BIOC 2300 Lecture 21: BIOC 2300 - Lecture 21
6 Sep 2019
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BIOC 2300 – Lecture 21 March 13, 2019 Gluconeogensis and Regulation of Glycolysis
• Regulatory steps of glycolysis
• Unique steps of gluconeogenesis
• Regulation of phosphofructokinase
• Interpreting and describing a diagram
How can the patient be intoxicated without having ingested alcohol?
A. He had an alcohol storage disease, which made alcohol last much longer in his body.
B. He ate too much yogurt, which can contain alcohol because of fermentation
C. Yeast growing in his intestine made alcohol from glucose***
D. He had been abducted by aliens
Gluconeogensis
• Humans use about 160 g of glucose per day. In the absence of dietary carbohydrates, glucose must
be made from non-carbohydrate precursors = GLUCONEOGENESIS.
• The brain needs most (75%) of the glucose.
• Gluconeogenesis occurs mostly in the liver, some in kidneys.
• Substrates for gluconeogenesis: pyruvate, lactate, glycerol, most amino acids, all citric acid cycle
intermediates. NOT fatty acids!
Glycolysis is the breakdown of glucose, which is catabolic pathway. Then, there is the complementary reaction
called gluconeogenesis which is anabolic. You don’t always have the input of dietary carbohydrates because you
are not constantly eating. Glucose level needs to be fairly constant even if you do not have constant intake of
carbohydrates. There must be a way to synthesize glucose because the brain relies on glucose. The key organ that
synthesize glucose is the liver; the kidney can only synthesize a bit.
Glycolysis Has Irreversible, Exergonic Steps.
How Can it Be Reversed?
• Diagram shows the ∆G for the full
reactions (Ex. coupled to ATP hydrolysis,
NAD+ reduction). The half reactions for the
intermediates alone would look different.
Not all carbon compound can be used in
gluconeogenesis. Lactate can go into pyruvate
and into the gluconeogenesis. There is also
glycerol and some amino acids. Compounds that can enter gluconeogenesis are called glycogenic.
Glycolysis as a whole pathways is exergonic. There are some irreversible steps in it so how can it be
reversed? ∆G is shown for the full reaction of 10 steps. There are a lot of reversible reactions that have
small ∆G. The larger ∆G are irreversible. There is a big difference between the reactions.
There are 3 steps that are difficult to circumvent when you go from pyruvate and synthesize glucose.
Reactions Spontaneity
• Reactions spontaneously occur towards equilibrium concentrations and lower free energy.
• If glycolysis is an energy-generating process with an overall negative ∆G, how can glucose synthesis
become thermodynamically possible?
Thermodynamically Possible
Thermodynamically Not Spontaneous
• Negative ∆G
• Keq > 1
• More products than substrates in equilibrium
• Exergonic reaction towards products
• Favorable reaction towards products
• Positive ∆G
• Keq < 1
• More substrates than products in equilibrium
• Endergonic reaction towards products
• Non-favorable reaction towards products
• Possible only when linked to an exergonic
reaction
How Are Reactions With a Large
Negative ∆G Reversed?
• They are not exactly reversed!
Different enzymes and different side
products make the reaction
possible.
• Steps 1, 3 and 10 of glycolysis have
a large negative ∆G:
o Regulatory steps
o Irreversible
o Different enzymes catalyze different combined reverse reactions for gluconeogenesis.
These 3 irreversible reactions are not exactly reversed in the generation of glucose. The side-products
are different. These are also regulatory steps, which means you can regulate the flux or the rate of the
pathway by regulating the enzyme. We can also regulate flux by regulating the amounts of products or
substrates of the reversible reactions. For irreversible reaction, flux, direction, speed is regulated
through the enzyme activity. The reaction is not exactly reversed but it is only reversed regarding the
carbon skeleton (carbon metabolite).
Gluconeogenesis
• Net reaction glycolysis: Glucose + 2 ADP + 2 NAD+ + 2 Pi -> 2 pyruvate + 2 ATP + 2 NADH
• Net reaction gluconeogenesis: 2 Pyruvate + 2 NADH + 6 ATP -> Glucose + 2 NAD+ + 6 ADP + 6 Pi
• Therefore, gluconeogenesis is not an exact reversal of glycolysis. Three steps are catalyzed by non-
glycolytic enzymes and generate different side products.
***H2O and H+ are omitted from the equations.
In gluconeogenesis, you need 6 ATP to get to glucose. In glycolysis, 2 ATP is generated. Hence, this
shows that it is not an exact reversal.
Gluconeogenesis
• If glycolysis and gluconeogenesis
occurred simultaneously, there would be
a net consumption of ATP!
• If glycolysis and gluconeogenesis occurred simultaneously it would be a futile cycle.
• Glycolysis and gluconeogenesis are reciprocally regulated based on the cell’s needs and on the
organism’s needs.
If we combine the reaction and run them through a cycle, the NADH would also even out. What does not even out
is the ATP. If you ran a futile cycle from glucose to pyruvate back to glucose, you lose 4 ATP. If the cell was to run
the two pathways at the same time, you are using energy all the time. This cell nee ds to have a regulatory
mechanism to avoid this futile cycle since energy is used up but the metabolites are not changed.
Gluconeogenesis: Synthesis of Glucose
• 2 Pyruvate + 2 NADH + 4H+ + 4 ATP + 2 GTP + 6 H2O -> Glucose + 2
NAD+ + 4 ADP + 2 GDP + 6 Pi
• Not a straightforward reversal of glycolysis
o The highly exergonic steps of glycolysis must be circumvented in
gluconeogenesis.
o Reversible reactions (near equilibrium) are the same for
glycolysis and gluconeogenesis.
• Steps shown in red are unique to gluconeogenesis
• Glycolytic enzymes not used in gluconeogenesis: Hexokinase,
phosphofructokinase, pyruvate kinase
• Unique gluconeogenic enzymes: Glucose phosphatase,
fructobisphosphatase, phosphoenolpyruvate carboxykinase
(PEPCK), pyruvate carboxylase
This is the depiction of glycolysis in blue (right). Glucose in on the top, pyruvate
is on the bottom. All the reversible reactions, going back from pyruvate to
glucose, involves one enzyme. For the irreversible reactions, there are other
enzymes catalyzing the reverse reaction regarding the carbon metabolite. Sometimes you need two enzymes to
circumvent that step.
“Reversal” of Glycolytic ATP-consuming Reactions
• “Reversal” of glycolytic ATP-consuming reactions in
gluconeogenesis: Hydrolysis of phosphates, not a
regeneration of ATP.
On the left are the glycolytic enzymes. The phosphorylation
reaction catalyzed by kinases use up ATP to make ADP. On the
reverse pathway, they are hydrolyzing the one phosphate group; they are not getting an ATP out. The
phosphorylation of glucose to G6P is an endergonic half reaction that was made possible by linking it to ATP
hydrolysis. On its own, this half reaction is able to be reversed but you’ll get a phosphate out. If you run this as a
futile cycle, you have glucose + ATP. If you ran it back through, you are losing 1 ATP because afterwards you have
hydrolyzed it. This is where the ATP loss comes in.
