Final answer:
In stage 3 of fatty acid synthesis, the reduction reactions require NADPH, with key enzymes 3-ketoacyl-ACP reductase and enoyl-ACP reductase utilizing NADPH as an electron donor.
Step-by-step explanation:
In stage 3 of fatty acid synthesis, the types of reactions that require NADPH are reduction reactions. The first reduction reaction is catalyzed by 3-ketoacyl-ACP reductase, which reduces acetoaceyl-S-ACP to 3-hydroxyacyl-S-ACP. Another reduction reaction is catalyzed by enoyl-ACP reductase, converting trans-²-enoyl-S-ACP to saturated acyl-S-ACP. Both of these enzymes use NADPH as an electron donor to facilitate the reduction process.
The enzyme responsible for the fatty acid elongation process is fatty acid synthase (FAS), and it catalyzes the sequential addition of two-carbon units to the growing fatty acid chain. The specific reactions that demand NADPH are the reduction steps, where carbonyl groups are converted to hydroxyl groups. NADPH serves as a reducing agent, providing the necessary electrons for these reduction reactions.
NADPH, a reduced form of nicotinamide adenine dinucleotide phosphate, plays a crucial role in maintaining the reducing environment required for biosynthetic pathways, including fatty acid synthesis. Its involvement in fatty acid elongation ensures the production of saturated fatty acids, which can later undergo modifications to generate a variety of lipid molecules essential for cellular structure and function.