5-Methyltetrahydropteroyltriglutamate—homocysteine S-methyltransferase

In enzymology, a 5-methyltetrahydropteroyltriglutamate—homocysteine S-methyltransferase is an enzyme that catalyzes the chemical reaction


 * 5-methyltetrahydropteroyltri-L-glutamate + L-homocysteine $$\rightleftharpoons$$ tetrahydropteroyltri-L-glutamate + L-methionine

Thus, the two substrates of this enzyme are 5-methyltetrahydropteroyltri-L-glutamate and L-homocysteine, whereas its two products are tetrahydropteroyltri-L-glutamate and L-methionine. This enzyme participates in methionine metabolism. It has 2 cofactors: orthophosphate, and zinc.

Nomenclature
This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is 5-methyltetrahydropteroyltri-L-glutamate:L-homocysteine S-methyltransferase. Other names in common use include tetrahydropteroyltriglutamate methyltransferase, homocysteine methylase, methyltransferase, tetrahydropteroylglutamate-homocysteine transmethylase, methyltetrahydropteroylpolyglutamate:homocysteine methyltransferase, cobalamin-independent methionine synthase, methionine synthase (cobalamin-independent), and MetE.

Structure
The enzyme from Escherichia coli consists of two alpha8-beta8 (TIM) barrels positioned face to face and thought to have evolved by gene duplication. The active site lies between the tops of the two barrels, the N-terminal barrel binds 5-methyltetrahydropteroyltri-L-glutamic acid and the C-terminal barrel binds homocysteine. Homocysteine is coordinated to a zinc ion, as initially suggested by spectroscopy and mutagenesis.