Methionine synthase also known as MS, MeSe, MetH is responsible for the regeneration of methionine from homocysteine. In humans it is encoded by the MTRgene (5-methyltetrahydrofolate-homocysteine methyltransferase). Methionine synthase forms part of the S-adenosylmethionine (SAMe) biosynthesis and regeneration cycle. Both cobalamin-dependent and cobalamin-independent forms of methionine synthase, which do not rely on an intermediate methyl carrier, exist in nature with animals expressing the former, plants the latter, and microorganisms expressing both. 
The reaction reaction catalyzed by methionine synthase (click to enlarge)
Methionine synthase catalyzes the final step in the regeneration of methionine(Met) from homocysteine(Hcy). The overall reaction transforms 5-methyltetrahydrofolate(N5-MeTHF) into tetrahydrofolate (THF) while transferring a methyl group to Hcy to form Met. Methionine synthase is the only mammalian enzyme that metabolizes N5-MeTHF to regenerate the active cofactor THF. In cobalamin-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 N5-MeTHF to Co(I) in enzyme-bound cobalamin(Cob), 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. The cob-independent mechanism follows the same general pathway but with a direct reaction between the zinc thiolate and N5-MeTHF.
Scavenger Pathway of Methionine Synthase Reductase to Recover Inactivated Methionine Synthase
The mechanism of the enzyme depends on the constant regeneration of Co(I) in cob, but this is not always guaranteed. Instead, every 1-2000 catalytic turnovers, the Co(I) may be oxidized into Co(II), which would permanently shut down catalytic activity. A separate protein, Methionine Synthase Reductase, catalyzes the regeneration of Co(II) and the restoration of enzymatic activity. Because this process inevitably shuts down all methionine synthase activity, defects or deficiencies in methionine synthase reductase have been implicated in some of the disease associations for methionine synthase deficiency discussed below. The two enzymes form a scavenger network seen on the lower left.
Homocysteine Binding Domain in Methionine Synthase. His 618, Cys 620, and Cys704 bind Zn(purple) which binds to Homocysteine(Red)
Crystal structures for both cob-independent and cob-dependent MetH have been solved, with little similarity in the overall structure despite the identical net reaction being performed by each and similarities within binding sites such as Hcy binding site. Cob-dependent MetH is divided into 4 separate domains: Activation, Cobalamin-binding(Cob domain), Homocysteine binding(Hyc domain), and N 5-methylTHF binding(MeTHF domain). The activation domain is the site of interaction with Methionine Synthase Reductase and binds SAM that is used as part of the re-activation cycle of the enzyme. The Cob domain contains Cob sandwiched between several large alpha helices and bound to the enzyme so that the cobalt atom of the group is exposed for contact with other domains. The Hcy domain contains the critical zinc-binding site, made up of cysteine or histidine residues coordinated to a zinc ion that can bind Hcy, with an example from a non-Cob dependent MetH shown on the right. The N5-MeTHF binding domain contains a conserved barrel in which N5-MeTHF can hydrogen bond with asparagine, arginine, and aspartic acid residues. The entire structure undergoes a dramatic swinging motion during catalysis as the Cob domain moves back and forth from the Hcy domain to the Fol domain, transferring the active methyl group from the Fol to Hcy domain. 
Methionine synthase's main purpose is to regenerate Met in the S-Adenosyl Methionine cycle, which in a single turnover consumes Met and ATP and generates Hcy. This cycle is critical because S-adenosyl methionine is used extensively in biology as a source of an active methyl group, and so methionine synthase serves an essential function by allowing the SAM cycle to perpetuate without a constant influx of Met. In this way, methionine synthase also serves to maintain low levels of Hcy and, because methionine synthase is one of the few enzymes that used N5-MeTHF as a substrate, to indirectly maintain THF levels.
In plants and microorganisms, methionine synthase serves a dual purpose of both perpetuating the SAM cycle and catalyzing the final synthetic step in the de novo synthesis of Met. While the reaction is exactly the same for both processes, the overall function is distinct from methionine synthase in humans because Met is an essential amino acid that is not synthesized de novo in the body.
Mutations in the MTR gene have been identified as the underlying cause of methylcobalamin deficiency complementation group G, or methylcobalamin deficiency cblG-type. Deficiency or deregulation of the enzyme due to deficient methionine synthase reductase can directly result in elevated levels of homocysteine, which is associated with blindness, neurological symptoms, and birth defects. Most cases of methionine synthase deficiency are symptomatic within 2 years of birth with many patients rapidly developing severe encephalopathy. The consequence of reduced methionine synthase activity is megaloblastic anemia.
