Final answer:
FAD (Flavin Adenine Dinucleotide) is necessary for Reaction 2, the formation of a-ß unsaturated acyl-CoA in fatty acid catabolism, because it acts as an oxidizing agent that accepts hydrogens, allowing the reaction to proceed.
Step-by-step explanation:
The additional substance necessary for Reaction 2 to take place, which is the formation of a-ß unsaturated acyl-CoA (Enoyl-CoA), is Flavin Adenine Dinucleotide (FAD). During this step in fatty acid catabolism, fatty acyl-CoA undergoes dehydrogenation at the alpha and beta carbon atoms, forming trans alpha-beta unsaturated acyl-CoA.
This reaction involves the coenzyme FAD, which accepts the hydrogens being removed, thereby getting converted into FADH2. FAD is thus necessary for the reaction to proceed because it acts as an oxidizing agent that facilitates the dehydrogenation process, leading to the generation of FADH2.