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PDBsum entry 1x1c
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References listed in PDB file
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Key reference
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Title
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Crystal structures of bchu, A methyltransferase involved in bacteriochlorophyll c biosynthesis, And its complex with s-Adenosylhomocysteine: implications for reaction mechanism.
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Authors
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K.Wada,
H.Yamaguchi,
J.Harada,
K.Niimi,
S.Osumi,
Y.Saga,
H.Oh-Oka,
H.Tamiaki,
K.Fukuyama.
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Ref.
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J Mol Biol, 2006,
360,
839-849.
[DOI no: ]
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PubMed id
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Abstract
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BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing
methylation at the C-20 position of cyclic tetrapyrrole chlorin using
S-adenosylmethionine (SAM) as a methyl source. This methylation causes
red-shifts of the electronic absorption spectrum of the light-harvesting
pigment, allowing green photosynthetic bacteria to adapt to low-light
environments. We have determined the crystal structures of BchU and its complex
with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists
of two domains, an N-terminal domain and a C-terminal domain. Dimerization
occurs through interactions between the N-terminal domains and the residues
responsible for the catalytic reaction are in the C-terminal domain. The binding
site of SAH is located in a large cavity between the two domains, where SAH is
specifically recognized by many hydrogen bonds and a salt-bridge. The electron
density map of BchU in complex with an analog of bacteriochlorophyll c located
its central metal near the SAH-binding site, but the tetrapyrrole ring was
invisible, suggesting that binding of the ring to BchU is loose and/or occupancy
of the ring is low. It is likely that His290 acts as a ligand for the central
metal of the substrate. The orientation of the substrate was predicted by
simulation, and allows us to propose a mechanism for the BchU directed
methylation: the strictly conserved Tyr246 residue acts catalytically in the
direct transfer of the methyl group from SAM to the substrate through an
S(N)2-like mechanism.
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Figure 1.
Figure 1. Methylation catalyzed by Chl. tepidum BchU.
Compounds: 1, 8-ethyl-12-methyl-bacteriochlorophyllide d; 2,
8-ethyl-12-methyl-bacteriochlorophyllide c.
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Figure 9.
Figure 9. A proposed reaction mechanism for C-20
methylation of Zn BChlid d to Zn-BChlid c by SAM and BchU.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2006,
360,
839-849)
copyright 2006.
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