Regulation of Glycolysis

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Regulation of Glycolysis
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Approach

• (1) What is the function of glycolysis
• -Regulation must relate to function
• (2) What are the regulatory steps and how are
  they regulated?

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Function of glycolysis

• In most tissues it is part of ATP production
  system
• it must be sensitive to the energy
  status of the cell
• (ii) In liver and adipose tissue- it is often
  part of the process that converts excess carbs to
  fat
• In these tissues there will be different
  controls

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What are the potential regulatory steps?
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Calculate the free energy changes across the 10
reactions of glycolysis

• e.g. Phosphohexoisomerase
• Keq for G6P-F6P 0.5
• Concentrations
• G6P 8.3 x 10-5 M
• F6P 1.4 x 10-5 M
• ?Go -2.303 RT log Keq
• -(2.303 x 8.31 x 310 x (-0.3))
• 1776 J/mole
• ?G ?Go 2.303 RT log product/reactant
• -2.810 J/mole
• -2.81 kJ/mole

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• Similarly ?G can be calculated across all of the
  reactions

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• Free energy profile -
• almost all of the free-energy released during
  glycolysis occur in just 3 steps HK, PFK, PK
• These three reactions are displaced far from the
  thermodynamic equilibrium
• The other reactions of glycolysis are close to
  equilibrium

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So what?
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• Net Flux 1nmole/sec
• Which steps are likely to be rate limiting?
• The Far-from-equilibrium reactions are holding up
  the close-to-equilibrium reactions
• The far-from-equilibrium reactions are the rate
  limiting reactions

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• If the close-to-equilibrium reaction is activated
  it can not make a difference to the overall rate
  of the pathway
• However, if we can activate the
  far-from-equilibrium (irreversible) reactions ,
  we can make the pathway go faster.

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Glycolysis

• We have identified the irreversible steps HK,
  PFK, PK
• Control of these key enzymes can make the
  glycolysis proceed faster How?

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Regulation of glycolysis by Adenine Nucleotides

• In tissues where the primary function of
  glycolysis is ATP production (muscle, Brain etc),
  it need to be sensitive to ATP, directly or
  indirectly.
• ATP is too important to be allowed to undergo
  large changes in concentration.
• AMP can reflect ATP, inversely as follows

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• This reaction is close to equlibrium in vivo Keq
  0.44
• A small change in ATP translates into large
  increase in AMP

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How?

• Eg.
• Total Adenine nucleotide 5 mM
• ATP 4.8 ADP 0.185 AMP 0.015
• Due to the action of Adenylate kinase a 4
  reduction in ATP will lead to a 300 increase
  in AMP.
• ATP 4.6 ADP 0.34 AMP 0.06

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• Thus AMP serves as a very sensitive indicator
  of small changes in ATP .
• Are the rate limiting glycolytic steps sensitive
  to AMP?
• i) Phosphofructokinase is allosterically
  inhibited by ATP and activated by AMP
• ii) Hexokinase is allosterically inhibited by
  glucose-6-P

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Slow flux through Glycolysis
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Rapid Flux through Glycolysis
Increased demand for ATP Low ATP and High AMP