Regulation of Pathways

Allosteric Binding and Activity (Cont.)

How ATP/ADP Binding Should Alter Activities

In the case PFK-1 and FBPase It should be apparent how ATP SHOULD alter activities as a signal of cellular energy level .

  1. High ATP concentration

  2. The need for glycolsysis (More ATP) is not necessary and this should "slow down" the PFK-1 enzyme and "speed up" the FBPase enzyme.
  3. Low ATP concentration

  4. The need for glycolysis (More ATP) is urgent and this should have the effect of increasing glycolysis and decreasing gluconeogenesis.

With phosphofructokinase there is a conundrum here

ATP is a co-substrate and is required for the reaction to take place at all. In this role, the observed rate should increase as ATP concentration increases - until vmax is attained.

But I just indicated above that if the ATP concentration is high then it should DECREASE the observed reaction rate of this enzyme -How is that a compound that is required for activity actually descrease it?

With fructosebisphophatase there is also a conundrum

Here I have made a big about the fact that this enzyme does not require ATP as a substrate. And indeed this is the case. YET ATP is found to enhance its efficiency. How is that a compound that has no relation to the activity actually enhance it?

Allosteric sites

As indicated earlier the allosteric site is literally another site. In both of these enzyme ATP bonds to a site away from the active site

There are TWO places on the PFK-1 enzyme where ATP can bind.

  1. The active site (as a cosubstrate)
  2. The allostric binding site. When ATP binds to this site then the efficiency (kcat/Km) decreases.

Coopertivity and Allosteric Binding

Just like in cooperative enzymes (described for hemoglobin), the effect of allosteric binding on enzyme efficiency depends on there being "two forms" of the enzyme structure: one that is catalytically efficient (GOOD or R) and one that is very inefficient (POOR or T). AS ATP binds to the allosteric site of either enzyme it will stabilize one of these forms: in the case of PFK-1 the POOR (T) form and in the case of FBPase the GOOD (R) form.