Glycolysis
January 22, 2003
Bryant Miles
O
CH2OH
CH2OH
CH
O
O
H C OH
OH
OH
HO C H
OH
OH
OH
H C OH
OH
OH
H C OH
α-D-glucose
H2C OH
OH
β-D-glucose
D-glucose
D-Glucose is a major fuel for most organisms. D-Glucose metabolism occupies the center position for all
metabolic pathways. Glucose contains a great deal of potential energy. The complete oxidation of
glucose yields −2,840 kJ/mol of energy.
6CO2 + 6H2O ∆Go’ = −2,840 kJ/mol
Glucose + 6O2
Glucose also provides metabolic intermediates for
biosynthetic reactions. Bacteria can use the skeletal carbon
atoms obtained from glucose to synthesize every amino acid,
nucleotide, cofactor and fatty acid required for life.
For higher plants and animals there are three major
metabolic fates for glucose.
Nearly every living cell catabolizes glucose and other simple
sugars by a process called glycolysis. Glycolysis differs
from one species to another only in the details of regulation
and the fate of pyruvate. Glycolysis is the metabolic
pathway that catabolizes glucose into two molecules of
pyruvate.
CH2OH
O
H
H
OH
H
H
2
OH
OH
H
H3 C
O
O
C
C
O-
OH
Glycolysis occurs in the cytosol of cells and is essentially an anaerobic process since the pathway’s
principle steps do not require oxygen. The glycolytic pathway is often referred to as the EmbfenMeyerhof pathway in honor of the two of the three biochemical pioneers (What about Jacob Parnas?)
who discovered it. The glycolytic pathway is shown below.
Glycolysis consists of 10 enzyme catalyzed reactions. The pathway can be broken down into two phases.
The first phase encompasses the first five reactions to the point that glucose is broken down into 2
molecules of glyceraldehyde 3-phosphate. Phase 1 consumes two molecules of ATP. The second phase
includes the last five reactions converting glyceraldehyde 3-phosphate into pyruvate. Phase 2 produces 4
molecules of ATP and 2 molecules of NADH. The net reaction (Phase 1 + Phase 2) produces 2 molecules
of ATP and 2 molecules of NADH per molecule of glucose.
Phase 1
Preparatory Phase
CH2OH
O
OH
OH
OH
OH
The first reaction of glycolysis is the transfer of a
phosphoryl group from ATP to glucose.
ATP
Mg2+
Hexokinase
ADP
O
-
-
O P O
OO P O CH
2 O H2C OH
OH
OH
O
CH2
O
OH
OH
H
HO
phosphoglucose
isomerase
OH
H
OH
OH
ATP
2+
Mg
Phosphofructokinase
OO P O
O- H
H
H
HO
ADP
O
H
O
The second reaction is the isomerization of glucose 6phosphate to fructose 6-phosphate.
The third step is the first committed step of glycolysis.
This reaction is the transfer of a second phosphoryl group
from ATP to fructose 6-phosphate.
H2C O P OOH
H
O-
OH
The fourth step of glycolysis involves the aldolytic
cleavage of the C3-C4 bond to yield two triose
Fructobisphosphate aldolase phosphates.
O
O
CH
HC OH
H2C O
H2C O P O-
O
P OO-
+
O C
O-
H2C OH
The 5th Step of glycolysis is the rapid isomerization of
the triose phosphates
Triose Phosphate Isomerase
O
CH
HC OH
H2C O
O
P OO-
O
Phase II
CH
Payoff Phase
O
HC OH
P O- Note: two glyceraldehydes produced per 1 glucose.
H2C O
ONAD+ + Pi
NADH + H+
Glyceraldehyde 3-phosphate dehydrogenase
O
O
C
Step six: Oxidation and phosphorylation yielding a high-energy mixed anhydride
bond.
P O-
O
O-
HC OH
O
P O-
H2C O
O-
Step 7: transfer of the high-energy phosphoryl group to ADP to produce ATP.
ADP
Phosphoglycerate kinase
ATP
O
O-
C
HC OH
O
P O-
H2C O
O-
Step 8: Intramolecular phosphoryl-group transfer
Phosphoglycerate kinase
O
C
O-
HC
O
O
P O-
H2C OH
O-
Step 9: Dehydration to yield an energy rich enol phosphodiester.
