The question is testing which enzyme, out of four choices that all sound like ordinary sugar metabolism enzymes, is actually the cell's antioxidant defense enzyme. The trick is to remember that one of these four does not belong to glycolysis at all.
- Hexokinase: catalyzes the very first step of glycolysis, phosphorylating glucose to glucose 6-phosphate using ATP. This traps glucose inside the cell but has nothing to do with removing reactive oxygen species.
- Glucose 6-phosphate dehydrogenase: this is not a glycolytic enzyme, it belongs to the pentose phosphate pathway. It oxidizes glucose 6-phosphate and, in doing so, produces $NADPH$. Cells use this $NADPH$ to keep glutathione reduced through glutathione reductase, and reduced glutathione is what glutathione peroxidase uses to break down hydrogen peroxide. This is the direct route by which this enzyme shields the cell from oxidative damage.
- Enolase: converts 2-phosphoglycerate into phosphoenolpyruvate late in glycolysis, a dehydration reaction used purely for energy extraction, unrelated to any antioxidant pathway.
- Pyruvate kinase: catalyzes the final, ATP generating step of glycolysis, transferring a phosphate from phosphoenolpyruvate to ADP. Its deficiency causes hemolytic anemia through energy shortage, a completely different mechanism from oxidative stress.
Since three of the four enzymes, hexokinase, enolase, pyruvate kinase, are purely glycolytic and generate no reducing power for antioxidant use, and only glucose 6-phosphate dehydrogenase feeds the $NADPH$ and glutathione defense system, option (B) is the one that protects cells from reactive oxygen species.
Let's summarize:
- Glucose 6-phosphate dehydrogenase drives the pentose phosphate pathway and supplies $NADPH$.
- $NADPH$ keeps glutathione reduced, and reduced glutathione neutralizes reactive oxygen species like $H_2O_2$.
So the enzyme that protects cells from reactive oxygen species is glucose 6-phosphate dehydrogenase, option (B).