Difference Between NAD and NADH

Main Difference – NAD vs NADH

NAD (Nicotinamide Adenine Diphosphate) is a coenzyme used in the cellular respiration in eukaryotes. The main function of NAD is to carry hydrogen and electrons from one reaction to another. This means NAD is involved in oxidation-reduction reactions. Therefore, it contains an oxidized form and a reduced form. The oxidized form of the NAD is NAD⁺ whereas the reduced form is NADH. The main difference between NAD and NADH is that NAD is the coenzyme whereas NADH is the reduced form of the NAD. NADH is produced in the glycolysis and Krebs cycle. It is used in the production of ATP in the electron transport chain.

Key Areas Covered

1. What is NAD
      – Definition, Synthesis, Role
2. What is NADH
      – Definition, Synthesis, Role
3. What are the Similarities Between NAD and NADH
      – Outline of Common Features
4. What is the Difference Between NAD and NADH
      – Comparison of Key Differences

Key Terms: Dehydrogenases, Electron Transport Chain, Glycolysis, Krebs Cycle, NAD, NAD⁺, NADH, Oxidative Phosphorylation

What is NAD

NAD is a most abundant coenzyme that acts as the oxidizing-reducing agent inside the cell. NAD⁺, which is the oxidized form of NAD, is the naturally-occurring form of NAD inside the cell. It is involved in the reactions of the cellular respiration such as glycolysis and Krebs cycle. It acquires a hydrogen ion and two electrons and is reduced to NADH. NADH is used to generate ATP in the electron transport chain. Hydroxylases and reductases also use NAD⁺ as an electron carrier. Oxidation and reduction of NAD are shown in figure 1.

Figure 1: Oxidation and Reduction of NAD

NAD⁺ is synthesized in two different pathways inside the cell: Tryptophan pathway and vitamin B₃ pathway. The starting product of the tryptophan pathway is the amino acid, tryptophan while the starting product of the vitamin B₃ pathway is vitamin B₃ (niacin or nicotinic acid).

What is NADH

NADH refers to the reduced form of NAD+, which is produced in the glycolysis and Krebs cycle. In glycolysis, two NADH molecules are produced per glucose molecule. Six NADH molecules are produced in the Krebs cycle per glucose molecule. These NADH molecules are used in the electron transport chain to produce ATP molecules. The production of NADH in glycolysis and Krebs cycle and the usage of NADH in the electron transport chain are shown in figure 2.

Figure 2: Cellular Respiration

Proteins embedded in the inner membrane of mitochondria obtains electrons from NADH molecules. These electrons are transported through different protein molecules of the electron transport chain. Ultimately, they are obtained by oxygen molecules to form water. This means oxygen molecules are the final electron acceptors in the aerobic respiration. The energy released in the process is used to produce ATP by oxidative phosphorylation. In fermentation, other molecules serve as final electron acceptors since oxygen is absent in the medium. The regeneration of NAD⁺ occurs through substrate-level phosphorylation.

Similarities Between NAD and NADH

Both NAD and NADH carry hydrogen and electrons from one reaction to another.
Both NAD and NADH contain two ribose molecules attached to the phosphate groups, a nicotinamide, and an adenine base.
Both NAD and NADH are nucleotides.
Both NAD and NADH are involved in catabolic reactions.
Most of the dehydrogenases use NAD and NADH.

Definition

NAD: NAD is the most abundant coenzyme, which acts as the oxidizing-reducing agent inside the cell.

NADH: NADH is the reduced form of NAD+, which is produced in the glycolysis and Krebs cycle.

Correspondence

NAD: NAD is a coenzyme compound.

NADH: NADH is the reduced form of the NAD.

Synthesis

NAD: NAD is synthesized either by tryptophan pathway or vitamin B₃ pathway.

NADH: NADH is synthesized in glycolysis and Krebs cycle.

Existing Form

NAD: NAD⁺ is the naturally-occurring form of NAD inside the cell.

NADH: NADH is the reduced form of the NAD.

Serve as

NAD: NAD⁺ serves as an electron and hydrogen acceptor.

NADH: NADH serves as an electron and hydrogen donor.

Conclusion

NAD and NADH are two types of nucleotides involved in the oxidizing-reducing reactions of cellular respiration. The naturally-occurring form of NAD inside the cell is NAD+. It serves as a hydrogen and electron acceptor in both glycolysis and Krebs cycle. NADH is the reduced form of NAD. It is used in the electron transport chain to produce ATP by oxidative phosphorylation. The main difference between NAD and NADH is the role of both compounds in the cell.

Reference:

1.“ NAD ,NADH – Nicotinamide adenine dinucleotide.” Glutamate dehydrogenase structure, Available here.
2.“Role of NADH in Cellular Respiration.” Study.com, Available here.

Image Courtesy:

1. “NAD oxidation reduction” By Fvasconcellos 19:44, 9 December 2007 (UTC). w:Image:NAD oxidation reduction.png by Tim Vickers. – Vector version of w:Image:NAD oxidation reduction.png by Tim Vickers (Public Domain) via Commons Wikimedia
2. “Cellular respiration” By Darekk2 – Own work (CC BY-SA 3.0) via Commons Wikimedia