1x1a

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spinqualitylabelsall models 1x1a, resolution 2.60Å Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Crystal structure of BchU complexed with S-adenosyl-L-methionine

Overview

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.

About this Structure

1X1A is a Single protein structure of sequence from Chlorobaculum tepidum with SO4, SAM and GOL as ligands. Full crystallographic information is available from OCA.

Reference

Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism., Wada K, Yamaguchi H, Harada J, Niimi K, Osumi S, Saga Y, Oh-Oka H, Tamiaki H, Fukuyama K, J Mol Biol. 2006 Jul 21;360(4):839-49. Epub 2006 Jun 8. PMID:16797589

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