title: A BchD (magnesium chelatase) mutant of rhodobacter sphaeroides synthesizes zinc bacteriochlorophyll through novel zinc-containing intermediates.
abstract: Heme and bacteriochlorophyll a (BChl) biosyntheses share the same pathway to protoporphyrin IX, which then branches as follows. Fe(2+) chelation into the macrocycle by ferrochelatase results in heme formation, and Mg(2+) addition by Mg-chelatase commits the porphyrin to BChl synthesis. It was recently discovered that a bchD (Mg-chelatase) mutant of Rhodobacter sphaeroides produces an alternative BChl in which Mg(2+) is substituted by Zn(2+). Zn-BChl has been found in only one other organism before, the acidophilic Acidiphilium rubrum. Our objectives in this work on the bchD mutant were to 1) elucidate the Zn-BChl biosynthetic pathway in this organism and 2) understand causes for the low amounts of Zn-BChl produced. The bchD mutant was found to contain a Zn-protoporphyrin IX pool, analogous to the Mg-protoporphyrin IX pool found in the wild type strain. Inhibition of ferrochelatase with N-methylprotoporphyrin IX caused Zn-protoporphyrin IX and Zn-BChl levels to decline by 80-90% in the bchD mutant, whereas in the wild type strain, Mg-protoporphyrin IX and Mg-BChl levels increased by 170-240%. Two early metabolites of the Zn-BChl pathway were isolated from the bchD mutant and identified as Zn-protoporphyrin IX monomethyl ester and divinyl-Zn-protochlorophyllide. Our data support a model in which ferrochelatase synthesizes Zn-protoporphyrin IX, and this metabolite is acted on by enzymes of the BChl pathway to produce Zn-BChl. Finally, the low amounts of Zn-BChl in the bchD mutant may be due, at least in part, to a bottleneck upstream of the step where divinyl-Zn-protochlorophyllide is converted to monovinyl-Zn-protochlorophyllide.
authors.affiliation: Department of Microbiology and Immunology, University of British Columbia, Vancouver V6T 1Z3, British Columbia, Canada.
authors.forename: Paul R
authors.initials: PR
authors.lastname: Jaschke
authors.forename: Amelia
authors.initials: A
authors.lastname: Hardjasa
authors.forename: Elizabeth L
authors.initials: EL
authors.lastname: Digby
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
authors.forename: J Thomas
authors.initials: JT
authors.lastname: Beatty
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000096:biosynthesis*; D005294:Ferrochelatase / Q000235:genetics / Q000378:metabolism; D008190:Lyases; D008954:Models, Biological; D009154:Mutation; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics; D015032:Zinc / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D008190:Lyases; C086788:magnesium chelatase; D005294:Ferrochelatase; D015032:Zinc