GATE 2026 BT Q.16: solution and answer

Official answer: BSource: Official answer key, GATE 2026 BT (IIT Guwahati)

1. Cells defend against reactive oxygen species (ROS) with reduced glutathione and thioredoxin, together with enzymes such as catalase, superoxide dismutase and glutathione peroxidase. Regenerating the reduced forms of glutathione and thioredoxin needs a steady supply of $\text{NADPH}$.

2. Glutathione peroxidase uses reduced glutathione (GSH) to destroy ${\text{H}}_2{\text{O}}_2$, forming oxidised glutathione (GSSG). Glutathione reductase then regenerates GSH from GSSG and needs $\text{NADPH}$ to do so.

3. In red blood cells the oxidative (first) stage of the pentose phosphate pathway is the main source of $\text{NADPH}$. Its first, rate-limiting step is catalysed by glucose 6-phosphate dehydrogenase: $\text{glucose 6-phosphate}+{\text{NADP}}^{+}\to \text{6-phosphogluconolactone}+\text{NADPH}$.

4. Option B is correct. A deficiency of this enzyme leaves cells short of $\text{NADPH}$ and unable to cope with oxidative stress. This causes haemolytic anaemia after exposure to oxidants such as primaquine or fava beans.

5. Option A is wrong. Hexokinase phosphorylates glucose to glucose 6-phosphate at the start of glycolysis and makes no $\text{NADPH}$.

6. Options C and D are wrong. Enolase (2-phosphoglycerate to phosphoenolpyruvate) and pyruvate kinase (phosphoenolpyruvate to pyruvate, with ATP formed) are glycolytic enzymes of the pay-off phase. Neither makes $\text{NADPH}$ or takes part in ROS defence.