Question

6. In the glycolytic path from glucose to phosphoenolpyruvate, two steps are practically irreversible. What are these steps, and how is each bypassed in gluconeogenesis? What advantages does an organism gain from having separate pathways for anabolic and catabolic metabolism? What are the disadvantages? (4 marks)

Answer 1

The irreversible steps in glycolysis are : 1) conversion of glucose to glucose 6-phosphate which is catalyzed by hexokinase and 2) conversion of fructose 6-phosphate to fructose 1,6-bisphosphate which is catalyzed by phosphofructokinase-1.

During gluconeogenesis, the first reaction is bypassed by the reaction catalyzed by glucose 6-phosphatase, an enzyme which is unique to the liver. The second reaction is bypassed by fructose 1,6-bisphosphatase-1 (FBPase-1). By having separate pathways that employ different enzymes,the organism is able to control anabolic and catabolic processes separately, hence avoiding futile cycles. One of the potential disadvantage is the need to produce separate sets of enzymes for catabolism and anabolism.