In the Cori cycle, when glucose is degraded by glycolysis to lactate in muscle, the lactate is excreted into the blood and returns to the liver where it is converted back into glucose by gluconeogenesis. For each enzyme below, identify whether it is involved in the glycolysis pathyway, gluconeogenesis pathway, both pathways or neither pathway.

1. glyceraldehyde 3-phosphate dehydrogenase

a. glycolysis
b. gluconeogenesis
c. both
d. neither
2. glucose-6-phosphatase
a. glycolysis
b. gluconeogenesis
c. both
d. neither
3. alcohol dehydrogenase
a. glycolysis
b. gluconeogenesis
c. both
d. neither
4. phosphoenolpyruvate carboxykinase
a. glycolysis
b. gluconeogenesis
c. both
d. neither
5. phosphofructokinase-1
a. glycolysis
b. gluconeogenesis
c. both
d. neither
6. phosphoglycerate mutase
a. glycolysis
b. gluconeogenesis
c. both
d. neither
7. hexokinase
a. glycolysis
b. gluconeogenesis
c. both
d. neither
8. pyruvate dehydrogenase
a. glycolysis
b. gluconeogenesis
c. both
d. neither

Explanation:

Glycolysis is the breakdown of glucose while gluconeogenesis is the formation of glucose. Both pathways are almost the same except that the non-reversible reactions in glycolysis requires another enzyme during gluconeogenesis.

Glycolysis is a 10-step multi-enzyme pathway, in which three of these 10 steps are non-reversible. It is these three steps that basically differentiates the glycolytic pathway from gluconeogenesis. These three steps are bypassed by different enzymes during gluconeogenesis

1. The correct option here is c

Glyceraldehyde 3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate in glycolysis. This reaction is however a reversible reaction, hence this enzyme also converts 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate during gluconeogenesis.

2. The correct option here is b

Glucose 6-phosphatase converts glucose 6-phosphate to glucose during gluconeogenesis

3. The correct option here is d

Alcohol dehydrogenase converts ethanol into acetaldehyde. This reaction is not present in both glycolysis and gluconeogenesis and hence the enzyme is not involved/present in these pathways.

4. The correct option here is b

Phosphoenol pyruvate carboxykinase converts oxaloacetate to phosphoenol pyruvate during gluconeogenesis

5. The correct option here is a

Phosphofructokinase-1 converts fructose 6-phosphate to fructose 1,6-bisphosphate during glycolysis

6. The correct here is c

Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate during glycolysis. This reaction is however a reversible reaction and hence this same enzyme converts 2-phosphoglycerate to 3-phosphoglycerate during gluconeogenesis.

7. The correct option here is a

Hexokinase converts glucose to glucose 6-phosphate during glycolysis.

8. The correct option here is d

Pyruvate dehydrogenase converts pyruvate to acetyl-coA. This reaction is one of the fates of pyruvate. This reaction is however not present in both glycolysis and gluconeogenesis and hence the enzyme is not involved/present in both also.