Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria

ABSTRACT Hydrogen gas (H2) is a possible future transportation fuel that can be produced by anoxygenic phototrophic bacteria via nitrogenase. The electrons for H2 are usually derived from organic compounds. Thus, one would expect more H2 to be produced when anoxygenic phototrophs are supplied with i...

Descripción completa

Guardado en:

Detalles Bibliográficos
Autores principales:	James B. McKinlay, Caroline S. Harwood
Formato:	article
Lenguaje:	EN
Publicado:	American Society for Microbiology 2011
Materias:	Microbiology QR1-502
Acceso en línea:	https://doaj.org/article/f381e269485a4c41b4364a5e6f01197b
Etiquetas:	Agregar Etiqueta Sin Etiquetas, Sea el primero en etiquetar este registro!

id	oai:doaj.org-article:f381e269485a4c41b4364a5e6f01197b
record_format	dspace
spelling	oai:doaj.org-article:f381e269485a4c41b4364a5e6f01197b2021-11-15T15:39:08ZCalvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria10.1128/mBio.00323-102150-7511https://doaj.org/article/f381e269485a4c41b4364a5e6f01197b2011-04-01T00:00:00Zhttps://journals.asm.org/doi/10.1128/mBio.00323-10https://doaj.org/toc/2150-7511ABSTRACT Hydrogen gas (H2) is a possible future transportation fuel that can be produced by anoxygenic phototrophic bacteria via nitrogenase. The electrons for H2 are usually derived from organic compounds. Thus, one would expect more H2 to be produced when anoxygenic phototrophs are supplied with increasingly reduced (electron-rich) organic compounds. However, the H2 yield does not always differ according to the substrate oxidation state. To understand other factors that influence the H2 yield, we determined metabolic fluxes in Rhodopseudomonas palustris grown on 13C-labeled fumarate, succinate, acetate, and butyrate (in order from most oxidized to most reduced). The flux maps revealed that the H2 yield was influenced by two main factors in addition to substrate oxidation state. The first factor was the route that a substrate took to biosynthetic precursors. For example, succinate took a different route to acetyl-coenzyme A (CoA) than acetate. As a result, R. palustris generated similar amounts of reducing equivalents and similar amounts of H2 from both succinate and acetate, even though succinate is more oxidized than acetate. The second factor affecting the H2 yield was the amount of Calvin cycle flux competing for electrons. When nitrogenase was active, electrons were diverted away from the Calvin cycle towards H2, but to various extents, depending on the substrate. When Calvin cycle flux was blocked, the H2 yield increased during growth on all substrates. In general, this increase in H2 yield could be predicted from the initial Calvin cycle flux. IMPORTANCE Photoheterotrophic bacteria, like Rhodopseudomonas palustris, obtain energy from light and carbon from organic compounds during anaerobic growth. Cells can naturally produce the biofuel H2 as a way of disposing of excess electrons. Unexpectedly, feeding cells organic compounds with more electrons does not necessarily result in more H2. Despite repeated observations over the last 40 years, the reasons for this discrepancy have remained unclear. In this paper, we identified two metabolic factors that influence the H2 yield, (i) the route taken to make biosynthetic precursors and (ii) the amount of CO2-fixing Calvin cycle flux that competes against H2 production for electrons. We show that the H2 yield can be improved on all substrates by using a strain that is incapable of Calvin cycle flux. We also contributed quantitative knowledge to the long-standing question of why photoheterotrophs must produce H2 or fix CO2 even on relatively oxidized substrates.James B. McKinlayCaroline S. HarwoodAmerican Society for MicrobiologyarticleMicrobiologyQR1-502ENmBio, Vol 2, Iss 2 (2011)
institution	DOAJ
collection	DOAJ
language	EN
topic	Microbiology QR1-502
spellingShingle	Microbiology QR1-502 James B. McKinlay Caroline S. Harwood Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
description	ABSTRACT Hydrogen gas (H2) is a possible future transportation fuel that can be produced by anoxygenic phototrophic bacteria via nitrogenase. The electrons for H2 are usually derived from organic compounds. Thus, one would expect more H2 to be produced when anoxygenic phototrophs are supplied with increasingly reduced (electron-rich) organic compounds. However, the H2 yield does not always differ according to the substrate oxidation state. To understand other factors that influence the H2 yield, we determined metabolic fluxes in Rhodopseudomonas palustris grown on 13C-labeled fumarate, succinate, acetate, and butyrate (in order from most oxidized to most reduced). The flux maps revealed that the H2 yield was influenced by two main factors in addition to substrate oxidation state. The first factor was the route that a substrate took to biosynthetic precursors. For example, succinate took a different route to acetyl-coenzyme A (CoA) than acetate. As a result, R. palustris generated similar amounts of reducing equivalents and similar amounts of H2 from both succinate and acetate, even though succinate is more oxidized than acetate. The second factor affecting the H2 yield was the amount of Calvin cycle flux competing for electrons. When nitrogenase was active, electrons were diverted away from the Calvin cycle towards H2, but to various extents, depending on the substrate. When Calvin cycle flux was blocked, the H2 yield increased during growth on all substrates. In general, this increase in H2 yield could be predicted from the initial Calvin cycle flux. IMPORTANCE Photoheterotrophic bacteria, like Rhodopseudomonas palustris, obtain energy from light and carbon from organic compounds during anaerobic growth. Cells can naturally produce the biofuel H2 as a way of disposing of excess electrons. Unexpectedly, feeding cells organic compounds with more electrons does not necessarily result in more H2. Despite repeated observations over the last 40 years, the reasons for this discrepancy have remained unclear. In this paper, we identified two metabolic factors that influence the H2 yield, (i) the route taken to make biosynthetic precursors and (ii) the amount of CO2-fixing Calvin cycle flux that competes against H2 production for electrons. We show that the H2 yield can be improved on all substrates by using a strain that is incapable of Calvin cycle flux. We also contributed quantitative knowledge to the long-standing question of why photoheterotrophs must produce H2 or fix CO2 even on relatively oxidized substrates.
format	article
author	James B. McKinlay Caroline S. Harwood
author_facet	James B. McKinlay Caroline S. Harwood
author_sort	James B. McKinlay
title	Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
title_short	Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
title_full	Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
title_fullStr	Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
title_full_unstemmed	Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria
title_sort	calvin cycle flux, pathway constraints, and substrate oxidation state together determine the h<sub>2</sub> biofuel yield in photoheterotrophic bacteria
publisher	American Society for Microbiology
publishDate	2011
url	https://doaj.org/article/f381e269485a4c41b4364a5e6f01197b
work_keys_str_mv	AT jamesbmckinlay calvincyclefluxpathwayconstraintsandsubstrateoxidationstatetogetherdeterminethehsub2subbiofuelyieldinphotoheterotrophicbacteria AT carolinesharwood calvincyclefluxpathwayconstraintsandsubstrateoxidationstatetogetherdeterminethehsub2subbiofuelyieldinphotoheterotrophicbacteria
_version_	1718427803029864448

Calvin Cycle Flux, Pathway Constraints, and Substrate Oxidation State Together Determine the H<sub>2</sub> Biofuel Yield in Photoheterotrophic Bacteria

Ejemplares similares