What Are the Enzymes in Vitamin B12 and Their Functions?

November 19, 2025 •

3 min read

Humans rely on only two distinct vitamin B12-dependent enzymes for critical metabolic functions, a stark contrast to the diverse array found in microorganisms. These two key enzymes, methionine synthase and methylmalonyl-CoA mutase, are essential for processes ranging from DNA synthesis to energy production and are the answer to what are the enzymes in vitamin B12.

Quick Summary

Vitamin B12 functions as a coenzyme for two critical enzymes in humans: methionine synthase and methylmalonyl-CoA mutase. These enzymes require the active forms, methylcobalamin and adenosylcobalamin, respectively, to facilitate DNA synthesis, energy production, and homocysteine metabolism.

Key Points

Two Human Enzymes: In humans, only two enzymes are known to require vitamin B12: methionine synthase and methylmalonyl-CoA mutase.
Active Coenzyme Forms: These enzymes function with the active forms of B12, methylcobalamin and adenosylcobalamin, which are synthesized from inactive dietary forms like cyanocobalamin.
Methionine Synthase Action: Methionine synthase, using methylcobalamin, recycles homocysteine into methionine, an essential amino acid and precursor to the universal methyl donor SAM.
Methylmalonyl-CoA Mutase Action: Methylmalonyl-CoA mutase, using adenosylcobalamin, converts methylmalonyl-CoA to succinyl-CoA, a vital step in energy metabolism.
Deficiency Consequences: A deficiency in vitamin B12 impairs both enzyme pathways, leading to elevated homocysteine and methylmalonic acid levels, which contribute to megaloblastic anemia and neurological damage.
Enzyme Location: Methionine synthase works in the cytoplasm, while methylmalonyl-CoA mutase performs its functions within the mitochondria.

The Two Key Human B12 Enzymes

In humans, all metabolic pathways that rely on vitamin B12 are powered by just two core enzymes. These enzymes depend on the two biologically active coenzyme forms of vitamin B12, known as methylcobalamin and adenosylcobalamin. Inactive forms of B12, such as cyanocobalamin found in supplements, must first be converted into one of these two active variants before they can participate in enzymatic reactions.

Methionine Synthase

Methionine synthase (MS), also known as 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), is a cytosolic enzyme that utilizes methylcobalamin as its cofactor. Its primary role is to catalyze the transfer of a methyl group to homocysteine, producing methionine and tetrahydrofolate. This reaction connects the folate and methionine cycles.

Regenerates Methionine: As the body doesn't synthesize methionine, MS regenerates it from homocysteine.
Enables DNA Synthesis: The reaction catalyzed by methionine synthase is crucial for regenerating tetrahydrofolate (THF), necessary for synthesizing DNA building blocks. B12 deficiency impairs this, leading to megaloblastic anemia.
Regulates Homocysteine Levels: Compromised MS activity causes homocysteine accumulation, a risk factor for health issues.
Facilitates Methylation: Methionine is converted to SAM, the body's methyl donor. MS ensures SAM supply for critical methylation reactions.

Methylmalonyl-CoA Mutase

Methylmalonyl-CoA mutase (MCM or MMUT) is a mitochondrial enzyme requiring adenosylcobalamin. It catalyzes the isomerization of L-methylmalonyl-CoA to succinyl-CoA, which enters the citric acid cycle for energy.

Fat and Amino Acid Metabolism: This reaction is vital for breaking down odd-chain fatty acids and certain amino acids.
Energy Production: By converting L-methylmalonyl-CoA into succinyl-CoA, MCM feeds intermediates into the citric acid cycle.
Diagnosis of Deficiency: MCM deficiency or its cofactor leads to methylmalonic acid (MMA) accumulation. Elevated MMA is a sensitive marker for B12 deficiency.
Neurological Health: Buildup of methylmalonyl-CoA and MMA can lead to abnormal fatty acids being incorporated into nerve myelin, potentially causing neurological symptoms in severe B12 deficiency.

A Comparison of Key B12 Enzymes


Feature	Methionine Synthase (MS)	Methylmalonyl-CoA Mutase (MCM)
Location	Cytoplasm of cells	Mitochondrial matrix
B12 Cofactor	Methylcobalamin (MeCbl)	Adenosylcobalamin (AdoCbl)
Core Reaction	Converts homocysteine to methionine	Converts L-methylmalonyl-CoA to succinyl-CoA
Key Pathway	Methionine and folate metabolism	Odd-chain fatty acid and amino acid catabolism
Metabolic Output	Methionine, precursor to SAM for methylation	Succinyl-CoA, an intermediate for energy production
Marker of Deficiency	Elevated homocysteine	Elevated methylmalonic acid (MMA)

The Role of Active B12 Forms

Vitamin B12's function as a coenzyme depends on its specific form. Cyanocobalamin in supplements must be converted to active methylcobalamin or adenosylcobalamin. This conversion involves various transport proteins and chaperones. Impaired conversion due to malabsorption or genetic defects can cause deficiency in active coenzymes despite sufficient intake.

Interplay and Health Consequences

The two B12-dependent enzymes' actions are interconnected. MS produces SAM for methylation, impacting DNA synthesis and nerve health. MCM supports energy production through fat and amino acid metabolism. B12 deficiency impairs both pathways, leading to megaloblastic anemia and neurological damage from abnormal fatty acid production and high homocysteine.

Conclusion: The Pivotal Role of B12-Dependent Enzymes

The two main enzymes relying on vitamin B12—methionine synthase and methylmalonyl-CoA mutase—are crucial for human metabolism, mediating DNA synthesis, methylation, and energy production. Their need for active methylcobalamin and adenosylcobalamin highlights the complex processes of B12 utilization. Dysfunction in these pathways, often from B12 deficiency, can cause severe health issues like anemia and neurological disorders. Understanding these enzymes and their cofactors is key to appreciating B12's impact and diagnosing deficiency.

Visit the NIH website for more detailed information on vitamin B12's role in health.

Frequently Asked Questions

If vitamin B12-dependent enzymes malfunction, it leads to a buildup of their respective substrates. Specifically, dysfunctional methionine synthase results in high homocysteine levels, while issues with methylmalonyl-CoA mutase cause a rise in methylmalonic acid. Both can lead to severe health problems.

Methylcobalamin and adenosylcobalamin are the two active coenzyme forms of vitamin B12 in humans. Methylcobalamin functions as a cofactor for methionine synthase in the cytoplasm, while adenosylcobalamin serves as a cofactor for methylmalonyl-CoA mutase in the mitochondria.

Vitamin B12 deficiency leads to impaired methionine synthase activity, which disrupts the folate cycle and hinders DNA synthesis. This results in the production of abnormally large, immature red blood cells (megaloblasts), causing megaloblastic anemia.

B12 is crucial for nerve health because the methylmalonyl-CoA mutase enzyme is involved in fatty acid metabolism, which impacts the synthesis and maintenance of the myelin sheath that insulates nerve cells. Deficiency can lead to nerve damage and neurological symptoms.

Yes, high-dose oral vitamin B12 supplements can be effective even in some cases of malabsorption because a small percentage of the vitamin can be absorbed via passive diffusion, bypassing the intrinsic factor pathway. However, treatment should always be guided by a doctor.

Yes, methylmalonyl-CoA mutase is found in both mammals and bacteria, while methionine synthase is also present in other animals. However, the exact number and function of B12-dependent enzymes can vary significantly between species.

Methionine synthase is a cytosolic enzyme, meaning it is located in the cytoplasm of the cell.