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FADH2 – Function in Energy Metabolism

FADH2 is the reduced form of the coenzyme FAD and plays a central role in cellular energy metabolism, particularly in the electron transport chain and the citric acid cycle.

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Things worth knowing about "FADH2"

FADH2 is the reduced form of the coenzyme FAD and plays a central role in cellular energy metabolism, particularly in the electron transport chain and the citric acid cycle.

What is FADH2?

FADH2 stands for reduced flavin adenine dinucleotide and is a key high-energy molecule in human metabolism. It is the reduced form of the coenzyme FAD (flavin adenine dinucleotide), which is derived from riboflavin (vitamin B2). FADH2 is formed when FAD accepts two hydrogen atoms (specifically two electrons and two protons) and becomes reduced in the process.

As an important electron carrier, FADH2 transports electrons to the mitochondrial electron transport chain, where they are used to produce ATP (adenosine triphosphate) – the universal energy currency of the cell.

Formation of FADH2

FADH2 is produced mainly in two central metabolic pathways:

Citric acid cycle (Krebs cycle): In the eighth step of the citric acid cycle, the enzyme succinate dehydrogenase oxidizes succinate to fumarate. In this process, FAD is reduced to FADH2.
Beta-oxidation of fatty acids: During the stepwise breakdown of fatty acids in the mitochondria, one molecule of FADH2 is generated per degradation cycle.

Function and Mechanism of Action

FADH2 plays a decisive transport function in energy metabolism. It donates its electrons directly to Complex II (succinate dehydrogenase complex) of the electron transport chain in the inner mitochondrial membrane. This process proceeds as follows:

FADH2 is oxidized at Complex II and transfers two electrons to ubiquinone (coenzyme Q).
The electrons travel further through Complex III and Complex IV of the electron transport chain.
At the end of the chain, the electrons react with oxygen and protons to form water.
The resulting proton gradient drives ATP synthase, which phosphorylates ADP to ATP.

Compared to NADH, which donates its electrons to Complex I and generates approximately 2.5 ATP equivalents, FADH2 enters at Complex II and yields only about 1.5 ATP equivalents per molecule.

Significance for Overall Metabolism

FADH2 is an indispensable component of aerobic (oxygen-dependent) cellular metabolism. Without FADH2, cells could not efficiently harvest energy from nutrients. FADH2 plays a particularly important role in tissues with high energy demands, such as the heart muscle, skeletal muscle, and liver.

Since FAD is synthesized from riboflavin (vitamin B2), an adequate dietary intake of vitamin B2 is essential for maintaining this process. A deficiency in riboflavin can impair the formation of FAD and thus FADH2, negatively affecting energy metabolism.

FADH2 Compared to NADH

Both FADH2 and NADH (reduced nicotinamide adenine dinucleotide) are electron carriers in the electron transport chain, but they differ in important ways:

Entry point: NADH donates electrons to Complex I, FADH2 to Complex II.
ATP yield: NADH produces approximately 2.5 ATP, FADH2 approximately 1.5 ATP per molecule.
Origin: NADH is generated in the citric acid cycle and glycolysis, while FADH2 is produced mainly in the citric acid cycle and beta-oxidation.
Cofactor basis: NADH is based on niacin (vitamin B3), FADH2 on riboflavin (vitamin B2).

Clinical Relevance

Disorders of the FAD/FADH2 system can occur in the context of rare mitochondrial diseases or in cases of severe riboflavin deficiency. Defects in succinate dehydrogenase (Complex II), which directly processes FADH2, are associated with certain tumors (e.g., paragangliomas and pheochromocytomas). In clinical biochemistry, an understanding of FADH2 provides a foundation for the diagnosis and treatment of metabolic disorders.

References

Berg, J.M., Tymoczko, J.L., Stryer, L. (2015). Biochemistry. 8th Edition. W.H. Freeman and Company, New York.
World Health Organization (WHO) (2004). Vitamin and Mineral Requirements in Human Nutrition. 2nd Edition. WHO Press, Geneva.
Nicholls, D.G., Ferguson, S.J. (2013). Bioenergetics 4. Academic Press, London.

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