Enolase
H2O
O
ADP
C
O-
C
O
CH2
ATP
O
P OO-
Pyruvate Kinase
O
C
O-
C
O
CH3
Step 10: Transfer of a high energy phosphoryl
group to ADP to yield ATP.
Net Overall
Glucose + 2ADP + 2Pi + 2NAD+ 2Pyruvate + 2ATP + 2NADH +2H2O + 2H+
∆Go’ = -85 kJ/mol
9 of the ten metabolites of glycolysis are phosphorylated. Phosphorylated intermediates serve 3
functions.
1. The phosphoryl groups are ionized at physiological pH giving them a net negative electrostatic
charge. Biological membranes are impermeable to charged molecules. Intermediates are held
within the cell.
2. The transfer of phosphoryl groups conserves metabolic energy. The energy released in breaking
the phosphoanhydride bonds of ATP is partially conserved in the formation of phosphate esters.
High-energy phosphate compounds formed in glycolysis donate phosphoryl groups to ADP to
form ATP.
3. The enzymes of glycolysis use the binding energy of phosphate groups to lower the activation
energy and increase the specificity of the enzyme reactions.
The Reactions of Glycolysis
Step 1: Phosphorylation of Glucose
CH2OH
Hexokinase transfers the γ-phosphate of ATP to the C-6 hydroxyl group of
glucose to yield glucose 6-phosphate. The phosphorylation of glucose
commits it to the cell.
O
OH
OH
Mg2+
OH A Kinase is an enzyme that catalyzes the transfer of the terminal phosphoryl
group of ATP to some acceptor nucleophile. The acceptor in the case of
OH
hexokinase is the C-6 hydroxyl group of D-glucose, D-mannose or D-fructose.
ATP
Hexokinase requires Mg2+ for activity. The true substrate for this enzyme is
Hexokinase not ATP4- but MgATP2-.
ADP
O
-
O P O-
Hexokinase is the poster enzyme for the induced fit mechanism where
substrate binding induces conformational changes in the enzyme.
O
CH2
O
OH
OH
OH
OH
The C-6 hydroxyl group of D-glucose has similar reactivity as water. The enzyme active site is fully
accessible by water. Hexokinase discriminates against water by a factor greater than 106. The ability to
discriminate comes from the conformational changes in hexokinase when the correct substrates are bound.
Only when a hexose is bound does the conformation change and the enzyme becomes active.
The mechanism of the phorphoryl group transfer is the typical Sn2
nucleophilic substitution reaction. By using a chiral phosphate shown
to the left, it can be shown that transfer of the γ-phosphate of ATP
proceeds with inversion of configuration at the electrophilic center of
the phosphorous.
The kinetic mechanism of hexokinase is a random bi bi
Glucose
G-6-P
ATP
EA
E
ADP
EP
EAB--EPQ
E
EQ
EB
ATP
ADP
Glucose
G-6-P
The complete reaction for the hexokinase reaction is:
α-D-Glucose + MgATP2- α-D-Glucose-6-phosphate2- + MgADP1- + H+ ∆Go’ = -16.7 kJ/mol
K’eq = 850
Under cellular conditions, the first step of glycolysis is even more favorable than the standard state. Take
erythrocytes for example. The steady state concentration of [ATP] = 1.85 mM, [Glucose] = 5.0 mM, [G6-P] = 0.083 mM, and [ADP] = 0.14 mM. The free energy change under these concentrations is given by:
 [0.14 × 10 − 3 M ][0.083 × 10 − 3 M ] 
 [ ADP][G − 6 P] 
kJ
kJ

 = −16.7
∆G = ∆G o '+ RT ln
298o K ln
+ 8.3145
 [1.85 × 10 − 3 M ][5.0 × 10 − 3 M ] 
mol
 [ ATP][Glc] 
mol o K


kJ
kJ
kJ
∆G = −16.7
− 16.6
= −33.3
mol
mol
mol
(
)
Hexokinase is found in all cells of all organisms.
Isozymes of hexokinase are found in yeast and mammals. Glucokinase is an isozyme of hexokinase
sometimes called Hexokinase IV (other times called Hexokinase D). Glucokinase is found in hepatocytes
(liver cells). The hexokinase isozyme found in skeletal muscle has a Km for glucose of 0.1 mM. The
skeletal muscle maintains steady state concentration of glucose around 4 mM. This isozyme is
allosterically inhibited by high concentrations of glucose-6-phosphate.
