I. Glycolysis A. Purposes/Roles/Functions of Glycolysis 1. Produce ATP

I. Glycolysis
A. Purposes/Roles/Functions of Glycolysis
1. Produce ATP
2. Feed ATP production via PDH, TCA, ET, OP
3. Feed production of TCA intermediates
4. Feed FA synthesis via PDH
5. Feed AA synthesis via TCA intermediates
B. Glycolysis Net Rxn (obtained by adding all rxns)
ATP + G  G6P + ADP
G6P  F6P
ATP + F6P  F1,6BP + ADP
2Pi + 2NAD + 2GAP  2 (1,3BPG) + 2NADH
2 (1,3BPG) + 2ADP 2 (3PG) + 2ATP
(3PG)  2 (2PG)
(2PG)  2 PEP + 2H2O
2 PEP + 2 ADP  2pyr + 2ATP
G + 2Pi + 2 ADP + 2NAD+  2pyr + 2 ATP + 2NADH
C. Lactate Dehydrogenase (LDH)
1. Purpose of LDH: to covert the NADH produced in GAPDH back to NAD+ for use in
a. This is required to keep glycolysis (G’lys) going rapidly during rapid muscle activity.
b. The sum: [NAD+] + [NADH] is small. All of it would soon be converted to NADH if it
was not recycled.
c. In muscle, NADH is converted to NAD+ via ET: 2NADH + 2H+ + O2 ---> 2H2O +
2NAD ; but in active muscle, NADH production exceeds the capacity to deliver O2 to ET, so
LDH also converts NADH to NAD+. This removes the limitation that O2 delivery rate would
place on the maximum rate of muscle activity: we can sprint and go into O2 debt rather than just
2. LDH rxn: pyruvate + NADH + H+ <-----> lactate + NAD+
3. a. In active muscle, the capacity to consume O2 exceeds ability to deliver, so [O2]
decreases: anaerobic conditions.
b. But O2 delivery by heart, lungs, and Hb is at the maximum possible rate and O2
consumption in muscle is the highest ever: aerobic conditions.
c. Anaerobic glycolysis produces 2 ATP per G converted to lactate at a high rate only
when aerobic glycolysis is rapid (and only in addition to aerobic glycolysis). Aerobic glycolysis
(working with PDH, TCA, ET, and OP) produces 36 ATP per G converted to carbon dioxide.
4. Q = [lactate][NAD+] / [pyruvate][NADH][H+]
5. Muscle: [pyruvate], [H+], and [NADH] are all high from rapid production in glycolysis;
this pushes the LDH rxn to the right (LeChatelier). Also, [NAD+] is low from rapid use in
glycolysis so the concentrations of these four substances make the value of Q low.
6. Glucagon causes liver enzymes to convert pyruvate to glucose in gluconeogenesis (G’neo).
a. Glucagon is secreted in response to adrenalin or epinephrine, which is secreted during
rapid muscle activity:
b. Cori cycle: Lactate is transported in blood from muscle to liver, where it is converted
back to G via G’neo. Then G goes back out into the blood, and is taken back into muscle cells,
where it is then used or stored for later.
7. Liver: [pyr], [H+], and [NADH] are all low from rapid consumption in hormonally
activated G’neo and [NAD+] is high from rapid production in G’neo, so Q is high. (In
gluconeogenesis, GAPDH is reversed (as are all reversible reactions of glycolysis) so NAD+ is
produced, NADH is consumed.)
LDH rxn: pyruvate + NADH + H+ <-----> lactate + NAD+
Q = [lactate][NAD+] / [pyruvate][NADH][H+]
8. Sign of ΔG and direction of LDH rxn depends on Q vs K:
ΔG0’ = -RT ln K
ΔG = ΔG0’ + RT ln Q = -RT ln K + RT ln Q
= RT ln (Q/K)
Q < K  -ln; so ΔG is - and LDH rxn goes forward in active muscle where Q is low
Q > K  +ln; so ΔG is + and LDH rxn goes in reverse (L <-- R) in liver at the same
time as it is going forward in muscle
(If Q = K  ΔG = 0; rxn is at EQ)
D. Fates of Pyruvate
Depending on the organism in question and the conditions under which it is operating,
pyruvate may be converted to:
1. Lactate
2. Acetyl coenzyme A (ACoA)
3. Ethanol
4. Oxaloacetate
5. Alanine
II. Gluconeogenesis
A. Gluconeogenesis: in the liver pyruvate (or oxaloacetate) is converted to G to increase [G] in
the blood:
1. in short-term CH2O starvation: muscle protein is hydrolysed to AAs which go to the blood
and to the liver where they are converted to oxac  G to supply G to the brain when [G] is low in
the blood.
