title: Crystal structure of putative CbiT from Methanocaldococcus jannaschii: an intermediate enzyme activity in cobalamin (vitamin B12) biosynthesis.
abstract: BACKGROUND
  In the anaerobic pathway of cobalamin (vitamin B12) synthesis, the CbiT enzyme plays two roles, as a cobalt-precorrin-7 C15-methyltransferase and a C12-decarboxylase, to produce the intermediate, cobalt-precorrin 8.
  
  
  RESULTS
  The primary structure of the hypothetical protein MJ0391, from Methanocaldococcus jannaschii, suggested that MJ0391 is a putative CbiT. Here, we report the crystal structure of MJ0391, solved by the MAD procedure and refined to final R-factor and R-free values of 19.8 & 27.3%, respectively, at 2.3 Å resolution. The asymmetric unit contains two NCS molecules, and the intact tetramer generated by crystallographic symmetry may be functionally important. The overall tertiary structure and the tetrameric arrangements are highly homologous to those found in MT0146/CbiT from Methanobacterium thermoautotrophicum.
  
  
  CONCLUSIONS
  The conservation of functional residues in the binding site for the co-factor, AdoMet, and in the putative precorrin-7 binding pocket suggested that MJ0391 may also possess CbiT activity. The putative function of MJ0391 is discussed, based on structural homology.
authors.affiliation: Department of Biophysics, National Institute of Mental Health and Neuro Sciences (NIMHANS), Bangalore 560029, India. paddy@nimhans.kar.nic.in
authors.forename: Balasundaram
authors.initials: B
authors.lastname: Padmanabhan
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Yokoyama
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Bessho
mesh_terms: D000595:Amino Acid Sequence; D019843:Archaeal Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D018360:Crystallography, X-Ray; D063747:Methanocaldococcus / Q000201:enzymology*; D008780:Methyltransferases / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D012436:S-Adenosylmethionine / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D014578:Uroporphyrins / Q000737:chemistry / Q000378:metabolism; D014805:Vitamin B 12 / Q000096:biosynthesis*
chemical_list: D019843:Archaeal Proteins; D014578:Uroporphyrins; D012436:S-Adenosylmethionine; D008780:Methyltransferases; D014805:Vitamin B 12