Enzymes that are a key regulation control point for metabolism exhibit complex kinetics indeed. Lets take a look at the relationships with the enzyme phosphofructokinase-1. It exhibits both cooperative AND allosteric effects. A number of compounds bind to the single allosteric site and have varying effects depending on which form of the enzyme they stabilize. A few of the important effectors are listed below. To add to the complexity notice that ATP binds to BOTH the allosteric site and active site on the enzyme and exhibit opposite effects.
|Compound||Site||Role||Effect on Rate||Rationale|
|ATP*||active site||substrate||Observed rate increases as [ATP] increases (until vmax obtained) in Michaelis-Menten Fashion||Normal substrate behaviour|
|F6P||active site||substrate||exhibits Cooperativity; as F6P binds it shifts protein structure equilibrium towards GOOD (R)|
|AMP||allosteric site||GOOD (R)||increases activity||High AMP levels indicate that the cell is low in energy and that glycolysis should be favored over gluconeogenesis|
|ADP||allosteric site||GOOD (R)||increases activity||High ADP levels indicate that the cell is low in energy and that glycolysis should be favored over gluconeogenesis|
|F26BP||allosteric site||GOOD (R)||increases activity||We will have pages dedicated to this later. F26BP is made only in the liver and is not part of any metabolic pathway. Its formation is solely regulatory and its concetration is regulated by insulin and glucagon|
|ADP||active site||competes with ATP||decreases activity||ADP, beyond binding to the allosteric site, is also a product of the reaction. As its concetration rises it can compete with ARP for binding to the active site. As you may guess, this is a bit counterproductive and is overall a minor effect relative to the effects of it binding to the allosteric site.|
|ATP*||allosteric site||POOR (T)||decreases activity||High ATP levels indicate that the cell is replete with energy and that more energy production through glycolcysis is not wo urgent. This is of course the oposite of the effect of ATP binding to the active site noted above.|
|citrate||allosteric site||POOR (T)||decreases activity||High citrate levels indicate that the cell is replete with energy. This is because citrate is the first compound in the Kreb's (citric acid) cycle. If there is a lot of citrate then then glycolcysis has already produced enough acetyl-CoA (compound necessary to make citrate)|
|phosphglycolate**||allosteric site||POOR (T)||decreases activity||The relevance of this compound is beyonf the scope of this course. I include it here because it was used in the structures shown on the previous page.|
|* Notice ATP bonds to both the active site AND the allosteric site. ATP binds to the active site at lower concentrations than it does to the allosteric site. At these concentrations (Low) it acts as a Michaelis Menten substrate. As ATP concentration increases the population of protein that binds ATP in the allosteric site increases. Here it stabilizes the POOR catalytic form of the protein causes the observed activity of the protein to drop.|
**phosphglycolate is not a physiological substrate, nevertheless it binds to the allosteric site and elicits the same response as ATP... stabilizes the POOR catalytic form of the enzyme.
The line with the circles in each graph shows the Cooperativity with the addition of F6P. Notice that there is a decidedly sigmoidal curvature to this line
Qualitatively these lines resemble the shape of the O2 binding to hemoglobin.
Neither AMP nor f26BP bond to the active site... only to the allosteric site. Notice that their binding makes the observed kinetics appear much more like normal Michaelis Menten kinetics because they stabilize the Good form of the structure.
The line with the circles shows the complex nature of ATP binding to the enzyme. At low concentrations it is binding mostly to the active site. But as the concentration increases then it binds to the allosteric site. Since it stabilizes the POOR structural form of the enzyme we see that the activity actually starts to drop.
AMP and F26BP compete with ATP for the allosteric site. When these are added (the top two lines in A.) ATP has substantially less effect at higher concetrations. This is because the allosteric site is already filled. In this case with compounds that stailbize the Good form.
My goal here is not for you to memorize these complex relationships, but to see that observed kinetics are complex. Frequently, in text books or in summary literature, you see indications of enzyme activity being ON or OFF. It is not quite this simple of course. One can really only talk about increasing and decreasing enzyme efficiency. You can see from the plots above, though, that the effects can be sunstantial.