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FAD (Flavin Adenine Dinucleotide) – Function & Role

FAD (flavin adenine dinucleotide) is an essential coenzyme in human metabolism, derived from vitamin B2 (riboflavin), and plays a key role in cellular energy production.

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Things worth knowing about "FAD"

FAD (flavin adenine dinucleotide) is an essential coenzyme in human metabolism, derived from vitamin B2 (riboflavin), and plays a key role in cellular energy production.

What is FAD?

FAD stands for flavin adenine dinucleotide and is a biologically active coenzyme derived from the vitamin riboflavin (vitamin B2). It belongs to the group of flavin nucleotides and plays a central role in the cellular energy metabolism of the human body. Coenzymes such as FAD act as helper molecules for enzymes and are indispensable for numerous biochemical reactions.

Mechanism of Action

FAD functions as an electron carrier (redox coenzyme) in enzymatic reactions. It can accept and donate electrons and hydrogen atoms, cycling between its oxidized form (FAD) and its reduced form (FADH₂). This redox cycling is critical for cellular energy production.

Oxidized form: FAD accepts two electrons and two protons to become FADH₂.
Reduced form: FADH₂ donates these electrons to the respiratory chain, regenerating FAD.

Biological Functions

FAD is involved in a wide range of vital metabolic pathways:

Energy Metabolism

In the citric acid cycle (Krebs cycle), FAD serves as a coenzyme for the enzyme succinate dehydrogenase. It accepts electrons from succinate and transfers them into the mitochondrial electron transport chain, where ATP – the universal energy currency of the cell – is produced. Each molecule of FADH₂ yields approximately 1.5 molecules of ATP.

Fatty Acid Oxidation (β-Oxidation)

During the β-oxidation of fatty acids, FAD also acts as a coenzyme, accepting electrons released during fatty acid breakdown and feeding them into the respiratory chain.

Amino Acid and Protein Metabolism

FAD is a component of various enzymes involved in amino acid catabolism, including monoamine oxidase (MAO), which degrades important neurotransmitters such as serotonin and dopamine.

Antioxidant Defense

The enzyme glutathione reductase, which plays a key role in protecting cells from oxidative stress, requires FAD as a coenzyme. This means FAD indirectly contributes to the antioxidant defense system of the body.

Synthesis and Dietary Sources

FAD is synthesized in the body from riboflavin (vitamin B2). An adequate dietary intake of vitamin B2 is therefore essential for sufficient FAD availability. Riboflavin is found in the following foods:

Milk and dairy products (e.g., cheese, yogurt)
Meat and organ meats (especially liver)
Eggs
Fish
Whole grain products
Legumes and green vegetables

Clinical Relevance and Deficiency

Since FAD availability depends directly on riboflavin intake, a vitamin B2 deficiency also leads to reduced FAD levels. This can cause the following symptoms:

Fatigue and reduced physical performance
Inflammation of the oral mucosa and angular cheilitis (cracked corners of the mouth)
Light sensitivity of the eyes (photophobia)
Impaired energy metabolism
Skin changes (seborrheic dermatitis)

At-risk groups for FAD and riboflavin deficiency include elderly individuals, vegans, pregnant women, and people with chronic gastrointestinal disorders.

FAD in Medicine and Research

FAD and FADH₂ are important parameters in biochemical research and are used as biomarkers for mitochondrial metabolism. In clinical diagnostics, measuring riboflavin levels and the activity of FAD-dependent enzymes can provide valuable information about metabolic disorders. Additionally, FAD is used in photodynamic therapy and in diagnostic techniques such as fluorescence spectroscopy, owing to its natural fluorescent properties.

References

Berg, J. M., Tymoczko, J. L., Stryer, L. - Biochemistry. 8th edition. W. H. Freeman and Company, 2015.
Mosegaard, S. et al. - Riboflavin deficiency – implications for general human health and inborn errors of metabolism. International Journal of Molecular Sciences, 2020. DOI: 10.3390/ijms21093847
World Health Organization (WHO) - Riboflavin. In: Vitamin and Mineral Requirements in Human Nutrition. 2nd edition. WHO Press, 2004.

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