Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Key enzyme in citrate cycle also functions “backwards”

02.02.2018

The citrate cycle provides many organisms with energy by breaking down organic substances. A team of researchers has unexpectedly discovered that a central enzyme in the citrate cycle – citrate synthase – functions both “forwards” and “backwards”. This had previously been considered impossible.

The citrate cycle: most people will probably remember this metabolic pathway from biology lessons at school. It provides numerous organisms with energy by breaking down organic substances, thereby enabling them to live. Organisms ranging all the way from bacteria to humans use this pathway, consuming oxygen in the process.


Prof. Dr. Ivan Berg

© WWU/Laura Schenk

Some microorganisms even use the citrate cycle when oxygen is absent – in other words, under anaerobic conditions – in the opposite direction: They build up biomass from carbon dioxide by means of the “reductive” citrate cycle. This means that they fix inorganic carbon, just like plants do in photosynthesis.

A team of researchers from the University of Münster, headed by biologist Prof. Ivan Berg, has unexpectedly discovered that a central enzyme in the citrate cycle functions both “forward” and “backwards”. Citrate synthase catalyzes the first step in the citrate cycle – the formation of citrate – thereby giving it its name. It had previously been regarded impossible that this step could be reversed in living cells – but not any longer.

The researchers have discovered that in certain anaerobic bacteria, which are able to fix inorganic carbon by means of the reductive citrate cycle, citrate synthase functions backwards and cleaves citrate instead of forming it. The researchers’ findings have been published in the latest issue of the journal “Science”.

“Shortly before our discovery I was still telling students in my lectures that under physiological conditions – in other words, in living cells – the citrate synthase reaction can only take place in one direction,” recalls Ivan Berg from the Institute of Molecular Microbiology and Biotechnology at Münster University. “Our latest findings disprove a decade-old conventional wisdom.” In addition to the Münster team, other researchers were also involved in the study – from the University of Freiburg, the Technical University of Munich (co-corresponding author: Prof. Wolfgang Eisenreich) and the Water Technology Centre in Karlsruhe.

From an energetic point of view, the newly discovered variation in the reductive citrate cycle is the most efficient way to fix carbon. In contrast to the variations previously known, no energy is used in the form of the energy carrier adenosine triphosphate (ATP) to cleave the citrate into acetyl coenzyme A and oxaloacetate. The enzyme, which normally enables citrate to be cleaved in the reductive citrate cycle, is a so-called ATP-dependent citrate lyase. This enzyme was seen as the key enzyme of the “reverse” pathway.

Experts describe organisms which feed on organic compounds as “heterotrophic”, in contrast to organisms which build up biomass with the aid of light energy or chemical energy, which are called “autotrophic”. The results of this new study have implications for our understanding of their evolution.

“With the possibility of using the enzymes of the heterotrophic oxidative citrate cycle for autotrophic metabolism, an autotrophic organism can develop very easily from a heterotrophic one,” says Ivan Berg. “Our results indicate that, in the evolutionary process, the capability to grow autotrophically has developed independently on several occasions.” In the researchers’ view, many anaerobic microorganisms are potential autotrophs.

The researchers’ discovery arose by chance. “While we were looking for the autotrophic carbon fixation pathway in a bacterium called Desulfurella acetivorans, we unexpectedly observed ATP-independent citrate cleavage,” says lead author Achim Mall, a member of Ivan Berg’s team. Using nuclear magnetic resonance spectroscopy, the researchers demonstrated at a molecular level that citrate synthase makes this reaction possible.

“Our results show that unexpected discoveries are possible even when investigating thoroughly investigated metabolic pathways,” says Ivan Berg. “There are probably further surprises awaiting us in the field of metabolic biochemistry.”

Desulfurella acetivorans was discovered on the north-east Asian Kamchatka Peninsula in the late 1980s in a volcanic spring with a temperature of over 50°C. The bacterium is strictly anaerobic and respires elemental sulfur instead of oxygen. It can live either heterotrophically, feeding on acetate or other organic substrates, or autotrophically, fixing inorganic carbon and gaining energy from the oxidation of hydrogen.

The research work received funding from the German Research Foundation and the Hans Fischer Society, Munich.

Original publication:

Mall A. et al. (2018): Science 02 Feb 2018: Vol. 359, Issue 6375, pp. 563-567; DOI: 10.1126/science.aao2410

Weitere Informationen:

https://www.uni-muenster.de/Biologie.IMMB/en/Arbeitskreise/agberg/index.html Group Prof. Dr. Ivan Berg

Dr. Christina Heimken | idw - Informationsdienst Wissenschaft
Further information:
https://www.uni-muenster.de/

Further reports about: anaerobic bacterium biomass citrate enzyme inorganic living cells microorganisms

More articles from Life Sciences:

nachricht Colorectal cancer risk factors decrypted
13.07.2018 | Max-Planck-Institut für Stoffwechselforschung

nachricht Algae Have Land Genes
13.07.2018 | Julius-Maximilians-Universität Würzburg

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Research finds new molecular structures in boron-based nanoclusters

13.07.2018 | Materials Sciences

Algae Have Land Genes

13.07.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>