Lecture 18 Flashcards
ΔG of rxn
concentration independent
This is the change in ΔG between the standard state concentrations and equilibrium concentrations
concentration dependent
concentration dependent
This is the influence on ΔG of the actual concentration of substrates and products in the cell
Aldolase– 4th step in Glycolysis
But in a living cell:
ΔGo’ = +22.8 kJ/mol
ΔG = -5.9 kJ/mol
TIM - 1st step
ΔGo’ = +7.9kJ/mol ΔG = 0 kJ/mol
GAPDH + PGK - 6+7 step
ΔGo’ = -16.7 kJ/mol ΔG = -1.1 kJ/mol
PK- step 10
ΔGo’ = +23 kJ/mol ΔG = -13.9 kJ/mol
Regulation of Metabolism
- Global regulation (at the organ level)
- Transcriptional or translational control
- Covalent post-translational modifications
- Allosteric regulation
Transcriptional or translational control
Expression of glucose metabolism genes is regulated by a specific transcription factor HIF-1. (Hypoxia Inducible Factor)
inactive to active enzyme
kinase= atp–>adp
active to inactive enzyme
phosphatase= minu Pi
Allosteric Regulation
• T state: inactive
• R state: active
• Equilibrium: T R
• Allosteric effectors shift the equilibrium
– Inhibitor shifts the equilibrium towards T
– Activator shifts the equilibrium towards R
Regulation of Glycolysis
ΔGo’ and ΔG for the rxb in heart muscle
hexokinase
PFK
PK
- 27.2
- 25.9
- 13.9
Best control is at the irreversible steps of a pathway
- ∆G is very negative
* Virtually all regulation occurs at the irreversible steps
• What are the irreversible steps of glycolysis?
– Steps 1, 3, and 10.
- Hexokinase: Glucose Glucose-6-P
- Phosphofructokinase: F-6-P F-1,6-BP
- Pyruvate kinase: PEP Pyruvate
Regulation of Glycolysis: Hexokinase
–HK is inhibited by G6P
–Product negative feedback inhibition.
Example of how HK is controlled by PFK.
F6P builds up then G6P builds up then glycolysis slows and glucose remains in the blood or G6P gets converted to glycogen.
Note: In LIVER Glucokinase is NOT inhibited by G6P
(encourages glycogen storage)
Regulation of Glycolysis: Pyruvate Kinase
PEP + ADP Pyruvate + ATP
Pyruvate kinase has several different forms
- Liver form – PKL
- Erythrocyte form – PKR
- Muscle form – PKM1
- Cancer cell form – PKM2
– For all forms Km is lowered by Fruc-1,6-P2.
Regulation of Glycolysis: Pyruvate Kinase
Liver Pyruvate Kinase is regulated by blood glucose levels (via insulin, epinephrine or glucagon, GPCR, G-protein and cyclic AMP)
(liver uses pyruvate to make and store fat)
(liver uses PEP to make glucose via gluconeogenesis)
Other forms of PK are not regulated by Blood Glucose levels.
Regulation of Glycolysis: Pyruvate Kinase
Cancer cell pyruvate kinase (PKM2) is inhibited by phosphorylation on tyrosine.
– Tyrosine kinases are activated by growth factors,
– Some tumors have highly active tyrosine kinases
– Growth requires anabolic building blocks, not energy.
–When PKM2 is inhibited then glycolytic intermediates can be used to make building blocks for amino acids and phospholipids..
Regulation of Glycolysis: Phosphofructokinase
Allosteric regulation of the Km of PFK-1 by high ATP.
ATP raises the Km for F6P
During exercise [ATP] does not change much, but AMP increases a lot!
ATP ADP + Pi
These reactions restore ATP:
PCr + ADP Cr + ATP
ADP + ADP AMP + ATP (adenosine kinase)
When all equilibria involving ATP, ADP and AMP are
considered:
[ATP] decreases: 1.0 mM 0.9 mM –> 10% decrease
[ADP] increases: 0.1 mM 0.2 mM –> 100% increase
[AMP] increases: 0.02 mM 0.1 mM –> 400% increase
Regulation of Glycolysis: Phosphofructokinase
Allosteric regulation of the Km of PFK-1 by high ATP.
AMP diminishes the inhibitory effect of ATP.
Activation of PFK-1 by F-2,6BP.
The sigmoidal kinetics become hyperbolic at 1 μM F-2,6BP.
F-2,6BP is an important allosteric effector in liver that shifts the inactive state to the active state for PFK-1.
F-2,6BP is made in liver cells only when blood sugar is high
Note that Vmax increases (a little) and Km decreases for the F-6-P substrate.
Activation of PFK-1 by F-2,6BP.
ATP, acting as a substrate, initially stimulates the reaction rate. As the ATP concentration increases, it acts as an allosteric inhibitor. The inhibitory effect is reversed by F-2,6BP.
The inhibitory effect is reversed by F-2,6BP. WHY
- High levels of glucose in blood
- High insulin in blood.
- Insulin stimulates liver cells to make F-2,6BP.
- The extra glucose gets metabolized and ultimately stored as fat.
When there is average or low blood glucose there is less F-2,6BP and the liver does not store glucose carbons as fat.
Glucose vs. Fructose metabolism in the liver
High Fructose Corn Syrup consumption correlates with obesity. Why?
- Glucokinase Km for Glucose is ~10 mM
- Fructokinase Km for Fructose is ~0.5 mM (makes F1P)
- Liver uses Glucose only when blood glucose is very high
- Liver uses Fructose whenever it is available…
• Fructose-1-Phosphate has “its own” aldolase
- (so it does not need to be made into F1,6BP)
- Regulation of PFK-1 by ATP, AMP, F2,6BP is bypassed.
- Carbons from fructose are always stored as fat.
• Fructose does not stimulate insulin production.
- Its concentration in blood is not regulated.
Some key concepts about regulation of Glycolysis
- Regulation occurs at steps with the highest free energy difference.
- Hexokinase, PFK-1, Pyruvate Kinase
- Be familiar with positive and negative regulators.
- Be familiar with types of regulation.
- Regulation depends on the purpose of the tissue - example muscle vs. liver
- PFK-1 stimulation by F2,6BP and relationship to ATP and AMP regulation
- Fructose metabolism differs from glucose metabolism
