Question

What statement is true? Only one is false and one is true.

a.In the skeletal muscles, when the levels of ADP increase, PFK is stimulated.

a.In the skeletal muscles, when the levels of citrate increase, PFK is inhibited.

Answer 1

Phosphofructokinase is first key regulatory enzyme of Glycolysis. It converts fructose 6 phosphate to fructose 1,6 bisphosphate. It is the commited step towards glycolysis because before activity of this enzyme the Glucose 6 phosphate can either go to Pentose phosphate pathway or it can be converted to Glucose 1 phosphate and get converted to Glycogen.

Phosphofructokinase (PFK) has around 3 isoforms. It is regulated allosterically. ATP and AMP compete for same site in PFK. If ATP will bind it will signify that cell is rich in energy hence activity of PFK will be inhibited.

When AMP binds to the allosteric site it signifies depletion of energy inside the cell hence PFK will be activated and glycolysis will operate more effectively.

Hence first statement is true indirectly (as ADP is latter on converted to AMP )

Effect of citrate especially on regulation of muscle isoform of PFK is debatable. Although in vitro it inhibits activity of PFK because more citrate means end product of glycolysis is not being cleared by TCA cycle fast enough so PFK activity should slow down hence Glycolysis as a whole should operate slowly. But the same is not proven in vivo in case of skeletal isoform of PFK.

Second statement is false (especially in vivo it may be true in case of in vitro but since in the statement we are dealing with skeletal muscle which is an in vivo system)