NAD⁺/NADH and FAD/FADH₂

Why NAD and FAD matter

• Universal redox carriers across all of life. Every organism uses NAD⁺ and FAD; both predate the divergence of bacteria and archaea. Glycolysis, the citric acid cycle, β-oxidation, and the electron transport chain all depend on these two coenzymes — they are as conserved as ATP.
• The voltage span powers oxidative phosphorylation. NADH at −0.32 V donates to complex I; electrons flow through ubiquinone (~+0.07 V), complex III, cytochrome c (~+0.25 V), and complex IV to O₂ at +0.82 V. ΔE = 1.14 V × 2 electrons × 96.485 kC/mol = 220 kJ/mol per NADH oxidized.
• ~32 ATP per glucose comes out of these two coenzymes. Glycolysis nets 2 ATP and 2 NADH; pyruvate dehydrogenase yields 2 NADH; citric acid cycle yields 6 NADH, 2 FADH₂, and 2 GTP per glucose. At ~2.5 ATP/NADH and ~1.5 ATP/FADH₂, total = ~32 ATP per glucose oxidized.
• NADPH/NADP⁺ is kept reduced; NADH/NAD⁺ is kept oxidized. The same chemistry, opposite kinetic pools: cytoplasmic [NADPH]/[NADP⁺] ~100 drives biosynthesis; [NAD⁺]/[NADH] ~1000 drives catabolism. The 5-order-of-magnitude ratio difference is what makes the cell run reductive synthesis and oxidative breakdown in parallel.
• Sirtuins consume NAD⁺ as a metabolic sensor. SIRT1–7 are NAD⁺-dependent deacetylases that activate when NAD⁺ is high (fasting, exercise, calorie restriction). Their substrates include p53, FOXO, PGC-1α, and histones — coupling metabolic state to gene expression and longevity. Discovered by Imai and Guarente (2000).
• FAD is usually protein-bound for stability. Free FADH₂ auto-oxidizes by O₂ within minutes producing superoxide; tight binding (K_d < 1 nM, sometimes covalent) shields it. Succinate dehydrogenase has FAD covalently linked via 8α-histidyl-FAD; the cost is that bound FAD potentials shift to ~0 V or even +0.06 V depending on the active site.
• NAD biosynthesis is a metabolic bottleneck. Mammals make NAD⁺ from tryptophan (de novo, ~6% efficiency) or salvage it from nicotinamide/nicotinic acid/nicotinamide riboside (vitamin B3 family). Pellagra (severe niacin deficiency) was epidemic in the early 20th century; Goldberger's 1914 work tied it to dietary niacin.

Common misconceptions

• Calling NAD⁺ and NADP⁺ interchangeable. Their redox chemistry is identical but their enzymes recognize the 2'-phosphate of NADP⁺. The two pools are kinetically segregated; using NAD⁺ in NADPH-dependent reactions does not work.
• Writing NADH₂. The reduced form is NADH plus a free H⁺ in solution — only one proton ends up bound to the molecule. FADH₂ by contrast does carry both protons because it accepts 2H, not a hydride.
• Treating "high-energy electrons" as a special quantum state. The energy is in the standard reduction potential of the carrier-substrate couple. NADH electrons are "high-energy" only because their environment (the nicotinamide ring) holds them at −0.32 V — far above ground-state oxygen.
• Saying FADH₂ yields ~2 ATP. Modern stoichiometry from Hinkle and others gives ~1.5 ATP per FADH₂ (~2.5 per NADH), reflecting the proton-pumping efficiency of complexes I, III, IV and ATP synthase. Older textbooks quoted 2 and 3 respectively.
• Forgetting the semiquinone intermediate of FAD. FAD reduces in two one-electron steps with FADH• (semiquinone) in between. This is why flavoproteins can interface with one-electron carriers (cytochromes, iron-sulfur clusters) where NAD cannot — NAD only does two-electron hydride transfers.
• Assuming all dehydrogenases work the same. "Pro-R" vs "pro-S" hydride transfer geometry is enzyme-specific. Lactate dehydrogenase transfers the pro-R H to the A face of NAD⁺; alcohol dehydrogenase transfers pro-S to the B face. Westheimer mapped these stereochemical preferences in the 1950s.

