Lecture 18 Flashcards

1
Q

ΔG of rxn

A

concentration independent

This is the change in ΔG between the standard state concentrations and equilibrium concentrations

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2
Q

concentration dependent

A

concentration dependent

This is the influence on ΔG of the actual concentration of substrates and products in the cell

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3
Q

Aldolase– 4th step in Glycolysis

A

But in a living cell:
ΔGo’ = +22.8 kJ/mol
ΔG = -5.9 kJ/mol

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4
Q

TIM - 1st step

A
ΔGo’  = +7.9kJ/mol
ΔG = 0 kJ/mol
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5
Q

GAPDH + PGK - 6+7 step

A
ΔGo’  = -16.7 kJ/mol
ΔG = -1.1 kJ/mol
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6
Q

PK- step 10

A
ΔGo’  = +23 kJ/mol
ΔG = -13.9 kJ/mol
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7
Q

Regulation of Metabolism

A
  • Global regulation (at the organ level)
  • Transcriptional or translational control
  • Covalent post-translational modifications
  • Allosteric regulation
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8
Q

Transcriptional or translational control

A

Expression of glucose metabolism genes is regulated by a specific transcription factor HIF-1. (Hypoxia Inducible Factor)

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9
Q

inactive to active enzyme

A

kinase= atp–>adp

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10
Q

active to inactive enzyme

A

phosphatase= minu Pi

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11
Q

Allosteric Regulation

A

• 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

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12
Q

Regulation of Glycolysis
ΔGo’ and ΔG for the rxb in heart muscle

hexokinase
PFK
PK

A
  • 27.2
  • 25.9
  • 13.9
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13
Q

Best control is at the irreversible steps of a pathway

A
  • ∆G is very negative

* Virtually all regulation occurs at the irreversible steps

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14
Q

• What are the irreversible steps of glycolysis?

A

– Steps 1, 3, and 10.

  1. Hexokinase: Glucose Glucose-6-P
  2. Phosphofructokinase: F-6-P F-1,6-BP
  3. Pyruvate kinase: PEP Pyruvate
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15
Q

Regulation of Glycolysis: Hexokinase

A

–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)

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16
Q

Regulation of Glycolysis: Pyruvate Kinase

A

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.

17
Q

Regulation of Glycolysis: Pyruvate Kinase

A

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.

18
Q

Regulation of Glycolysis: Pyruvate Kinase

A

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..

19
Q

Regulation of Glycolysis: Phosphofructokinase

A

Allosteric regulation of the Km of PFK-1 by high ATP.

ATP raises the Km for F6P

20
Q

During exercise [ATP] does not change much, but AMP increases a lot!

A

ATP ADP + Pi

21
Q

These reactions restore ATP:

A

PCr + ADP Cr + ATP

ADP + ADP AMP + ATP (adenosine kinase)

22
Q

When all equilibria involving ATP, ADP and AMP are

considered:

A

[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

23
Q

Regulation of Glycolysis: Phosphofructokinase

A

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.

24
Q

Activation of PFK-1 by F-2,6BP.

A

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.

25
Q

The inhibitory effect is reversed by F-2,6BP. WHY

A
  • 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.

26
Q

Glucose vs. Fructose metabolism in the liver

High Fructose Corn Syrup consumption correlates with obesity. Why?

A
  • 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.

27
Q

Some key concepts about regulation of Glycolysis

A
  • 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