 glucose molecule
We are left with 2 G3P now
Time for energy payback, thus STAGE 3!
Recall that stage 3 happens in parallel to
the two G3P molecules
RXN 6: G3P is oxidized to 1,3-BPG
 Glyceraldehyde-3-phosphate to 1,3bisphosphoglycerate
 By glyceraldehyde 3-phosphate dehydrogenase
RXN 7: 1,3-BPG to 3-PG
1,3-bisphosphogycerate to 3phosphoglycerate
By phosphoglycerate kinase (ATP yield!)
RXN 8: 3PG to 2PG
Simply Phosphoryl group from C-3 to C-2
 Done by phosphoglycerate mutase
 Rationale for this enzyme - repositions the
phosphate to make PEP
RXN 9: 2PG to PEP
 2-phosphoglycerate to phosphoenolpyruvate
 By Enolase
 "Energy content" of 2-PG and PEP are similar
 Enolase just rearranges to a form from which more energy
can be released in hydrolysis
RXN 10: PEP to Pyruvate
 By pyruvate kinase
 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
Review stage 3
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 Rxns have large, negative G
 Large negative G Rxns are sites of regulation!
GLYCOLYSIS NET
 Glucose + 2Pi + 2 ADP + 2 NAD+ ->
2 pyruvate + 2 ATP + 2 NADH + 2H+ + 2H2O
What Now?: The Fate of NADH
and Pyruvate
 Aerobic or anaerobic, that is the question.
 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 Pyruvate
 Pyruvate is also energy
- two general possible
fates
(AEROBIC/ANAEROBIC):
 aerobic: citric acid
cycle
 anaerobic: to lactate
(lactic acid
fermentation) or to
ethanol (alcoholic
fermentation)