Photosynthesis Research

, Volume 41, Issue 1, pp 75–88

An enzyme and13C-NMR study of carbon metabolism in heliobacteria

  • Mark W. Pickett
  • Michael P. Williamson
  • David J. Kelly
Group 3: New Organisms, Ecology and Biochemistry Regular Papers

Abstract

Heliobacteria are a group of anoxygenic phototrophs that can grow photoheterotrophically in defined minimal media on only a limited range of organic substrates as carbon sources. In this study the mechanisms which operate to assimilate carbon and the routes employed for the biosynthesis of cellular intermediates were investigated in a newHeliobacterium strain, HY-3. This was achieved using two approaches (1) by measuring the activities of key enzymes in cell-free extracts and (2) by the use of13C nuclear magnetic resonance (NMR) spectroscopy to analyze in detail the labelling pattern of amino-acids of cells grown on [13C] pyruvate and [13C] acetate.Heliobacterium strain HY-3 was unable to grow autotrophically on CO2/H2 and neither (ATP)-citrate lyase nor ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPcase) were detectable in cell-free extracts. The enzyme profile of pyruvate grown cells indicated the presence of a pyruvate:acceptor oxidoreductase at high specific activity which could convert pyruvate to acetyl-Coenzyme A. No pyridine nucleotide dependent pyruvate dehydrogenase complex activity was detected. Of the citric-acid cycle enzymes, malate dehydrogenase, fumarase, fumarate reductase and an NADP-specific isocitrate dehydrogenase were readily detectable but no aconitase or citrate synthase activity was found. However, the labelling pattern of glutamate in long-term 2-[13C] acetate incorporation experiments indicated that a mechanism exists for the conversion of carbon from acetyl-CoA into 2-oxoglutarate. A 2-oxoglutarate:acceptor oxidoreductase activity was present which was also assayable by isotope exchange, but no 2-oxoglutarate dehydrogenase complex activity could be detected. Heliobacteria appear to use a type of incomplete reductive carboxylic acid pathway for the conversion of pyruvate to 2-oxoglutarate but are unable to grow autotrophically using this metabolic route due to the absence of ATP-citrate lyase.

Key words

photosynthesis carbon assimilation photosynthetic bacteria citric-acid cycle pyruvate synthase fermentation heliobacteria 

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Copyright information

© Kluwer Academic Publishers 1994

Authors and Affiliations

  • Mark W. Pickett
    • 1
    • 2
  • Michael P. Williamson
    • 1
  • David J. Kelly
    • 1
    • 2
  1. 1.Krebs Institute, Department of Molecular Biology and BiotechnologyUniversity of SheffieldSheffieldUK
  2. 2.Robert Hill Institute, Department of Molecular Biology and BiotechnologyUniversity of SheffieldSheffieldUK

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