Enzyme

2.5.1.6 - Methionine adenosyltransferase

Alternative Name(s)
  • AdoMet synthetase.
  • S-adenosylmethionine synthetase.

Catalytic Activity

ATP + H2O + L-methionine = diphosphate + phosphate + S-adenosyl-L-methionine

Cofactors

There are no Cofactors for this Enzyme

Reaction Mechanism

    methionine adenosyltransferase By Reference

    In biological systems, methyl groups are transferred from a small number of donors to a large number of acceptors. S-adenosylmethionine (AdoMet) is the most widespread of these donors, and is synthesised solely by the action of AdoMet synthase.

    The catalytic site of this enzyme, found in a cleft between two subunits, conducts an unusual reaction pathway where a triphosphate chain is cleaved from ATP as AdoMet is formed and the triphosphate is hydrolysed to diphosphate and phosphate before the product is released. There are three similar domains arranged around a pseudo-threefold symmetry axis.

    The reaction involves the cleavage of the triphosphate chain from ATP, in forming the product, and hydrolysis of the PPPi moiety to PPi and Pi before the AdoMet product is released.

    Mechanistic studies have shown the AdoMet forming reaction to follow an SN2 mechanism, with the S atom of methionine attacking the C5 atom of ATP directly. His29 acts as a general acid, activated by the surrounding basic backbone amide groups, towards the O5' as the C5'-O5' bond cleaves. Simultaneously, the methionine sulphur attacks the developing cation. This reaction is followed by the hydrolysis of triphosphate to phosphate and pyrophosphate, providing energy for the removal of the reaction product from the active site.

    The reaction requires divalent metal cations for activity, two binding sites have been identified both by structural information and EPR studies.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Glu P31153 57 5a1i 57
    Lys P31153 32 5a1i 32
    His P31153 29 5a1i 29
    Asp P31153 31 5a1i 31
    Lys P31153 181 5a1i 181
    Phe P31153 250 5a1i 250
    Asp P31153 258 5a1i 258
    Ala P31153 259 5a1i 259
    Arg P31153 264 5a1i 264
    Lys P31153 265 5a1i 265
    Lys P31153 285 5a1i 285
    Lys P31153 289 5a1i 289
    Glu P31153 70 5a1i 70
    Asp P31153 291 5a1i 291
    Step Components

    intermediate formation, intermediate terminated, native state of enzyme regenerated, dephosphorylation, overall reactant used, hydrolysis, bimolecular nucleophilic substitution, rate-determining step, intermediate collapse, proton transfer, overall product formed

    Step 1.

    The C5'-O5' bond of ATP dissociates, the phosphate of the leaving group deprotonates His29, the anionic histidine is stabilised by the main chain amides of Lys32 and Asp31. A simultaneous change in the ribose ring conformation from C4'-exo to C3'-endo occurs, and the SD of methionine makes a nucleophilic attack on the C5' to form S-adenosylmethionine. His29 deprotonates the leaving group in an overall nucleophilic substitution reaction.

    Step 2.

    Triphosphate hydrolyses to form diphosphate and monophosphate.

    Products.

    The products of the reaction.

Reaction Parameters

  • Kinetic Parameters
    Organism KM Value [mM] Substrate Comment
    Homo sapiens 0.076 S-(-)-methioninol pH 8.0, 37°C, wild-type enzyme
    Saccharolobus solfataricus 0.43 ATP pH 8.0, 37°C, cosubstrate: L-ethionine
    Plasmodium falciparum 1.5 ATP pH 8.4, 37°C, mutant enzyme
    Pyrococcus furiosus 65.2 ATP pH 8.0, 80°C
  • Temperature
    Organism Temperature Range Comment
    Streptomyces avermitilis 20 - 50
    Lactiplantibacillus paraplantarum 20 - 50 more than 50% activity between 20 and 50°C. The activity is low below 20°C and above 55°C with less than 20% relative activity
    Escherichia coli 20 - 60 about 20% activity at 20°C, about 30% activity at 30°C, about 80% activity at 35°C, about 90% activity at 40°C, 100% at 45°C, about 850% activity at 50°C, about 45% activity at 60°C, and no activity at 70°C
    Bacillus subtilis 25 - 55
    Thermus thermophilus 30 - 90 the enzyme shows activity at a broad temperature range, from 30 to 90°C. The reaction rate increases significantly with temperature, and reaches maximum at 80°C
  • pH
    Organism pH Range Comment
    Streptomyces avermitilis 5 - 10
    Corynebacterium glutamicum 6 - 11 the activity is significantly reduced at pH values below 6.0 or above 11.0
    Bacillus subtilis 6 - 10
    Pyrococcus furiosus 7 - 9 half of the maximal activity at pH 7 and 9
    Saccharomyces cerevisiae 7 - 10 more than 60% of maximum activity within

Associated Proteins

Protein name Organism
Probable S-adenosylmethionine synthase 5 Caenorhabditis elegans
S-adenosylmethionine synthase 2 Mouse-ear cress
S-adenosylmethionine synthase isoform type-1 Human
Probable S-adenosylmethionine synthase 1 Caenorhabditis elegans
S-adenosylmethionine synthase Escherichia coli (strain K12)

Citations