1x19: Difference between revisions

[[Image:1x19.gif|left|200px]]<br /><applet load="1x19" size="350" color="white" frame="true" align="right" spinBox="true"  

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.

1X19 is a [http://en.wikipedia.org/wiki/Single_protein Single protein] structure of sequence from [http://en.wikipedia.org/wiki/Chlorobaculum_tepidum Chlorobaculum tepidum] with <scene name='pdbligand=SO4:'>SO4</scene> as [http://en.wikipedia.org/wiki/ligand ligand]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1X19 OCA].  

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