ICAR AIEEA PG Animal Biotechnology 09 July 2023 (Shift 1) Paper

Inhibition of mitochondrial electron transport chain by Piericidin A:

• Assertion (A) is true: Piericidin A is a microbial antibiotic structurally homologous to ubiquinone (coenzyme Q) that potently arrests mitochondrial respiration and ATP synthesis.
• Reason (R) is true: Piericidin A competitively binds to the ubiquinone reduction site in NADH:ubiquinone oxidoreductase (Complex I), blocking electron transfer from iron-sulfur clusters (N2) to ubiquinone.
• Because blocking Complex I electron entry into the ubiquinone pool arrests the downstream cascade of the respiratory chain when NADH is the primary electron donor, Reason (R) directly explains Assertion (A).

Auxiliary pathway enzymes for $\beta$-oxidation of unsaturated fatty acids:

• $\Delta^3$-$\Delta^2$-enoyl CoA isomerase converts cis-$\Delta^3$ double bonds into trans-$\Delta^2$ enoyl CoA intermediates suitable for enoyl CoA hydratase.
• NADPH-dependent 2,4-dienoyl CoA reductase reduces conjugated double bonds formed during oxidation of polyunsaturated fatty acids.
• Enoyl CoA isomerase is the general class encompassing the essential isomerization activity for unsaturated fatty acid catabolism.
• Methylmalonyl-CoA mutase is an enzyme of odd-chain fatty acid and branched-chain amino acid metabolism (converting methylmalonyl-CoA to succinyl-CoA), not an enzyme required for double bond processing in polyunsaturated fatty acids.

Caloric density of fats versus carbohydrates:

• Assertion (A) is true: Complete oxidative combustion of fatty acids yields approximately $9\text{ kcal/g}$ ($38\text{ kJ/g}$) compared to only $4\text{ kcal/g}$ ($17\text{ kJ/g}$) for carbohydrates and proteins.
• Reason (R) is true: Carbohydrates (polyhydroxy aldehydes/ketones) already have one oxygen atom for every carbon (partially oxidized state), whereas long hydrocarbon chains of fatty acids consist almost entirely of fully reduced methylene ($-CH_2-$) groups. Oxidizing more highly reduced carbons down to $ ext{CO}_2$ releases substantially more free energy and generates more reducing equivalents ($NADH$ and $FADH_2$).
• Therefore, the more reduced oxidation state of carbon in fats directly explains their higher energy yield.