Question

Of the three regulated steps of the glycolysis pathway, one is unusual. Which step is unusual as a regulated step, and what is unusual about it?

Answer 1

Glycolysis is regulated by three enzymes- hexokinase, phosphofructokinase-1 and pyruvate kinase. Hexokinase catalyzes the irreversible step of glucose conversion to glucose 6 phosphate. This glucose 6 phosphate can be used either for glycolysis or glycogenesis. Pyruvate kinase converts PEP to pyruvate and is the last step of glycolysis.

Phopshofructokinase-1 (PFK-1) is the enzyme that converts fructose-6-phosphate to fructose, 1, 6 bisphosphate. It is most important regulatory step of glycolysis. It is also the most unusual enzyme in glycolysis. It is unusual because it can be regulated by its own substrate ATP. ATP is utilized when fructose-6 –phosphate is phosphorylated to Fructose-1, 6 bisphosphate. One molecule of ATP binds to the catalytic site of PFK-1 for catalysis. However when ATP levels increase in the cell, two molecules of ATP bind to the enzyme. One ATP binds catalytic site while the other binds to the allosteric regulatory site. This will lead to inhibition of the enzyme. This step is unusual because the same substrate is binding to catalytic and regulatory site of the enzyme. Normally allosteric regulators are not substrates for the enzyme.