1RG9: S-Adenosylmethionine synthetase complexed with SAM and PPNP

S-Adenosylmethionine synthetase (MAT) catalyzes formation of S-adenosylmethionine (SAM) from ATP and l-methionine (Met) and hydrolysis of tripolyphosphate to PP(i) and P(i). Escherichia coli MAT (eMAT) has been crystallized with the ATP analogue AMPPNP and Met, and the crystal structure has been determined at 2.5 A resolution. eMAT is a dimer of dimers and has a 222 symmetry. Each active site contains the products SAM and PPNP. A modeling study indicates that the substrates (AMPPNP and Met) can bind at the same sites as the products, and only a small conformation change of the ribose ring is needed for conversion of the substrates to the products. On the basis of the ternary complex structure and a modeling study, a novel catalytic mechanism of SAM formation is proposed. In the mechanism, neutral His14 acts as an acid to cleave the C5'-O5' bond of ATP while simultaneously a change in the ribose ring conformation from C4'-exo to C3'-endo occurs, and the S of Met makes a nucleophilic attack on the C5' to form SAM. All essential amino acid residues for substrate binding found in eMAT are conserved in the rat liver enzyme, indicating that the bacterial and mammalian enzymes have the same catalytic mechanism. However, a catalytic mechanism proposed recently by Gonzalez et al. based on the structures of three ternary complexes of rat liver MAT [Gonzalez, B., Pajares, M. A., Hermoso, J. A., Guillerm, D., Guillerm, G., and Sanz-Aparicio. J. (2003) J. Mol. Biol. 331, 407] is substantially different from our mechanism.
PDB ID: 1RG9Download
MMDB ID: 26545
PDB Deposition Date: 2003/11/11
Updated in MMDB: 2017/11
Experimental Method:
x-ray diffraction
Resolution: 2.5  Å
Source Organism:
Similar Structures:
Biological Unit for 1RG9: tetrameric; determined by author and by software (PISA)
Molecular Components in 1RG9
Label Count Molecule
Proteins (4 molecules)
S-adenosylmethionine Synthetase(Gene symbol: metK)
Molecule annotation
Chemicals (20 molecules)
* Click molecule labels to explore molecular sequence information.

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