Question

The enzymes involved in glycolysis are.....

• A.

  constitutive proteins

• B.

  inducible proteins

• C.

  repressed proteins

• D.

  regulated proteins

Answer 1

D. regulated proteins

In the glycolytic pathway, the enzymes involved in 3 of the 8 steps of reactions catalyze irreversible reactions. So these three steps are the key control sites in the pathway involving the three regulatory enzymes.The amounts of these enzymes vary by the regulation of transcription to meet changing metabolic needs.

Hexokinase catalyzes the first step of glycolysis in formation of glucose 6-phosphate from glucose. Hexokinase is inhibited by its own product glucose 6-phosphate; so concentrations of glucose 6-phosphatesignals that the cell no longer requires glucose for energy.

The second and most important regulatory protein involved in glycolysis is Phosphofructokinase. It catalyses the conversion of β-D-Fructose 6-phosphate into β-D-Fructose 1,6-bisphosphate. This enzyme reaction is coupled by hydrolysis of ATP molecule. High concentration of ATP allostearically inhibits the enzyme, lowering its affinity for the substrate molecule. Glycolysis is stimulated as the energy charge falls which is indicated by an increase in AMP levels.

The third regulatory protein is pyruvate kinase which catalyses the conversion of phosphoenolpyruvate to pyruvate and ATP, which is the last step of glycolysis. ATP allosterically inhibits pyruvate kinase to slow glycolysis when the energy charge is high. Increase in blood sugar leads to secretion of insulin, which activates phosphoprotein phosphatase I, leading to dephosphorylation of pyruvate kinase.