^ abZhang, Z.; Tian, C.; Zhou, S.; Wang, W.; Guo, Y.; Xia, J.; Liu, Z.; Wang, B.; Wang, X.; Golding, B. T.; Griff, R. J.; Du, Y.; Liu, J. (2012). "Mechanism-based design, synthesis and biological studies of N5-substituted tetrahydrofolate analogs as inhibitors of cobalamin-dependent methionine synthase and potential anticancer agents". European Journal of Medicinal Chemistry58: 228–236. doi:10.1016/j.ejmech.2012.09.027. PMID23124219.edit
^Matthews, R. G.; Smith, A. E.; Zhou, Z. S.; Taurog, R. E.; Bandarian, V.; Evans, J. C.; Ludwig, M. (2003). "Cobalamin-Dependent and Cobalamin-Independent Methionine Synthases: Are There Two Solutions to the Same Chemical Problem?". Helvetica Chimica Acta86 (12): 3939. doi:10.1002/hlca.200390329.edit
^Wolthers, K. R.; Scrutton, N. S. (2007). "Protein Interactions in the Human Methionine Synthase−Methionine Synthase Reductase Complex and Implications for the Mechanism of Enzyme Reactivation†". Biochemistry46 (23): 6696–6709. doi:10.1021/bi700339v. PMID17477549.edit
Matthews RG, Sheppard C, Goulding C (1998). "Methylenetetrahydrofolate reductase and methionine synthase: biochemistry and molecular biology.". Eur. J. Pediatr. 157 Suppl 2: S54–9. doi:10.1007/PL00014305. PMID9587027.
Garovic-Kocic V, Rosenblatt DS (1992). "Methionine auxotrophy in inborn errors of cobalamin metabolism.". Clinical and investigative medicine. Médecine clinique et experimentale15 (4): 395–400. PMID1516297.
O'Connor DL, Moriarty P, Picciano MF (1992). "The impact of iron deficiency on the flux of folates within the mammary gland.". International journal for vitamin and nutrition research. Internationale Zeitschrift für Vitamin- und Ernährungsforschung. Journal international de vitaminologie et de nutrition62 (2): 173–80. PMID1517041.
Everman BW, Koblin DD (1992). "Aging, chronic administration of ethanol, and acute exposure to nitrous oxide: effects on vitamin B12 and folate status in rats.". Mech. Ageing Dev.62 (3): 229–43. doi:10.1016/0047-6374(92)90109-Q. PMID1583909.
Vassiliadis A, Rosenblatt DS, Cooper BA, Bergeron JJ (1991). "Lysosomal cobalamin accumulation in fibroblasts from a patient with an inborn error of cobalamin metabolism (cblF complementation group): visualization by electron microscope radioautography.". Exp. Cell Res.195 (2): 295–302. doi:10.1016/0014-4827(91)90376-6. PMID2070814.
Li YN, Gulati S, Baker PJ, et al. (1997). "Cloning, mapping and RNA analysis of the human methionine synthase gene.". Hum. Mol. Genet.5 (12): 1851–8. doi:10.1093/hmg/5.12.1851. PMID8968735.
Leclerc D, Campeau E, Goyette P, et al. (1997). "Human methionine synthase: cDNA cloning and identification of mutations in patients of the cblG complementation group of folate/cobalamin disorders.". Hum. Mol. Genet.5 (12): 1867–74. doi:10.1093/hmg/5.12.1867. PMID8968737.
Chen LH, Liu ML, Hwang HY, et al. (1997). "Human methionine synthase. cDNA cloning, gene localization, and expression.". J. Biol. Chem.272 (6): 3628–34. doi:10.1074/jbc.272.6.3628. PMID9013615.
Salomon O, Rosenberg N, Zivelin A, et al. (2002). "Methionine synthase A2756G and methylenetetrahydrofolate reductase A1298C polymorphisms are not risk factors for idiopathic venous thromboembolism.". Hematol. J.2 (1): 38–41. doi:10.1038/sj/thj/6200078. PMID11920232.
De Marco P, Calevo MG, Moroni A, et al. (2002). "Study of MTHFR and MS polymorphisms as risk factors for NTD in the Italian population.". J. Hum. Genet.47 (6): 319–24. doi:10.1007/s100380200043. PMID12111380.