Role of Reduced Coenzymes NADH and FADH₂

NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are coenzymes derived from B vitamins that act as mobile electron carriers in biochemical reactions. During oxidation, these molecules accept electrons and become reduced to NADH and FADH2. They then release these electrons during reduction reactions, converting back to their oxidized forms and enabling energy extraction from nutrients.

NADH and FADH2 are produced in two major metabolic pathways. Glycolysis in the cytosol reduces NAD+ to NADH during glucose oxidation. The TCA cycle in the mitochondria produces three NADH molecules and one FADH2 molecule per cycle, making it the primary source of reduced coenzymes for carbohydrate metabolism and energy production.

NADH produced during glycolysis in the cytosol cannot cross the mitochondrial membrane directly. Instead, two carrier pathways—the malate-aspartate shuttle and the glycerol phosphate shuttle—transfer electrons to NAD+ or FAD molecules already present inside the mitochondria, allowing energy from cytosolic glucose breakdown to be utilized in the electron transport chain.

Reduced coenzymes NADH and FADH2 donate their high-energy electrons to the electron transport chain, which generates a proton motive force across the mitochondrial membrane. This electrochemical gradient drives ATP synthase to produce ATP, making reduced coenzymes essential for converting chemical energy stored in nutrients into usable cellular energy.

NAD+ and NADH exist in a reversible redox pair. During oxidation reactions, NAD+ accepts electrons and hydrogen ions, becoming reduced to NADH. During reduction reactions, NADH donates these electrons back, converting to NAD+. This cycle allows these coenzymes to shuttle electrons between metabolic pathways and store reducing power for energy extraction.

NADH and FADH2 are primarily used in catabolic pathways to extract energy from nutrients through the electron transport chain. NADPH, the reduced form of NADP+, plays a different role in anabolic reactions and photosynthesis. While all three carry reducing power, NADH and FADH2 are central to ATP generation, whereas NADPH supports biosynthetic reactions.

NADH, FADH2, and NADPH are called reducing agents because they possess high-energy electrons capable of donating to various chemical reactions. This reducing power enables them to drive oxidation-reduction reactions throughout metabolism, making them essential for both energy extraction and biosynthetic pathways in living systems.