2 research outputs found
The Structure of an As(III) <i>S</i>‑Adenosylmethionine Methyltransferase with 3‑Coordinately Bound As(III) Depicts the First Step in Catalysis
Arsenic
is a ubiquitous environmental toxic substance and a Class 1 human carcinogen. Arsenic methylation
by the enzyme AsÂ(III) <i>S</i>-adenosylmethionine (SAM)
methyltransferase (ArsM in microbes or AS3MT in animals) detoxifies
AsÂ(III) in microbes but transforms it into more toxic and potentially
more carcinogenic methylated species in humans. We previously proposed
a reaction pathway for ArsM/AS3MT that involves initial 3-coordinate
binding of AsÂ(III). To date, reported structures have had only 2-coordinately
bound trivalent arsenicals. Here we report a crystal structure of
CmArsM from <i>Cyanidioschyzon</i> sp.5508 in which AsÂ(III)
is 3-coordinately bound to three conserved cysteine residues with
a molecule of the product <i>S</i>-adenosyl-l-homocysteine
bound in the SAM binding site. We propose that this structure represents
the first step in the catalytic cycle. In a previously reported SAM-bound
structure, a disulfide bond is formed between two conserved cysteine
residues. Comparison of these two structures indicates that there
is a conformational change in the N-terminal domain of CmArsM that
moves a loop to allow formation of the 3-coordinate AsÂ(III) binding
site. We propose that this conformational change is an initial step
in the AsÂ(III) SAM methyltransferase catalytic cycle
Reorientation of the Methyl Group in MAs(III) is the Rate-Limiting Step in the ArsM As(III) <i>S</i>‑Adenosylmethionine Methyltransferase Reaction
The
most common biotransformation of trivalent inorganic arsenic
(AsÂ(III)) is methylation to mono-, di-, and trimethylated species.
Methylation is catalyzed by AsÂ(III) <i>S</i>-adenosylmethionine
(SAM) methyltransferase (termed ArsM in microbes and AS3MT in animals).
Methylarsenite (MAsÂ(III)) is both the product of the first methylation
step and the substrate of the second methylation step. When the rate
of the overall methylation reaction was determined with AsÂ(III) as
the substrate, the first methylation step was rapid, whereas the second
methylation step was slow. In contrast, when MAsÂ(III) was used as
the substrate, the rate of methylation was as fast as the first methylation
step when AsÂ(III) was used as the substrate. These results indicate
that there is a slow conformational change between the first and second
methylation steps. The structure of CmArsM from the thermophilic alga Cyanidioschyzon merolae sp. 5508 was determined with
bound MAsÂ(III) at 2.27 Ă… resolution. The methyl group is facing
the solvent, as would be expected when MAsÂ(III) is bound as the substrate
rather than facing the SAM-binding site, as would be expected for
MAsÂ(III) as a product. We propose that the rate-limiting step in arsenic
methylation is slow reorientation of the methyl group from the SAM-binding
site to the solvent, which is linked to the conformation of the side
chain of a conserved residue Tyr70