Difference Between NADH and FADH2

The main difference between NADH and FADH₂ is that every NADH molecule produces 3 ATP molecules during oxidative phosphorylation whereas every FADH₂ molecule produces 2 ATP molecules. Furthermore, NADH transfers electrons to Cytochrome complex I while FADH₂ transfers electrons to Cytochrome complex II.

NADH and FADH₂ are the reduced forms of coenzymes, known as NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), respectively. They play a crucial role in cellular energy production.

Key Areas Covered

1. What is NADH
     – Definition, Structure, Function
2. What is FADH2
     – Definition, Structure, Function
3. What are the Similarities Between NADH and FADH2
     – Outline of Common Features
4. What is the Difference Between NADH and FADH2
     – Comparison of Key Differences

Key Terms

Cellular Respiration, Coenzymes, FAD, FADH₂, NAD, NADH, Oxidative Phosphorylation

What is NADH

NADH is the reduced form of NAD (nicotinamide adenine dinucleotide), which is a crucial coenzyme involved in the transfer of energy between biochemical reactions that occur in the cell. The structure of NAD is made up of two nucleotides: adenine and nicotinamide, joined through their phosphate groups. The oxidized form of DNA is NAD⁺.

Figure 1: NAD⁺

The main function of NAD is its role in the oxidation-reduction reactions inside the cell, serving as a coenzyme to enzymes such as dehydrogenases, reductases, and hydroxylases, in major metabolic processes such as glycolysis, Krebs cycle, fatty acid synthesis, and steroid synthesis.

Figure 2: Redox Metabolism Linked by NAD⁺ and NADH

During glycolysis, two NADH molecules are produced while during Krebs cycle, six NADH molecules are produced per glucose molecule. These eight NADH molecules move to the electron transport chain to produce ATP. Three ATP molecules are produced per NADH molecule. However, in fermentation, two NADH molecules are produced during glycolysis and their regeneration occurs through substrate-level phosphorylation.

What is FADH₂

FADH₂ is the reduced form of FAD (flavin adenine dinucleotide) made up of two joined nucleotides: adenine and flavin mononucleotide. Flavin-N (5)-oxide, quinone, semiquinone, and hydroquinone are the four, redox forms of FAD. Quinone is the fully-oxidized form while hydroquinone or FADH₂ is the fully-reduced from, which has accepted two electrons (2e^–) and two protons (2H⁺). FAD, along with proteins, form flavoproteins.

Figure 3: Role of NADH and FADH₂ in Electron Transport Chain

Two FADH₂ molecules are produced during the Krebs cycle per glucose molecule. These two molecules carry electrons to the electron transport chain and produce two ATP molecules per FADH₂.

Similarities Between NADH and FADH₂

• NADH and FADH₂ are reduced forms of coenzymes.
• They are produced during glycolysis and Krebs cycle.
• Further, both are made up of two nucleotides joined together by their phosphate groups.
• Both contain an adenine nucleotide.
• They carry hydrogen and electrons.
• Also , both of them can take up two electrons.
• Both carry electrons for the production of ATP during oxidative phosphorylation.
• Additionally, they are involved in the oxidation-reduction reactions as electron carriers.

Difference Between NADH and FADH₂

Definition

NADH refers to the reduced form of the ubiquitous coenzyme NAD, comprising of two nucleotides: adenine and nicotinamide while FADH₂ refers to the reduced form of the coenzyme FAD in which riboflavin is the core component.

Production

While NADH is produced during both glycolysis and Krebs cycle, FADH₂ is produced during Krebs cycle.

Nucleotides

There are two joined nucleotides in NADH: adenine and nicotinamide while FADH₂ contains the two joined nucleotides adenine and flavine mononucleotide.

Electron Transfer

During oxidative phosphorylation, NADH transfers its electrons to the Cytochrome complex I while FADH₂ transfers its electrons to the Cytochrome complex II.

Conclusion

NADH is the reduced form of NAD, which produces 3 ATP molecules during oxidative phosphorylation while FADH₂ is the reduced form of FAD, which produces 2 ATP molecules during oxidative phosphorylation. Both NADH and FADH₂ are involved in other oxidation-reduction reactions that occur in the cell as well. The main difference between NADH and FADH₂ is the number of ATP molecules produced by oxidative phosphorylation.

Reference:

1. Berg, Jeremy M. “NAD , FAD, and Coenzyme A Are Formed from ATP.” Biochemistry. 5th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available Here

Image Courtesy:

1. “NAD+ phys” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia 
2. “Catabolism schematic” By Tim Vickers, vectorized by Fvasconcellos – w:Image:Catabolism.png (Public Domain) via Commons Wikimedia    
3. “2508 The Electron Transport Chain” By OpenStax College – Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia