NADH (Nicotinamide adenine dinucleotide)
Nicotinamide adenine dinucleotide (NADH) is the biochemical active form of vitamin B3. It is found in every human cell and is essential in energy production. NADH is found exogenously in muscle tissue of fish, cattle, poultry and products containing yeast.
Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are two important coenzymes found in cells. NADH is the reduced form and NAD+ is the oxidized form of NAD. Nicotinamide adenine dinucleotide is a coenzyme derived from the vitamin, niacin. It is important as a hydrogen carrier and alternates between the oxidized state (NAD+) (left) and reduced state (NADH + H+) at the right. A related compound, nicotinamide adenine dinucleotide phosphate (NADP+ is the oxidized form) has an additional phosphate group, and is important in photosynthesis.
NAD is used extensively in glycolysis and the citric acid cycle of cellular respiration. It forms NADP with the addition of a phosphate group (much as ADP forms ATP). NADP is produced in the preliminary cycles of photosynthesis, and is used in the later Calvin cycle of photosynthesis. It is used in many other anabolic reactions in various organisms as well.
Because of the positive charge on the nitrogen atom in the nicotinamide ring, the oxidized forms of these important redox reagents are often depicted as NAD+ and NADP+ respectively. In cells, most oxidations are accomplished by the removal of hydrogen atoms. Both of these coenzymes play crucial roles in this. Each molecule of NAD+ (or NADP+) can acquire two electrons; that is, be reduced by two electrons. However, only one proton accompanies the reduction. The other proton produced as two hydrogen atoms are removed from the molecule being oxidized is liberated into the surrounding medium. NADP has the same structure with the addition of an extra phosphate group to AMP.
NAD can be reduced to NADH during coupling with reactions which oxidize various organic substrates. For example, the reaction catalyzed by glyceraldehyde phosphate dehydrogenase during glycolysis. NADH then passes to the inside of mitochondria where it donates the electrons it is carrying to the electron transport chain. In this manner, NAD acts as an intermediate energy storage compound that indirectly generates ATP.
Generally, NADP accepts electrons from catabolic reactions to form NADPH. An example is its coupling with the conversion of glucose-6-phosphate to ribose-5-phosphate in the pentose phosphate pathway. NADPH has a slightly different role to NADH in that it does not donate electrons to the electron transport chain. Instead, it tends to reduce intermediates in anabolic pathways e.g. fatty acid synthesis.