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. In the liver, lactate is converted back into glucose by gluconeogenesis. For each given enzyme, identify whether it is involved in the glycolysis pathyway, gluconeogenesis pathway, both pathways, or neither pathway. a. glyceraldehyde 3-phosphate dehydrogenase
b. phosphofructokinase-1
c. phosphoenolpyruvate carboxykinase
d. alcohol dehydrogenase

Answer and explanation:

Glycolysis and gluconeogenesis are metabolic pathways of great importance in the regulation of glycemia (glucose levels in the blood).

While glycolysis, a catabolic pathway that occurs when the glucose levels are high and is used to generate energy inside our cells; gluconeogenesis, an anabolic pathway, happens because glucose levels aren't high enough for our tissues to perform their energy-demanding tasks.

Glycolysis is stimulated by insulin and transforms 1 glucose molecule into 2 pyruvate molecules, which can then be further metabolized to produce ATP. Gluconeogenesis does the opposite: being stimulated by glucagon and epinephrine, it converts pyruvate molecules into glucose molecules to elevate glycemia.

Partaking in antagonizing pathways, glycolysis and gluconeogenesis appear to be the exact reverse pathway for one another, except for a few specific enzymes that are unique to each one of them.

a. glyceraldehyde 3-phosphate dehydrogenase - both pathways. This enzyme catalyzes the conversion of glyceraldehyde 3-phosphate into 1, 2 biphosphoglycerate, and backward.

b. phosphofructokinase-1 - glycolysis. This kinase phosphorylates (adds a phosphate group) fructose 6-phosphate into fructose 1, 6 biphosphate. In gluconeogenesis, the opposite reaction occurs thanks to the fructose 1, 6 biphosphatase (which removes a phosphate group from fructose 1, 6 biphosphate).

c. phosphoenolpyruvate (PEP) carboxykinase - gluconeogenesis. In glycolysis, phosphoenolpyruvate is transformed to pyruvate in one single reaction, catalyzed by enzyme pyruvate kinase. But in gluconeogenesis, pyruvate needs first to be converted into oxalacetate by the pyruvate carboxylase, so can then be transformed into phosphoenolpyruvate by the PEP carboxykinase.

d. alcohol dehydrogenase - neither pathway. Alcohol dehydrogenase is an enzyme that can be found in Ethanol metabolism, catalyzing the forming of acetaldehyde from an ethanol molecule.

substitution is the answer in this case