The isozyme glucokinase catalyzes the same reaction but has a very high Km for glucose of 10.0 mM.
This enzyme is not allosterically regulated by high glucose-6-phosphate concentrations. The high Km,
means that glucokinase only becomes metabolically important when glucose levels are high. This
enzyme produces glucose-6-phosphate which is then stored by the liver in the form of glycogen.
Glucokinase is an inducible enzyme. The amount of this enzyme present in hepatocyte cells is regulated
by insulin.
Step 2: Isomerization of Glucose-6-Phosphate
In the second reaction of glycolysis the aldose glucose-6-phosphate is isomerized into the ketose, fructo6-phosphate. The carbonyl is shifted from the C1 of glucose to the C2 of fructose. This reaction serves
two functions. Function 1: The next step of glycolysis is phophorylation at the C1 position. The
hemiacetal hydroxyl group of glucose is a poor nucleophile, while the primary hydroxyl group of fructose
is a good nucleophile. Function 2: The isomerization to fructose puts the carbonyl at the C2 position
which activates the C3 carbon for aldolytic cleavage.
The isomerization is catalyzed by the enzyme phosphoglucoisomerase.
O
-
∆Go’ = 1.67 kJ/mol
O P OO
In erythrocytes
OO P O CH
2 O H2C OH ∆G = -2.92 kJ/mol
OH
OH
CH2
O
OH
OH
OH
OH phosphoglucose isomerase
H
HO
H
OH
This enzyme functions near
equilibrium under cellular
concentrations.
The predominant forms of glucose and fructose in solution are the ring forms shown above.
Phosphoglucoisomerase must begin with the opening of the ring to interconvert G-6-P to F-6-P.
Every enzyme mechanism begins with the binding of the substrate.
Step A. A general acid, presumably a lys ε-amino group catalyzes the ring opening.
Step B. A base presumably a carboxylate of glutamate abstracts the proton of C2 to form a cis-enediolate
intermediate,
Step C: The proton abstracted from C2 is replaced on the C1 carbon.
Ring closure then produces the products.
Reaction 3: Phosphorylation of Fructose-6-Phosphate
This is the second of the two priming reactions of glycolysis. The phosphorylation is catatyzed by the
enzyme phosphofructokinase-1.
∆Go’ = -14.2 kJ/mol
OO P O CH
2 O H2C OH
OH
OH
H
HO
H
In erythrocytes
OH
∆G = -18.8 kJ/mol
ATP
Far from equilibrium.
ADP
This reaction is the first committed step of glycolysis.
Phosphofructokinase commits the glucose towards glycolysis.
2+
Mg
Phosphofructokinase
-
O
O
This is the enzyme that regulates the flux of metabolites through
O P O CH
H
C
O
P
O
the pathway. It is allosterically regulated.
2
2 O
OO
H
OH
H
HO
H
ATP AMP Citrate β-D-fructose 2,6 bisphosphate OH
Reaction 4: Cleavage of Fructose-1,6-bisphosphate.
OO P O
O- H
O
H
H
O
H
HO
H2C O P O
O-
OH
H
-
OH
Fructobisphosphate aldolase
O
O
CH
HC OH
H2 C O
H2C O P O-
O
P OO
-
+
O C
H2C OH
O-
The enzyme fructobisphosphate aldolase catalyzes a
reversible aldol condensation. The enzyme is
commonly called aldolase. Fructose-1,6bisphosphate is cleaved to yield two triose
phosphates, glyceraldehydes 3-phosphate and
dihydroxyacetone phosphate.
∆Go’ = 23.8 kJ/mol
In erythrocytes
∆G = -0.23 kJ/mol
Another near equilibrium reaction.
This reaction is an aldol cleavage reaction, mechanism shown on the following page. The bond cleave is
the one between the C3 and C4 carbons. The aldol cleavage of fructose-1-6-bisphosphate results in two
interconvertible C3 compounds that can enter a common degradative pathway. The enolate intermediate
is stabilized by resonance.
Two classes of aldolase enzymes are found in nature. Animal tissues have Class I aldolase. Class I
aldolases form a covalent Shiff base between the carbonyl of thesubstrate and an active site lysine.
