Question

PEK-1 is an enzyme that catalyzes step 3 of glycolysis. The activity of this enzyme is regulated in the following ways: O either ADP or ATP can bind a particular regulatory site on this allosteric enzyme. O if ATP is bound to the regulatory site, the enzyme's activity rate increases. if ADP is bound to the regulatory site, the enzyme's activity rate increase. high levels of citrate indicate that the CAC is not keeping up, citrate binds to the regulatory site on PFK-1 and slows its activity.

Answer 1

Phosphofructokinase 1 is an allosteric enzyme involved in conversion of fructose 6 phosphate and ATP to fructose 1, 6 bisphosphate and ADP. PFK-1 has four subunits where each subunit has three binding sites for ligands. Each subunit has two beta sheet domains. PFK-1 is allosterically inhibited (decreased activity) by ATP and citrate while fructose 2, 6 bisphosphate and ADP are allosteric activator (increase activity)

Fructose 6 phosphate and ATP bind to two different sites on PFK-1 called domain 1 and 2. The catalytic site lies between these 2 domains. Domain one binds ATP while the other domain2 will bin fructose 6 phosphate. Substrate fructose 6 phosphate binds domain 2 only in the R state of enzyme.

However, when there is excess ATP in the cell, instead of binding to one site, ATP will bind to two sites- one the catalytic site while the other is the regulatory site. ATP can bind to the regulatory sites only in T state of enzyme while it can bind the catalytic domain 1 in T and R state of enzyme. In excess ATP, there is shift of conformation toward the T state. Thus, the enzyme can no longer bind to fructose 6 phosphate, but can bind ATP at both sites. As a result, the enzyme activity is inhibited. AMP and ADP can reverse the effect of ATP inhibition. They can also bind to the allosteric site and reverse the inhibition by converting the T state to R state. Thus, ADP can also bind to the regulatory ATP site, but it also has another ADP binding site as well.

Fructose 1, 6 bisphosphatase is enzyme that is activated when there are high levels of citrate. Fructose 1, 6 bisphosphatase converts fructose 1, 6 bisphosphate to fructose-6 phosphate via dephosphorylation. Thus, citrate will increase the effects of ATP inhibition. Citrate does not bind PFK-1. It is fructose 2,6 bisphosphate that binds and increases affinity

There are two correct choices:

1. Either ATP or ADP can bind to a particular regulator site on the allosteric enzyme.

2. If ADP is bound to the regulatory site, the enzyme’s activity rate increases.