The interconversion of fructose 6-phosphate to fructose 1,6-bisphosphate is a critical step in central metabolism in plants. Followings are certain statements regarding this interconversion:

A. Phosphofructokinase catalyzes the C6 phosphorylation of fructose 6-phosphate.
B. Plastid phosphofructokinase is activated by Pi while cytosolic phosphofructokinase is activated by phosphoenolpyruvate.
C. Cytosolic fructose 1,6-bisphosphatase is strongly inhibited by fructose 2,6-bisphosphate.
D. Pyrophosphate-dependent phosphofructokinase catalyzes a reversible reaction of interconversion of fructose 6-phosphate to fructose 1,6-bisphosphate.

Which one of the following options is a combination of all correct statements?

Correct option is D

Statement A:

"Phosphofructokinase catalyzes the C6 phosphorylation of fructose 6-phosphate."

• Incorrect:

  • Phosphofructokinase (PFK) phosphorylates the C1 position, not C6.

  • The reaction it catalyzes is:

    $$\text{Fructose-6-phosphate (F6P)}+\text{ATP}\to \text{Fructose-1,6-bisphosphate (F1,6BP)}+\text{ADP}$$

  • The “C6 phosphorylation” is factually wrong — the phosphate is added at carbon 1 of fructose.

Statement B:

"Plastid phosphofructokinase is activated by Pi while cytosolic phosphofructokinase is activated by phosphoenolpyruvate."

•  Incorrect:

  • In plant cells:

    • Plastid PFK is usually not regulated by Pi (inorganic phosphate); rather, it is less sensitive to regulation compared to cytosolic PFK.

    • Cytosolic PFK is not activated by phosphoenolpyruvate (PEP). In fact, PEP can inhibit PFK depending on the context (e.g., in bacterial systems).

  • Instead, in many systems, ATP, AMP, citrate, or fructose 2,6-bisphosphate are common regulators.

Statement C:

"Cytosolic fructose 1,6-bisphosphatase is strongly inhibited by fructose 2,6-bisphosphate."

•  True
  Fructose 2,6-bisphosphate is a potent allosteric inhibitor of fructose 1,6-bisphosphatase (FBPase).

  • This regulation is crucial in plants for balancing glycolysis and gluconeogenesis.

  • Inhibition of FBPase prevents futile cycling.

Statement D:

"Pyrophosphate-dependent phosphofructokinase catalyzes a reversible reaction of interconversion of fructose 6-phosphate to fructose 1,6-bisphosphate."

• True
  In plants, an alternative to ATP-dependent PFK is pyrophosphate (PPi)-dependent phosphofructokinase (PFP).

  • This enzyme catalyzes:

    $$\text{Fructose-6-P}+\text{PPi}\leftrightarrow \text{Fructose-1,6-BP}+\text{Pi}$$

  • It is reversible, unlike ATP-dependent PFK.