Mechanism: hydride transfer and the proton-electron split

NAD⁺ reduction occurs at the C4 position of the nicotinamide ring. A substrate alcohol R–CH(OH)–R' loses the alpha hydrogen as a hydride (H⁻) to NAD⁺, while the OH proton dissociates separately into solution as H⁺. The reaction is therefore R–CH(OH)–R' + NAD⁺ → R–C(=O)–R' + NADH + H⁺. The hydride goes to one specific face of the nicotinamide (A or B face, depending on the enzyme), with strict stereochemistry that Frank Westheimer demonstrated in 1953 using deuterium labeling. Reduction collapses the pyridinium ring's aromaticity into a 1,4-dihydropyridine — the ring loses its quaternary nitrogen positive charge and gains a sp³ carbon at C4.

FAD reduction occurs at the isoalloxazine ring's N5 and N10 (in some flavins, N1 instead). FAD picks up two electrons and two protons to become FADH₂, going through the semiquinone radical FADH• in two one-electron steps. The intermediate is detectable spectroscopically — oxidized FAD is yellow (λ_max ~450 nm), semiquinone FADH• is blue (~580 nm) or red (~370 nm) depending on protonation, and FADH₂ is colorless. This three-state behavior makes FAD the universal interface between two-electron substrates (NADH, succinate) and one-electron carriers (cytochromes, iron-sulfur clusters).

Otto Warburg isolated NAD (then called "co-zymase" after Harden and Young's 1906 work on yeast fermentation) in 1929 and identified its reduced form. Hugo Theorell crystallized FAD-binding "old yellow enzyme" in 1934, the first flavoprotein shown to be a colored protein because of its bound flavin. Warburg won the 1931 Nobel for cellular respiration, Theorell the 1955 Nobel for oxidation enzymes. Mitchell's 1961 chemiosmotic hypothesis, awarded the Nobel in 1978, finally tied NADH oxidation through the electron transport chain to ATP synthesis via a proton gradient — closing the loop between the redox coenzymes and the energy currency.

Variant comparison: redox coenzymes

Coenzyme	Full name	E°' (V, free)	Electrons/protons	Cellular role	Key feature
NAD+/NADH	Nicotinamide adenine dinucleotide	−0.32	2e− + 1H+ (hydride)	Catabolism, ETC input	[NAD+]/[NADH] ~1000
NADP+/NADPH	NAD with 2'-phosphate	−0.32	2e− + 1H+ (hydride)	Anabolism, biosynthesis	[NADPH]/[NADP+] ~100
FAD/FADH2	Flavin adenine dinucleotide	−0.22 (free)	2e− + 2H+ (2H)	Succinate DH, fatty acid β-oxidation	Tightly enzyme-bound; semiquinone intermediate
FMN/FMNH2	Flavin mononucleotide	−0.22 (free)	2e− + 2H+	Complex I prosthetic group	FAD without the AMP; same isoalloxazine
CoQ/CoQH2 (ubiquinone)	Coenzyme Q10	+0.07	2e− + 2H+ (2H)	ETC mobile lipid carrier	Diffuses in inner mitochondrial membrane
Cytochrome c	Heme c protein	+0.25	1e−	ETC complex III to IV	One-electron Fe3+/Fe2+
O2/H2O	Molecular oxygen	+0.82	4e− + 4H+	Terminal electron acceptor	Reduced by complex IV to water
Glutathione GSSG/2 GSH	γ-Glu-Cys-Gly disulfide	−0.24	2e− + 2H+	Cytoplasmic antioxidant buffer	~5 mM in cytoplasm; reduced by NADPH

Applications and examples

• Lactate dehydrogenase clinical assay. Pyruvate + NADH → lactate + NAD⁺; NADH absorbs at 340 nm but NAD⁺ does not, so the rate of A₃₄₀ change measures enzyme activity. The standard assay since Warburg's 1936 spectrophotometric method.
• Sirtuin activators in healthspan research. Resveratrol, NMN (nicotinamide mononucleotide), and nicotinamide riboside raise NAD⁺ levels in mice and improve metabolic markers; human trials are ongoing. The mechanism rests on sustaining sirtuin activity as cellular NAD⁺ declines with age.
• Metformin and complex I. The world's most prescribed diabetes drug partially inhibits complex I, lowering hepatic ATP and raising AMPK activation. The downstream effect is reduced gluconeogenesis — metformin's NADH/NAD⁺ ratio shift in liver is now seen as central to its mechanism.
• Fluorescent flavin imaging. NADH and FAD have distinguishable autofluorescence (NADH blue, FAD green); two-photon microscopy of NADH/FAD ratio (the redox ratio) reveals metabolic state in live tissues without staining. Warburg-effect tumor imaging uses this directly.
• Vitamin B3 fortification. Niacin (nicotinic acid) is added to flour worldwide because dietary deficiency causes pellagra (dermatitis, dementia, diarrhea, death). The 1937 USDA recommendation followed Goldberger's 1914 epidemiological work.