Question

Compare glycolysis with gluconeogenesis: location, common enzymes, explain how the same enzyme can be used in both pathways, describe which steps of the two pathways require different enzymes and explain why, describe in detail how the liver regulates the two pathways.

Answer 1

Glycolysis is the conversion of glucose to pyruvate and the conversion of pyruvate to glucose, is called gluconeogenesis

• glycolysis a catabolic process of metabolism, while gluconeogenesis is anabolic.
• glycolysis occurs in the cytoplasm of all cells, gluconeogenesis is confined mainly to the liver and kidney.

Most of the enzymes required for gluconeogenesis are the same ones in glycolysis. This is because they are exact opposite processes, the glucose is converted to pyruvate in glycolysis and pyruvate is converted to glucose. The substrates and enzymes are mostly same except for the direction of reaction.

Example: phosphoglucose isomerase converts glucose 6-phosphate to fructose 6-phosphate in glycolysis and vice versa in gluconeogenesis.

• The enzymes which are same in both pathways are phosphoglucose isomerase,aldolase,glyceraldehyde 3 phosphate dehydrogenase,phosphoglycerate kinase, phosphoglycerate mutase and enolase.

The different Enzymes required in both pathways are of the irreversible steps namely pyruvate kinase, 6-phosphofructokinase, and hexokinase.

The irreversibility of the glycolytic pathway is due to three reactions, that cannot be used in gluconeogenesis. They are

• The phosphorylation of glucose to glucose 6-phosphate, catalyzed by hexokinase
• The phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate, catalyzed by phosphofructokinase-1
• The conversion of phosphoenolpyruvate or PEP to pyruvate, catalyzed by pyruvate kinase.

In gluconeogenesis, these three steps are catalyzed by enzymes like glucose 6 phosphatase, fructose 1,6 bisphosphatase and pyruvate carboxylase that catalyze irreversible steps of gluconeogenesis. This ensures the irreversibility of the metabolic pathway.

• The processes of gluconeogenesis and glycolysis are regulated in a reciprocal way. That means that when one process is active, the other one is inhibited.

It is mainly regulated by hormones such as insulin, which stimulates glycolysis, and glucagon, which stimulates gluconeogenesis.