title: Rhodobacter capsulatus magnesium chelatase subunit BchH contains an oxygen sensitive iron-sulfur cluster.
abstract: Magnesium chelatase is the first unique enzyme of the bacteriochlorophyll biosynthetic pathway. It consists of three subunits (BchI, BchD, and BchH). Amino acid sequence analysis of the Rhodobacter capsulatus BchH revealed a novel cysteine motif (393CX2CX3CX14C) that was found in only six other proteobacteria (CX2CX3CX11-14C). The cysteine motif is likely to coordinate an unprecedented [Fe-S] cluster. Purified BchH demonstrated absorbance in the 460 nm region. This absorbance was abolished in BchH proteins with alanine substitutions at positions Cys396 and Cys414. These modified proteins were also EPR silent. In contrast, wild type BchH protein in the reduced state showed EPR signals resembling those of a [4Fe-4S] cluster with rhombic symmetry and g values at 1.90, 1.93, and 2.09, superimposed with a [3Fe-4S] cluster centered at g = 2.02. The [3Fe-4S] signal was observed independently of the [4Fe-4S] signal under oxidizing conditions. Mg-chelatase activity assays showed that the cluster is not catalytic. We suggest that the [4Fe-4S] and [3Fe-4S] signals originate from a single coordination site on the monomeric BchH protein and that the [4Fe-4S] cluster is sensitive to oxidation. It is speculated that the cluster participates in the switching between aerobic and anaerobic life of the proteobacteria.
authors.affiliation: Department of Biochemistry, Center for Molecular Protein Science, Lund University, PO Box 124, 221 00 Lund, Sweden. Nick.Sirijovski@biochemistry.lu.se
authors.forename: Nick
authors.initials: N
authors.lastname: Sirijovski
authors.forename: Fikret
authors.initials: F
authors.lastname: Mamedov
authors.forename: Ulf
authors.initials: U
authors.lastname: Olsson
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.forename: Mats
authors.initials: M
authors.lastname: Hansson
mesh_terms: D020816:Amino Acid Motifs; D001429:Bacteriochlorophylls / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism; D003545:Cysteine / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008190:Lyases / Q000737:chemistry* / Q000378:metabolism; D010100:Oxygen / Q000737:chemistry*; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D013057:Spectrum Analysis / Q000379:methods
chemical_list: D001429:Bacteriochlorophylls; D007506:Iron-Sulfur Proteins; D008190:Lyases; C086788:magnesium chelatase; D003545:Cysteine; D010100:Oxygen