LeChatelier's Principle - Phosphofructokinase in Glycolysis
PFK plays a central role in the control of glycolysis because it
  catalyses one of the pathway’s rate-determining reactions. The control
  of Phosphofructokinase (PFK) is exquisitely complex.


To analyse this complicated enzymatic reaction pathway, we have to revisit important aspects of metabolic control mechanism:


Thus the flux (rate of flow) of intermediates through a metabolic pathway is constant; that is, the rates of synthesis and breakdown of each pathway intermediate maintain it at a constant concentration.
Regulation of the steady state (homeostasis) must be maintained in the face of changes in flux through the pathway in response to changes in demand. In an open system, such as metabolism, the steady state is the state of maximum thermodynamic efficiency.


This explanation provides a somewhat agreement of Le Chatlier’s principle. (i.e. where a dynamic equilibrium is affected by changing conditions, the position of equilibrium moves to counteract the change)

Consider this simple hypothesis:


  J = vf – vr
  
  Since a metabolic pathway is a series of enzyme-catalysed reactions,
  it is easiest to describe the flux of metabolites through the pathway
  by considering its reaction steps individually. The flux of
  metabolites, J, through each reaction step is the rate of the forward
  reaction, vf, less that of the reverse reaction, vr:
  
  At equilibrium, by definition, there is no flux (J=0), although vf and
  vr may be quite large. At the other extreme, in reactions that are far
  from equilibrium, vf>> vr, so that the flux is essentially equal to
  the rate of the forward reaction, J~vf.


The flux throughout a steady-state pathway is constant and is set (generated) by the pathway’s rate determining step (or steps). 

Consequently, control of flux through a metabolic pathway requires: 


that the flux through this flux-generating step vary in response to the organism’s metabolic requirements and
that this change in flux be communicated throughout the pathway to maintain a steady state


Phosphofructokinase is a non-equilibrium enzyme of glycolysis

One important thing to take note of is that the enzyme phosphofructokinase, a highly controlled enzyme functioning far from equilibrium (but ironically itself operates via an equilibrium of two conformational states), evidently is the major target for regulating glycolysis.

PFK is a tetrameric enzyme with two conformational states, R and T, that are in equilibrium. The substrate site binds ATP equally well in either conformation, but the inhibitor site binds ATP almost exclusively in the T state. The other substrate of PFK, F6P, preferentially binds to the R state. Consequently, at high concentrations, ATP acts as a heterotrophic allosteric inhibitor of PFK by binding to the T state, thereby shifting the T &lt;=> R equilibrium in favour of the T state and thus decreasing PFK’s affinity for F6P.

Increase in concentration of PFK does not increase the rate of glycolysis.

In this scenario I believe the Le Chatlier’s principle applies little here. The reason is because the control is very complicated Although PFK has a major role in regulating the flux through glycolysis, it is controlled, in vivo, by factors outside the pathway.

An increase in the in vivo concentration of PFK will therefore not increase the flux through the pathway because these controlling factors adjust the catalytic activity of PFK only to meet the needs of the cell.

Let’s consider another illustration showing enzyme activity versus substrate concentration:



The inhibition of PFK by ATP is relieved by AMP. This results from AMP’s preferential binding to the R state of PFK. If a PFK solution containing 1 mMATP and 0.5 mM F6P is brought to 0.1 mM in AMP, the activity of PFK rises from 10 to 50% of its maximal activity, a 5-fold increase. when [F6P] =0.5 mM (the dashed line), the enzyme is nearly maximally active, but in the presence of 1 mM ATP, the activity drops to 15% of its original level (a nearly 7-fold decrease).

However, measurements of [ATP] in vivo at various levels of metabolic activity indicate that [ATP] varies &lt;10% between rest and vigorous exertion. Yet there is no known allosteric mechanism that can account for a 100-fold change in flux of a nonequilibrium reaction with only 10% change in effector concentration. 

Thus, some other mechanism, or mechanisms, must be responsible for controlling glycolytic flux. Let’s discuss the action of adenylate cyclase; adenylate cyclase catalyses the following reaction:

2ADP &lt;=> ATP + AMP (K=0.44)

as a result of the adenylate kinase reaction, a 10% decrease in [ATP] will cause over a 4-fold increase in [AMP]. Consequently, a metabolic signal consisting of a decrease in [ATP] too small to relieve PFK inhibition is amplified significantly by the adenylate kinase reaction, which increases [AMP] by an amount sufficient to produce a much larger increase in PFK activity.

In summary the control of the PFK (non-equilibrium enzyme) has limited Le Chatlier’s principle application as other crucial regulation mechanism exist through the reaction pathway. Even if you were to consider temperature changes, these control enzymes operate far from equilibrium (these have very negative delta G°) and the actual free energy changes in vivo differ from experimental ones making it difficult to simulate actual effects in equilibrium states. It is more likely based on supply-demand basis. One evidence of this is seen through the variation of metabolic flux  throughout glycolysis (may vary by 100-fold or more), depending on the metabolic demand for ATP.

However, for pathways which operate close to equilibrium it may be applied, for example, the delta G° for aldolase is endergonic, whereas under physiological conditions in heart muscle it is close to zero, indicating that the in vivo activity of aldolase is sufficient to equilibrate its substrates and products. Their forward and reverse rates are much faster than the actual flux through the pathway (although their forward rates must be at least slightly greater than their reverse rates). Consequently, these near-equilibrium reactions are very sensitive to changes in the concentration of pathway intermediates.

References

Biochemistry Voet and Voet