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NAD⁺, derived from niacin, is freely diffusible and accepts a hydride ion to form NADH. NADH can transfer electrons to the respiratory chain through complex I, while cellular pathways such as shuttles or lactate formation help reoxidize it. NADP⁺ is associated with reductive biosynthesis and antioxidant defense. In contrast, FAD, derived from riboflavin, is commonly bound to enzymes and accepts two hydrogen atoms to form FADH₂. FAD-linked dehydrogenases participate in reactions where NAD⁺ is less suited to accept electrons, and flavoproteins also contribute to electron transport. These coenzymes temporarily hold reducing equivalents, coupling catabolism to ATP synthesis. Their vitamin origins connect coenzyme availability to clinical effects, while disrupted electron transfer can contribute to oxidative stress.