2. in rapid muscle activity, lactate from muscles is converted back to G to go back to the
3. Reactions: 7 of the reactions are the same reactions (same E) as in glycolysis: they just go
faster in reverse during gluconeogenesis (G’neo). (Q becomes greater than K)
4. The irreversible reactions of glycolysis (G’lys) are bypassed (NOT reversed)
B. Net Reaction for G’neo
1. To bypass PK (twice/G) consumes 2ATP + 2GTP.
2. Reverse of PGK (2(1,3BPG) + 2ADP 2 (3PG) + 2ATP) twice/G consumes 2ATP.
3. No ATP is produced in the bypass rxns of G’neo, nor in the reversed rxns of G’lys.
4. So the net rxn for G’neo is:
2pyr + 4ATP + 2GTP + 2NADH  G + 4ADP + 2GDP + 6Pi + 2NAD+
C. Energy Released Uphill and Down?
1. Both glycolysis and gluconeogenesis are energy releasing processes that occur rapidly
when their regulated enzymes are active.
2. But the enzymes have no effect on the energy, so how can both directions (G  2pyr; 2pyr
 G) have negative ΔGs?
3. The net reaction for glycolysis has ΔG ~ -100 kJ:
G + 2ADP + 2Pi + 2NAD+  2pyr + 2ATP + 2NADH
And 2ATP  2ADP + 2Pi has ΔG ~ -100 kJ; so the sum:
G + 2NAD+  2pyr + 2NADH has ΔG ~ -200 kJ
4. So 2pyr + 2NADH  G + 2NAD+ has ΔG ~ +200 kJ
Add: 4ATP + 2GTP  4ADP + 2GDP + 6Pi ~ -300 kJ
This yields ~ -100 kJ for the net rxn for G”neo
5. So how can both directions (G  2pyr; 2pyr  G) have negative ΔGs?
a. G  2pyr releases about 200 kJ. Of this, 100 is used to make 2ATP and the other 100 is
released in G’lys.
b. 2pyr  G consumes 200 kJ, so 6 ATP equivalents (4ATP + 2GTP) are used to “pay”
that 200 kJ and have 100 kJ left to be released.
III. “Irreversible” Reactions of Glycolysis
A. 1. We can divide the reactions into two groups, based on their ΔG values: “large negatives” or
“near zero”. Table 17-1 pg 613
2. Unlike the LDH reaction, the reactions with large negative values of ΔG never have a
large enough value of Q to cause their net direction to be in reverse in a cell. (Like all enzymes,
these enzymes do catalyze the reverse reaction. It has a nonzero rate, but a rate that is always slower
than that of the forward rxn.)
a. For this reason they are said to be “physiologically irreversible”.
b. These reactions a catalyzed by regulated enzymes.
3. The reactions with values of ΔG that are near zero are easily shifted one way or the other
by changes in the value of Q that occur in cells. These “reversible” reactions are catalyzed by
enzymes that are not regulated: these reactions are always moving rapidly toward equilibrium.
B. Regulation of Glycolysis Enzymes
1. Hexokinase (HXK) is inhibited by glucose-6-phospate (G6P)
a. Metabolic Relationship (MR): G6P is a product of the HXK reaction (“direct product”)
b. Metabolic Logic (ML): if [G6P] is high, there is no need for HXK to produce more.
c. What does this mean: “[G6P] is high”? The regulated enzymes have evolved so that
their affinity for their effectors causes minimal binding when the [effector] is barely enough to
meet needs, but binding to most of the molecules of E when the [effector] is ample to meet needs.
d. A similar situation exists for rate and the binding of substrate: the steep part of the rate
curve usually extends across the usual range of [substrate] in the cell where that enzyme exists.
2. PFK is inhibited by ATP.
a. MR: ATP is a product of this pathway and of ATP producing processes fed by this
pathway: PDH, TCA, ET, OP. (“indirect product”)
b. ML: When the [ATP] is high there is no need for PFK to feed production of more.
3. PFK is inhibited by citrate.
a. MR: citrate is an indirect product of PFK. Note the metabolic relationship diagram
b. ML: When [citrate] is high, this indicates that the TCA and all processes fed by it are
well supplied, especially ET/OP and FA synthesis, and that PFK does not need to produce more.
4. F1, 6BP is an activator of PK
a. MR: F1, 6BP is an indirect substrate (it will be converted to PEP, the substrate) of PK.
Note the MRD
b. ML: when [F1, 6BP] is high, PFK is rapid, there is plenty of PEP, and there is a need
for PK to be active to be in sync w/PFK
5. HXK also “follows” PFK rate:
a. When PFK is rapid, F6P is consumed rapidly and G6P is “pulled” to F6P, so [G6P]
does not build up and HXK is not inhibited
b. HXK has a very high affinity for G; at “any” [G] the v  90% Vmax; main effect on
rate is [G6P]. HXK is an exception to point 1d above.
