Question

Explain why 1 additional net ATP is produced when the beginning substrate is glycogen compared to glucose. In your answer provide the name of the enzyme responsible for this difference.

Answer 1

The breakdown of glycogen to provide glucose is known as glycogenolysis. This process can take place in muscles, liver etc. , i.e., in places where there are glycogen reserves present in the body. Glycogen reserves are usually broken down in the body, when glucose is limiting. In skeletal muscles, glycogen is broken down to glucose-1-phosphate, which is then converted to glucose-6-phosphate, and this then enters the glycolytic pathways to release energy. But, in the liver, the glucose-6-phosphate is first converted to glucose by an enzyme glucose-6-phosphatase, so that it can be transported to other cells of the body, and then this glucose can enter the various catabolic pathways to produce energy. In this process inside the hepatic cells, under the action of the enzyme glucose-6-phosphatase, when glucose-6-phosphate is converted to glucose, one molecule of ATP is liberated. So, an additional net ATP molecule is produced when the beginning substrate is glycogen compared to glucose(in the conversion of glucose-6-phosphate to glucose by glucose-6-phosphatase), but this is true only when this glycogen comes from the liver.