Phosphosfructokinase, PFK for short, is an enzyme that catalyses the addition of a phosphate to fructose-6-phosphate (F6P) in the third step of glycolysis, forming fructose-1,6-bisphosphate (F-1,6-BP).

The regulation of this step is very significant to the rate of glycolysis as a whole. It includes both activation and negative feedback. Steps 4-9 are reversible and so are basically at equilibrium; an increase in F-1,6-BP leads to an increase in each of those other products.

Because the purpose of glycolysis is to make ATP for the cell, a high concentration of ATP inhibits PFK by binding to an allosteric site. However, high concentrations of AMP block this inhibition, because when a cell is low on ATP but has lots of ADP due to ATP hydrolysis, another enzyme combines two molecules of ADP to make one ATP and one AMP.

Citrate also inhibits PFK. This is because the final product of glycolysis, pyruvate, goes into the citric acid cycle (aka Krebs cycle or TCA cycle), one of the molecules in which is (big surprise) citrate. So if there is plenty of citrate, glycolysis doesn't need to happen quickly.

One other molecule involved in PFK regulation is fructose-2,6-bisphosphate. It is another potential product of reactions of F6P (produced when F6P concentrations are high). It directly increases PFK activity and also reduces the inhibition caused by high ATP.