The insulin receptor is a transmembrane heterotetrameric ($(\alpha\beta)_2$) Receptor Tyrosine Kinase (RTK). Upon insulin binding to the extracellular $\alpha$ subunits, the intracellular $\beta$ subunit intrinsic tyrosine kinase undergoes autophosphorylation on specific tyrosine residues.

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.

Reactions and substrate specificity in the $\beta$-oxidation cycle:

• Acyl-CoA is the initial substrate acted upon by acyl-CoA dehydrogenase.
• Trans-$\Delta^2$-enoyl-CoA is the substrate for enoyl-CoA hydratase.
• L-$\beta$-hydroxyacyl-CoA is the substrate for $\beta$-hydroxyacyl-CoA dehydrogenase.
• Acyl-CoA acetyltransferase (commonly known as $\beta$-ketothiolase or thiolase) catalyzes the fourth and final thiolytic cleavage step of the $\beta$-oxidation cycle, acting specifically on $\beta$-ketoacyl-CoA in the presence of free coenzyme A ($CoA\text{-SH}$) to yield acetyl-CoA and an acyl-CoA shortened by two carbons.