R'
O H
H
C C
C O H
H
R
The mechanism for base catalyzed aldol cleavage reation is
shown to the left. The reverse of this reaction is an aldol
condensation.
:B
H
C O
R
Resonance Stabilized
Enolate Carbanion
O R
R'
C C:
-
H
O- R
R'
H
This reaction has a high activation energy because of the
high energy carbanion character of the transition state.
There two types of aldolase enzymes that catalyze this
reaction.
C C
H
H
B
B
O- R
R'
C C
H
H
B
O R
R'
C C
H
:B
Class I aldolases are found in plants and animals. To lower
the activation energy they use covalent catalysis. An active
site lysine attacks the carbonyl to form a Schiff base. The
Schiff base withdraws electrons from the reaction center
stabilizing the carbanion character of the transition state and
lowering the activation energy.
H
Shiff base formation
Adolytic Cleavage
2-
H2C OPO3
2-
H2C OPO3
C O
NH2
+
C
N
H
HO C H
H A
HO C H
-
H C O H
H C OH
O
H C OH
H C OH
2-
H2C OPO3
2-
H2C OPO3
O
H C
H C OH
2-
H
+
N
H2C OPO3
2-
H2C OPO3
2-
H2C OPO3
C OH
H
C
N
HO C H
H C OH
H
HO C H
H2O
H C OH
H
2-
H2C OPO3
O
2-
H2C OPO3
+
N
2-
H2C OPO3
C
H
+
HO C H
N
H C OH
H C OH
2-
H2C OPO3
C
H
HO C H
H
-
O
Hydrolysis of the Schiff base.
Class II aldolases
2-
H2C OPO3
2-
H2C OPO3
+
N
C
O H
H
:B
H
Zn2+
C O
HO C H
HO C H
:B
H C O H
H
H C OH
2-
H2C OPO3
B
H
O
2-
H2C OPO3
N
H C
H C OH
C O H
2-
H2C OPO3
2-
H2C OPO3
H
HO C H
C O-
H
H
B:
+
Zn2+
HO C H
B
H
2-
H2C OPO3
H
N
C O
H
HO C H
H
2-
H2C OPO3
C
O
HO C H
H
Class II aldolases are found in fungi, algae and some bacteria. They do not form a Schiff base with the
substrate. Instead they lower the activating energy by polarizing the carbonyl bond and providing
electrostatic stabilization of the carbanion character of the transition state with divalent metal ions such as
Zn2+ or Fe2+.
Reaction 5: Triose Phosphate Isomerization.
O
Of the two products formed in the aldolase reaction, only glyceraldehydes goes on into
the second phase of glycolysis. The other triose phosphate, dihydroxyacetone phosphate
must be converted into glyceraldehydes 3-phosphate by the enzyme triose phosphate
isomerase.
H2C O P OO C
-
O
H2C OH
By this reaction, C1,C2 and C3 of the starting glucose become indistinguishable from
C6,C5 and C4 respectively.
∆Go’ = 7.5 kJ/mol
O
CH
HC OH
H2C O
O
-
P O
O-
In erythrocytes ∆G = 2.7 kJ/mol
Another reaction near equilibrium.
The mechanism of triose phosphate isomerase (TIM) is shown above. The mechanism proceeds with the
formation of an enediol or enediolate intermediate similar to that of phosphoglucose isomerase. Glu-165
has been shown to function as a general acid/base catalyst. The pKa of Glu-165 has been perturbed to
6.5. In the first step of the reaction Glu-165 acts as a general base abstracting the proton from the C3
position of DHAP. The general acid catalyst protonating the carbonyl group is His-95. The result is the
enediol intermediate. His-95 then functions as a general base abstracting the hydrogen from the hydroxyl
group with Glu-165 functioning as a general acid donating its proton to the C2 carbon forming G-3-P.
TIM was the first enzyme discovered to have an α/β barrel tertiary structure. A cylinder of 8 parallel βstrands is surrounded by eight α-helices. Since the discovery numerous other proteins have been found to
have this tertiary structure including the glycolytic enzymes, aldolase, enolase and pyruvate kinase.
Triose phosphate isomerase is an enzyme that has evolved into catalytic perfection. It catalyzes the
interconversions at the diffusion limit. Meaning the rate determining step is the diffusion of the substrate
to the active site.