Methionine synthase

Methionine synthase (MS, MeSe, MTR) is responsible for the regeneration of

Methionine synthase catalyzes the final step in the regeneration of

Under physiological conditions, approximately once every 2000 catalytic turnovers the Co(I) may be oxidized into inactive Co(II) in cob-dependent MetH. To account for this effect, the protein contains a self-reactivation mechanism, a reductive methylation process that uses S-adenosylmethionine as a distinct methyl donor. In humans, the enzyme is reduced in this process by

The mechanism of the cobalamin-independent (MetE) form, by contrast, proceeds through a direct methyl transfer from the activated N⁵-MeTHF to zinc thiolate homocysteine. Although the mechanism is considerably simpler, the direct transfer reaction is much less favorable than the cobalamin-mediated reactions and as a result the turnover rate for MetE is ~100x slower than that of MetH. As it does not contain the cobalamin cofactor, the cobalamin-independent enzyme is not prone to oxidative inactivation [21]^[8][22][23]

Structure

High-resolution structures have been solved by X-ray crystallography for intact MetE both in the absence and presence of substrates^[23][22] and for fragments of MetH,^{[24][25][26][27]} although no structural description exists of a fully intact MetH enzyme. The available structures and accompanying bioinformatic analysis indicate minimal similarity in the overall structure, although there are similarities within the substrate-binding sites themselves.^[28] Cob-dependent MetH is divided into 4 separate domains. The domains, from N- to C-terminus, are denoted homocysteine binding (Hcy domain), N⁵-methylTHF binding (MTHF domain) Cobalamin-binding (Cob domain) and the S-adenosymethionine-binding or reactivation domain. The reactivation domain binds SAM and is the site of interaction with flavodoxin or Methionine Synthase Reductase during the reactivation cycle of the enzyme.^[17][16][20] The cobalamin-binding domain contains two subdomains, with the cofactor bound to the Rossman-fold B₁₂-binding subdomain, which is in turn capped by the other subdomain, the four-helix bundle cap subdomain.^[25] The four-helix bundle serves to protect the cobalamin cofactor from unwanted reactivity, but can significantly change conformations to expose the cofactor allow it access to the other substrates during turnover.^[26] Both the Hcy and N⁵-MeTHF domains adopt a TIM barrel architecture; the Hcy domain contains the zinc-binding site, which in MetH consists of three cysteine residues coordinated to a zinc ion which in turn binds and activates Hcy. The N⁵-MeTHF binding domain binds and activates N⁵-MeTHF via a hydrogen bonding network with several asparagine, arginine, and aspartic acid residues. During turnover, the enzyme undergoes significant conformational changes that involve moving the Cob-domain back and forth from the Hcy domain to the N⁵-MeTHF domain in order for the two methyl transfer reactions to proceed.^[24]

The cob-independent MetE consists of two TIM-barrel domains that bind homocysteine and N⁵-MeTHF individually. The two domains adopt a face-to-face double barrel architecture, which requires a "closing" of the structure upon binding of both substrates to enable the direct methyl transfer.^[22] Substrate-binding strategies are similar to MetH, although in the case of MetE the zinc atom is instead coordinated to two cysteines, a histidine and a glutamate,^[23] for which an example is shown on the right.

Biochemical function

In humans the enzyme's main purpose is to regenerate Met in the S-adenosylmethionine (SAM) cycle. The SAM cycle in a single turnover consumes Met and ATP and generates Hcy, and can involve any of a number of critical enzymatic reactions that use S-adenosylmethionine as the source of an active methyl group for methylation of nucleic acids, histones, phospholipids and various proteins.^[29][30] As such, methionine synthase serves an essential function by allowing the SAM cycle to perpetuate without a constant influx of Met. As a secondary effect, 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.^[31][32]

In bacteria and plants, 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, which is one of the 20 canonical amino acids.^[33][11] While the chemical 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.^[34]

Clinical significance

Mutations in the MTR gene have been identified as the underlying cause of methylcobalamin deficiency complementation group G, or methylcobalamin deficiency cblG-type.^[5] Deficiency or deregulation of the enzyme due to deficient methionine synthase reductase can directly result in elevated levels of homocysteine (hyperhomocysteinemia), which is associated with blindness, neurological symptoms, and birth defects.^[35][36] Methionine synthase reductase (MTRR) or methylene-tetrahydrofolate reductase (MTHFR) deficiencies can also result in the condition. Most cases of methionine synthase deficiency are symptomatic within 2 years of birth with many patients rapidly developing severe encephalopathy.^[37] One consequence of reduced methionine synthase activity that is measurable by routine clinical blood tests is megaloblastic anemia.

Genetics

Several cblG-associated polymorphisms in the MTR gene have been identified.^[38]

• 2756D→G (Asp⁹¹⁹Gly)
• 3804C→T (Pro¹¹³⁷Leu)
• Δ2926A-2928T (ΔIle⁸⁸¹)

See also

• Methyltransferase
• Arakawa's syndrome II
• Betaine—homocysteine S-methyltransferase

References