What Coenzyme Does Riboflavin Make? A Detailed Guide

January 13, 2026 •

3 min read

Over 90% of dietary riboflavin is found in the form of its coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). Riboflavin, also known as Vitamin B2, serves as the critical precursor for these two powerful coenzymes that are essential for numerous metabolic processes throughout the body, particularly energy production.

Quick Summary

Riboflavin, or Vitamin B2, is converted into two primary coenzymes: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These crucial molecules serve as electron carriers, driving oxidation-reduction reactions vital for energy metabolism and cellular function within the body.

Key Points

FAD and FMN: Riboflavin is the precursor for two main coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN).
Two-step conversion: Riboflavin is first converted to FMN by flavokinase, and FMN is subsequently converted to FAD by FAD synthetase.
Electron carriers: Both FAD and FMN function as crucial electron carriers in redox reactions within numerous metabolic pathways.
Energy metabolism: These coenzymes are vital for energy production, particularly in the electron transport chain and the Krebs cycle.
Nutrient interactions: FAD and FMN are required for the metabolism of other nutrients, including Vitamin B6 and niacin.
Deficiency symptoms: Inadequate riboflavin (ariboflavinosis) can lead to skin disorders, sore throat, cheilosis, and fatigue due to impaired energy production.

The Biochemical Conversion of Riboflavin

To understand what coenzyme does riboflavin make, it's important to look at the process by which Vitamin B2 is transformed into its active coenzyme forms. This conversion involves two enzymatic steps, resulting in flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These transformations primarily occur within the cytoplasm of cells, particularly in organs like the liver, kidneys, and heart.

Step 1: Riboflavin to Flavin Mononucleotide (FMN)

This initial step is catalyzed by the enzyme flavokinase (riboflavin kinase). The reaction involves the phosphorylation of riboflavin, using ATP to add a phosphate group to the ribityl side chain. This process yields Flavin Mononucleotide (FMN), also referred to as riboflavin-5'-phosphate.

Step 2: FMN to Flavin Adenine Dinucleotide (FAD)

Following the creation of FMN, the enzyme FAD synthetase catalyzes the second step. Here, an adenylyl group from another ATP molecule is transferred to FMN, forming FAD and releasing pyrophosphate. FAD is the more prevalent of the two flavin coenzymes.

The Critical Role of FAD and FMN in Metabolism

FAD and FMN are vital flavocoenzymes that function as electron carriers in numerous oxidation-reduction (redox) reactions across various metabolic pathways. Their ability to accept and donate electrons is fundamental to energy production, especially during cellular respiration in the mitochondria.

Functions in Energy Production

Electron Transport Chain: FMN is a key element in Complex I (NADH dehydrogenase) of the mitochondrial electron transport chain. It receives electrons from NADH and transmits them to subsequent components, initiating ATP production through oxidative phosphorylation.
Krebs Cycle: FAD acts as a prosthetic group for succinate dehydrogenase within Complex II of the Krebs cycle. It accepts electrons from succinate, converting it to fumarate and becoming FADH2, which then delivers electrons to the electron transport chain to support ATP synthesis.
Fatty Acid Oxidation: FAD is necessary for acyl-CoA dehydrogenases, enzymes critical for breaking down fatty acids through beta-oxidation to generate energy.

Roles in Nutrient Metabolism

Beyond energy production, FAD and FMN are essential for the metabolism of other vital nutrients:

Vitamin B6: FMN is required for the conversion of vitamin B6 into its active coenzyme form, pyridoxal 5'-phosphate.
Niacin: FAD is needed to transform the amino acid tryptophan into niacin (Vitamin B3).
Folate Metabolism: The FAD-dependent enzyme methylenetetrahydrofolate reductase (MTHFR) plays a role in folate metabolism and managing homocysteine levels, which impacts cardiovascular health.

Comparison of FMN vs. FAD


Feature	Flavin Mononucleotide (FMN)	Flavin Adenine Dinucleotide (FAD)
Structure	Riboflavin plus one phosphate group.	Riboflavin, one phosphate group, and one adenine group.
Primary Role	Electron carrier, especially in the initial step of the electron transport chain (Complex I).	Electron carrier, notably involved in the Krebs cycle (Complex II) and fatty acid oxidation.
Redox States	Cycles between oxidized (FMN), semiquinone radical (FMNH•), and reduced (FMNH2) forms.	Cycles between oxidized (FAD), semiquinone radical (FADH•), and reduced (FADH2) forms.
Binding	Often non-covalently but tightly bound to its apoenzyme.	Can be either non-covalently or covalently bound to its apoenzyme.
Location	Present in various cellular compartments, including mitochondria.	Found in various tissues, and is the more abundant form of flavin in the body.

The Consequences of Riboflavin Deficiency

Given the vital functions of FAD and FMN, a lack of riboflavin, known as ariboflavinosis, can disrupt metabolic processes and cause health issues. Symptoms may include:

Skin problems like seborrheic dermatitis
Soreness and swelling in the throat and mouth
Cracking at the corners of the mouth (angular cheilitis) and swollen lips (cheilosis)
Hair loss and reproductive complications
Fatigue resulting from impaired energy metabolism
Anemia and cataracts in prolonged and severe cases

Obtaining Riboflavin from the Diet

To maintain sufficient levels of FAD and FMN, a consistent dietary intake of riboflavin is necessary as the body's storage is limited. Good sources of this vitamin include:

Dairy products (milk, yogurt, cheese)
Lean meats, especially organ meats
Eggs
Fortified cereals and breads
Green leafy vegetables such as spinach

Conclusion

In conclusion, riboflavin is the precursor to two vital coenzymes: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These flavocoenzymes are crucial for numerous biochemical reactions, acting primarily as electron carriers in cellular respiration and various metabolic pathways. The conversion of riboflavin into these active forms underscores its fundamental role in cellular function and energy production throughout the body. Understanding the specific functions of FAD and FMN helps explain the critical importance of riboflavin for human health and why its deficiency can lead to significant metabolic issues.

For additional details on metabolic pathways involving these coenzymes, the Linus Pauling Institute at Oregon State University offers comprehensive resources.

Frequently Asked Questions

The primary function of the coenzymes FAD and FMN is to act as electron carriers in metabolic processes, facilitating oxidation-reduction reactions necessary for energy production within the cell.

Riboflavin is converted into FMN through a phosphorylation reaction catalyzed by the enzyme flavokinase, which adds a phosphate group to the riboflavin molecule.

The conversion of riboflavin to its coenzyme forms, FMN and FAD, takes place within the cytoplasm of most cells, with high concentrations in the liver, heart, and kidneys.

Yes, FAD is involved in the Krebs cycle. It is a prosthetic group for the enzyme succinate dehydrogenase (Complex II), where it is reduced to FADH2 during the conversion of succinate to fumarate.

A riboflavin deficiency can lead to impaired energy metabolism, skin disorders, mouth and throat swelling, sores at the corners of the mouth, hair loss, and, in severe cases, anemia and cataracts.

Good dietary sources of riboflavin include dairy products (milk, cheese), eggs, lean meats (especially organ meats), fortified cereals and bread, and green leafy vegetables like spinach.

No, the human body cannot produce its own riboflavin and must obtain it from dietary sources. However, certain bacteria in the large intestine can produce some riboflavin that can be absorbed.