5-Methyltetrahydrofolate-homocysteine methyltransferase or (MTR) is an enzyme responsible for the production of methionine from homocysteine. MTR forms part of the S-adenosyl methionine cycle and is also called methionine synthase.

Function

The enzyme works in two steps in a ping-pong reaction. First, methylcobalamin is formed by a methyl group transfer from N⁵-mTHF with formation of MeB₁₂ and tetrahydrofolate (THF). In the second step, MeB₁₂ transfers this methyl group to (homocysteine), regenerating the cofactor cobalamin and releasing the product methionine

MTR is the only mammalian enzyme that metabolizes 5-methylTHF to regenerate the active cofactor, tetrahydrofolate. Deficiency in MTR function may be due to genetic mutations, reduced levels of its cobalamin cofactor (vitamin B₁₂), or decreased levels of the enzyme methionine synthase reductase (required for the sustained activity of MTR).

The consequence of reduced MTR activity is megaloblastic anemia.

Genetics

Several polymorphisms in MTR have been identified. 2756A->G (Asp⁹¹⁹Gly)

See also

• methyltransferase
• Arakawa's syndrome II
• 5-methyltetrahydrofolate

References

Further reading

External links

• ENZYME: EC 2.1.1.13