A BchD (Magnesium Chelatase) Mutant of Rhodobacter Sphaeroides Synthesizes Zinc Bacteriochlorophyll Through Novel Zinc-Containing Intermediates

J Biol Chem. 2011 Jun 10;286(23):20313-22. doi: 10.1074/jbc.M110.212605. Epub 2011 Apr 18.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Bacteriochlorophylls / biosynthesis*
  • Ferrochelatase / genetics
  • Ferrochelatase / metabolism
  • Lyases*
  • Models, Biological*
  • Mutation*
  • Rhodobacter sphaeroides / enzymology*
  • Rhodobacter sphaeroides / genetics
  • Zinc / metabolism*

Substances

  • Bacterial Proteins
  • Bacteriochlorophylls
  • Lyases
  • magnesium chelatase
  • Ferrochelatase
  • Zinc