Energy conservation involving 2 respiratory circuits

Marie Charlotte Schoelmerich; Alexander Katsyv; Judith Dönig; Timothy J. Hackmann; Volker Müller

doi:10.1073/pnas.1914939117

New Research In

Edited by Caroline S. Harwood, University of Washington, Seattle, WA, and approved November 27, 2019 (received for review August 28, 2019)

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Significance

The chemiosmotic mechanism is a central mode of energy conservation for microorganisms. It relies on a respiratory chain that couples electron flow at the membrane to the transport of ions across the cytoplasmic membrane. This electrochemical potential fuels a rotary machine, the ATP synthase, to make intracellular ATP. Here, we show that a strictly anaerobic rumen bacterium uses 2 different ion circuits for energy conservation. This is achieved by employing 2 ATP synthases that are driven by a H+ or Na⁺ gradient. The mixed gradient is established by 2 distinct ion pumps. The H+ gradient is formed by the Ech complex, and the Na⁺ gradient is formed by the Rnf complex.

Abstract

Chemiosmosis and substrate-level phosphorylation are the 2 mechanisms employed to form the biological energy currency adenosine triphosphate (ATP). During chemiosmosis, a transmembrane electrochemical ion gradient is harnessed by a rotary ATP synthase to phosphorylate adenosine diphosphate to ATP. In microorganisms, this ion gradient is usually composed of H+, but it can also be composed of Na⁺. Here, we show that the strictly anaerobic rumen bacterium Pseudobutyrivibrio ruminis possesses 2 ATP synthases and 2 distinct respiratory enzymes, the ferredoxin:NAD+ oxidoreductase (Rnf complex) and the energy-converting hydrogenase (Ech complex). In silico analyses revealed that 1 ATP synthase is H+-dependent and the other Na⁺-dependent, which was validated by biochemical analyses. Rnf and Ech activity was also biochemically identified and investigated in membranes of P. ruminis. Furthermore, the physiology of the rumen bacterium and the role of the energy-conserving systems was investigated in dependence of 2 different catabolic pathways (the Embden–Meyerhof–Parnas or the pentose–phosphate pathway) and in dependence of Na⁺ availability. Growth of P. ruminis was greatly stimulated by Na⁺, and a combination of physiological, biochemical, and transcriptional analyses revealed the role of the energy conserving systems in P. ruminis under different metabolic scenarios. These data demonstrate the use of a 2-component ion circuit for H+ bioenergetics and a 2nd 2-component ion circuit for Na⁺ bioenergetics in a strictly anaerobic rumen bacterium. In silico analyses infer that these 2 circuits are prevalent in a number of other strictly anaerobic microorganisms.

Footnotes

↵¹Present address: Microbiology & Biotechnology, Institute of Plant Sciences and Microbiology, Universität Hamburg, 22609 Hamburg, Germany.
↵²To whom correspondence may be addressed. Email: vmueller{at}bio.uni-frankfurt.de.

Author contributions: M.C.S., A.K., T.J.H., and V.M. designed research; M.C.S., A.K., J.D., and T.J.H. performed research; M.C.S., A.K., J.D., T.J.H., and V.M. analyzed data; and M.C.S. and V.M. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1914939117/-/DCSupplemental.

Published under the PNAS license.

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