NAD+

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme found in all living cells that acts as an electron acceptor. It is the oxidized form of NAD (lacking electrons). NAD+ can accept two electrons and one proton (H+) to become NADH. In metabolic pathways (like glycolysis and the citric acid cycle), NAD+ is essential for capturing electrons from nutrient molecules; those electrons are carried as NADH and later used to produce ATP.

• Think of NAD+ as an 'empty shuttle' for electrons: it picks up electrons during catabolic reactions, turning into NADH (the 'full shuttle'). If NAD+ is unavailable or all in the NADH form, pathways like glycolysis cannot proceed because there-s no carrier to take the electrons. That-s why cells must regenerate NAD+ continually.
• Under aerobic conditions, NAD+ is regenerated when NADH donates its electrons to the electron transport chain (with oxygen ultimately accepting those electrons). Under anaerobic conditions (no oxygen), cells regenerate NAD+ via fermentation (e.g., converting pyruvate to lactate or ethanol, which oxidizes NADH back to NAD+).
• NAD+ and NADH are part of the same redox couple. NAD+ is the oxidizing agent (electron acceptor) and NADH is the reducing agent (electron donor). A classic exam question might ask which form has more energy (answer: NADH contains the high-energy electrons, whereas NAD+ is energy-poor until it-s reduced).

• Oxidation-reduction questions: You may see something like 'which cofactor is required for this dehydrogenase enzyme-' In many cases, the answer is NAD+ (as it will be reduced to NADH). Recognize NAD+ as a common coenzyme for dehydrogenase enzymes (like those in glycolysis and the TCA cycle).
• NAD+ vs NADH: Questions often test knowing which is oxidized vs reduced. NAD+ is oxidized (no electrons attached) and NADH is reduced (carrying electrons). For instance, 'which form of NAD is regenerated during fermentation-' Answer: NAD+ (because NADH is oxidized back to NAD+).
• Vitamin link: Some exam questions connect vitamins to coenzymes. Remember that NAD+ is derived from niacin (vitamin B3). A severe niacin deficiency (pellagra) affects NAD+/NADH levels and thereby impairs energy production. So, if asked 'which vitamin deficiency would directly reduce the cell-s ability to carry out oxidation-reduction reactions in catabolism-', niacin (B3) is the answer (due to its role in NAD+).