C. Reciprocal Regulation of G’lys and G’neo Enzymes
1. AMP activates PFK by overriding the ATP inhibition of PFK; and AMP inhibits FBPase.
a. MR for PFK: its activity feeds ATP production in glycolysis AND via PDH + TCA +
b. ML: if [AMP] is high then ATP is being consumed rapidly and needs to be produced,
which it will be by PFK activation
c. MR for FBPase: G’neo consumes 6ATP/G
d. ML: cell can’t afford to do G’neo when ATP consumption is rapid and ATP production
is needed, G’neo slows in response to AMP
D. AMP vs ATP Effects
1. PFK is more responsive to AMP than to ATP
2. Adenylate Kinase (myokinase): 2ADP  ATP + AMP
3. The [AMP] changes by a much larger factor (though not by a greater amount) than [ATP].
a. This is because ATP is at a much higher concentration and only changes by ~10%.
b. The [AMP] is only about 1/50th (2%) of the [ATP] at rest; so if [ATP] goes from 2 mM to
1.8 mM, the [AMP] may go from about 0.04 mM to .16 - .20 mM, a factor of 4 or 5
c. Because the factor change is greater for [AMP], it has more effect on PFK than ATP,
(PFK has higher affinity for allosteric binding of AMP than it does for ATP)
IV. Hormonal Regulation of G’lys and G’neo In Liver
A. Glucagon and insulin exert their effects on these pathways by regulating the “tandem enzyme”
1. PFK2 and FBPase2 activities are at 2 different active sites on each of the polypeptide
chains of “the tandem E”, a homodimer (two identical subunits).
a. PFK2 catalyzes: F6P + ATP  F2,6BP + ADP
b. FBPase2 catalyzes: F2,6BP  F6P + Pi
2. a. F2,6BP activates PFK (overrides PFK inhibition by ATP)
b. F2,6BP inhibits FBPase
c. There is only one purpose (and only one effect) of F2,6BP: to regulate PFK and
B. Glucagon is secreted in short-term CH2O starvation in response to low [G] in blood; its
purpose is to cause the liver to release G into the blood by activating G’neo and glycogenolysis.
1. When the tandem E is phosphorylated in response to glucagon, only its FBPase2 is active
(PFK2 is NOT); this causes the [F2,6BP] to decrease.
2. This removes the inhibition of FBPase by F2,6BP and G’neo goes rapidly.
3. Without F2,6BP, G’lys is slowed by ATP inhibition of PFK.
4. G’lys has to go slow (NOT consume G when [G] is low) so as to not oppose G’neo.
C. Insulin is secreted after a CH2O meal in response to high [G] in blood; its purpose is to cause
the liver to store fuel by G’lys, PDH, CS, and FA synthesis, and by glycogen synthesis.
1. When the tandem E is dephosphorylated in response to insulin, only its PFK2 is active
(FBPase2 is NOT) ); this causes the [F2,6BP] to increase.
2. PFK is activated by the [F2, 6BP] increase, and glycolysis goes rapidly to “feed” FA
synthesis (The E of FA synth is also dephos, active).
3. FBPase is inhibited by F2,6BP, to keep it from opposing G’lys
D. 1. Like the tandem E, liver PK is also phosphorylated in response to glucagon. This inhibits
PK to slow G’lys and keep it from working against G’neo.
2. Liver PK is also dephosphorylated in response to insulin. This activates PK to work with
PFK to make G’lys go to feed FA synthesis.
E. In heart muscle, the tandem E has the opposite response to phosphorylation, which occurs in
response to adrenalin, as compared to tandem E in liver:
1. The heart muscle tandem E’s PFK2 is active (its FBPase2 is inactive), so that the [F2,6BP]
increases and PFK is activated.
2. The purpose here is to activate glycolysis, so that ATP can be rapidly produced (in G’lys
and in PDH, TCA, and ET + OP) so that heart muscle can have rapid activity.
V. HXK is NOT an ATPase
1. HXK can bind ATP and G in either order, but if ATP binds first, it is not in contact with
HXK’s catalytic groups.
2. G must bind to HXK and cause a conformational change to put ATP in position for catalysis.
3. Otherwise, ATP could bind to HXK (if HXK had only one, active conformation) and the
gamma phosphate would usually be transferred to water:
ATP + H2O  ADP + Pi (this is an ATPase activity)
ATP produced from fuel (food) burning would be wasted.
4. Xylose converts HXK to an ATPase. It lacks the #6C of glucose (-H instead of -CH2OH) (it is
otherwise the same).
VI. Substrate Cycling
Effect of “substrate cycling” on relative rate of enzymes:
1. Consider rapid PFK with high [AMP], say 90 units, while FBPase is slow, say 1 unit; so net is
89 units of G consumed.
2. At rest, with low [AMP], PFK will be much lower, say 10 units, and FBPase may be 9 units,
net is 1 unit G consumed.
3. Difference is 89 fold net increase in G consumed, even though rate actually only differed by
VII. Regulation of PC, Pyruvate Carboxylase
1. PC is the “anaplerotic” (filling up) reaction for the TCA; it is also a G’neo rxn:
pyr + ATP + CO2  oxac + ADP
(Oxidation of 18 of the 20 AA also produces TCA ints.)
2. PC requires that ACoA be bound to it in order to be active. ACoA activates PC.
a. MR (for G’neo): ACoA is the product of PDH an alternative use of pyr
b. ML: If [ACoA] is high, then pyr is not needed for PDH, can be “conserved as CH2O” and
converted to G
c. ACoA cannot be converted to G: PDH is irreversible AND THERE IS NO BYPASS