Editing Methylation (section)

====Methionine synthase====
[[File:VitaminB12 2.png|thumb|right|The methylation reaction catalyzed by [[methionine synthase]]]]

[[Methionine synthase]] regenerates [[methionine]] (Met) from [[homocysteine]] (Hcy). The overall reaction transforms [[5-methyltetrahydrofolate]] (N<sup>5</sup>-MeTHF) into [[tetrahydrofolate]] (THF) while transferring a methyl group to Hcy to form Met. Methionine Syntheses can be cobalamin-dependent and cobalamin-independent: Plants have both, animals depend on the methylcobalamin-dependent form.

In methylcobalamin-dependent forms of the enzyme, the reaction proceeds by two steps in a ping-pong reaction. The enzyme is initially primed into a reactive state by the transfer of a methyl group from N<sup>5</sup>-MeTHF to Co(I) in enzyme-bound [[cobalamin]] ((Cob), also known as vitamine B12)) ,
, forming methyl-cobalamin(Me-Cob) that now contains Me-Co(III) and activating the enzyme. Then, a Hcy that has coordinated to an enzyme-bound [[zinc]] to form a reactive thiolate reacts with the Me-Cob. The activated methyl group is transferred from Me-Cob to the Hcy thiolate, which regenerates Co(I) in Cob, and Met is released from the enzyme.<ref name = Zinc>{{Cite journal | last1 = Matthews | first1 = R. G. | last2 = Smith | first2 = A. E. | last3 = Zhou | first3 = Z. S. | last4 = Taurog | first4 = R. E. | last5 = Bandarian | first5 = V. | last6 = Evans | first6 = J. C. | last7 = Ludwig | first7 = M. | doi = 10.1002/hlca.200390329 | title = Cobalamin-Dependent and Cobalamin-Independent Methionine Synthases: Are There Two Solutions to the Same Chemical Problem? | journal = Helvetica Chimica Acta | volume = 86 | issue = 12 | pages = 3939–3954 | year = 2003 }}</ref>