A bchD (magnesium chelatase) mutant of Rhodobacter sphaeroides synthesizes zinc bacteriochlorophyll through novel zinc-containing intermediates

Paul R. Jaschke, Amelia Hardjasa, Elizabeth L. Digby, C. Neil Hunter, J. Thomas Beatty

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

Heme and bacteriochlorophyll a (BChl) biosyntheses share the same pathway to protoporphyrin IX, which then branches as follows. Fe2+ 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 Mg2+ is substituted by Zn2+. 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.

Original languageEnglish
Pages (from-to)20313-20322
Number of pages10
JournalJournal of Biological Chemistry
Volume286
Issue number23
DOIs
Publication statusPublished - 10 Jun 2011
Externally publishedYes

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