There are two key regulatory steps in glycolysis: one mediated by phosphofructokinase (PFK) and the other by pyruvate kinase. In addition, there are other types of modulation involving cofactors and enzyme structural changes reported to be involved in glycolytic control.
ATP levels increase during ripening. However, in fruits, this does not cause a feedback inhibition of phosphofructokinase as observed in animal systems. There are two isozymes of PFK in plants: one localized in plastids and the other localized in the cytoplasm.
These isozymes regulate the flow of carbon from the hexose phosphate pool to the pentose phosphate pool. PFK isozymes are strongly inhibited by phosphoenol pyruvate.
Thus, any conditions that may cause the accumulation of phosphoenol pyruvate will tend to reduce the carbon flow through glycolysis. By contrast, inorganic phosphate is a strong activator of PFK. Thus, the ratio of PEP to inorganic phosphate would appear to be the major factor that regulates the activity of PFK and carbon flux through glycolysis. Structural alteration of phosphofructokinase, which increases the efficiency of utilization of fructose-6-phosphate, is another means of regulation that can activate the carbon flow through the glycolytic pathway.