Question

What should be the net ATP yield for glycolysis when fructose, mannose, and galactose are used as the starting compounds? Justify your answer.

Short answer

The net ATP yield for glycolysis starting with fructose, mannose, or galactose is 2 ATP.

Step-by-step solution

Understand glycolysis

Glycolysis is a metabolic pathway that converts glucose into two molecules of pyruvate. This process generates a net gain of 2 ATP molecules per glucose molecule.

Fructose metabolism

Fructose is phosphorylated to fructose-1-phosphate by fructokinase, then split into glyceraldehyde and dihydroxyacetone phosphate (DHAP) by aldolase B. Both molecules enter the glycolysis pathway as glyceraldehyde-3-phosphate (GAP), contributing to the same net ATP yield as glucose, which is 2 ATP.

Mannose metabolism

Mannose is phosphorylated by hexokinase to form mannose-6-phosphate, which is then isomerized to fructose-6-phosphate by phosphomannose isomerase. Fructose-6-phosphate then enters the glycolysis pathway, resulting in the same net ATP yield as glucose, which is 2 ATP.

Galactose metabolism

Galactose is phosphorylated by galactokinase to form galactose-1-phosphate. This is then converted to glucose-1-phosphate through the Leloir pathway, which is further converted to glucose-6-phosphate. Glucose-6-phosphate enters glycolysis, yielding a net gain of 2 ATP, the same as when starting with glucose.

Summarize all net ATP yields

The net ATP yield for glycolysis when fructose, mannose, and galactose are used as the starting compounds is the same for each: 2 ATP per molecule.

Key concepts

glycolysis pathway

Glycolysis is the first step in cellular respiration. It's a key metabolic pathway that converts one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). This conversion yields energy, which is captured in the form of ATP (adenosine triphosphate). During glycolysis, 4 ATP molecules are produced, but since 2 ATP molecules are consumed, the net gain is 2 ATP. This process also generates NADH, a carrier of electrons used in further energy production steps.

fructose metabolism

Fructose metabolism involves a slightly different pathway compared to glucose. Fructose is first phosphorylated by the enzyme fructokinase, yielding fructose-1-phosphate. Aldolase B then splits fructose-1-phosphate into two three-carbon molecules: glyceraldehyde and dihydroxyacetone phosphate (DHAP). Both of these molecules are converted into glyceraldehyde-3-phosphate (GAP), which then enters the latter stages of glycolysis. The net result is the same as with glucose: a net gain of 2 ATP per molecule of fructose processed.

mannose metabolism

Mannose is metabolized by being phosphorylated by hexokinase to form mannose-6-phosphate. This compound is then isomerized to fructose-6-phosphate by the enzyme phosphomannose isomerase. Fructose-6-phosphate is then fed into the glycolysis pathway the same way glucose-6-phosphate would be. Because mannose is processed to enter the glycolysis pathway as fructose-6-phosphate, the net gain in ATP remains consistent with glucose metabolism, which is 2 ATP per molecule.

galactose metabolism

Galactose metabolism starts with its phosphorylation by the enzyme galactokinase, producing galactose-1-phosphate. This is then converted into glucose-1-phosphate by the Leloir pathway. Glucose-1-phosphate is subsequently converted into glucose-6-phosphate, which feeds into the glycolysis pathway. Thus, starting with galactose and following these steps until it integrates into glycolysis yields the same net ATP production as glucose, which is 2 ATP per molecule. This shows how galactose, despite being a different sugar, ends up contributing similarly to the cell’s energy currency.