Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

The roots of respiration

07.03.2011
The structure of a greenhouse gas-producing bacterial enzyme may yield insights into the evolution of our earliest oxygen-breathing ancestors

Every year, nitrogen-metabolizing bacteria in the soil and seas churn out more than ten billion kilograms of nitrous oxide (N2O) gas as they respire in these oxygen-deficient environments.

Nitric oxide reductase (NOR) enzymes are the powerhouse underlying production of this gas, taking pairs of nitric oxide (NO) molecules and transforming them into N2O and water via a chemical reaction known as ‘reduction’. These enzymes also help pathogenic bacteria to evade destruction by the immune system, as some T cells use NO as a chemical weapon against infectious agents.

More generally, scientists are interested in the potential to employ these enzymes as a tool for synthesizing useful, customized molecules for a variety of applications. “The nitrogen-oxygen bond cleavage and nitrogen–nitrogen bond formation reactions executed by these enzymes are the essence of chemistry,” says Yoshitsugu Shiro of the RIKEN SPring-8 Center in Harima.

Although a great deal is known about the biochemical properties of these proteins, scientists have found it challenging to determine the structure of bacterial NOR in fine detail. Now, after seven years of hard work, Shiro and colleagues have finally obtained the first such structure for NOR from Pseudomonas aeruginosa1, a pathogenic bacterium associated with opportunistic infections in immune-compromised patients (Fig. 1).

Not-so-distant relations

Although all NOR enzymes execute essentially the same chemical reaction to produce N2O, they can be subdivided into three major classes: cNOR, qNOR and qCuNOR. The enzyme crystallized by Shiro and colleagues (Fig. 2) belongs to the cNOR family, and contains a subunit known as cytochrome c that also enables bacteria to engage in aerobic (oxygen-driven) respiration. This ability to switch from oxygen-based to nitrogen-based respiration is highly beneficial for survival in the oxygen-poor conditions deep in the soil or beneath the waves.

Accordingly, cNORs are thought to be closely related to the cytochrome oxidases (COX), enzymes that play a central role in aerobic respiration, and these new findings have revealed a number of structural parallels between the two. “There is a long history of research into these respiratory enzymes, COX and NOR, and a lot of knowledge on NOR has been accumulated by biochemical, chemical, molecular biology and microbiological studies,” says Shiro. “From these points of view, our NOR structure is not surprising, but seeing is believing!”

The reduction process is dependent on the directional transport of electrons and protons, and COX and cNOR appear to closely resemble one another in terms of the structure of their electron-transfer networks. Both enzymes depend on precisely positioned metal ions to enable electron transport, and the four iron atoms contained within cNOR are arranged in a configuration that closely resembles COX, maintained via interactions between these positively charged iron atoms and a set of evolutionarily conserved, negatively charged histidine and glutamate amino acids.

On the other hand, the researchers observed some notable differences with regard to the movement of protons. Both COX and cNOR are bound within membranes, but COX contains channels that are believed to direct the flow of protons across the membrane from the interior of the cell. This flow helps generate electrical potential that subsequently powers a variety of cellular motors. However, cNOR lacks such membrane-spanning channels, and protons entering the enzyme from the exterior of the cell only make it as far as the membrane interior, where the reductase catalytic site is located.

Back to the beginning

Even with a structure in hand for this well-studied enzyme, a number of mysteries remain to be addressed. For example, these data are insufficient to resolve an ongoing debate over the fine details of the N2O production mechanism. Shiro and colleagues were readily able to identify the two iron atoms involved in catalysis, but their structure reveals insufficient space at this ‘active site’ to accommodate the two molecules of NO believed to be required for this reaction.

“The NOR active site is tightly packed and very crowded,” says Shiro. “This observation suggests that some conformational change [is] needed to achieve catalytic turnover, but no one knows of any such conformational change so far.” Resolving this issue will require the acquisition of additional, high-resolution structures that might offer clear snapshots of the enzyme at intermediate stages in the catalytic process.

This structure offers tentative support for the hypothesis that the COX aerobic respiratory machinery originally evolved from NOR enzymes, although additional work will clearly be required to confirm this. Unlike NOR, which exclusively employs iron ions, COX makes use of both copper and iron for catalysis, and the researchers have tentatively identified a few amino acid changes that might have enabled this transition to take place. In addition, although NOR lacks the ‘K-channel’ that allows COX to deliver protons from the cytoplasm, Shiro’s team has identified some structural elements that could potentially represent early evolutionary precursors in the formation of this channel.

In future studies, Shiro plans to develop experimental tests for some of these still-speculative models. “We want to follow the molecular evolution of the respiratory enzymes from anaerobic to aerobic conditions on Earth, from NOR to COX,” he says. “Using mutagenesis, based on our structural comparisons, we are hoping to convert NO-reducing NOR into oxygen-reducing COX.” For the present, though, he is optimistic that this structure will give a boost to researchers seeking to understand and manipulate this enzymatic process. “Scientists worldwide who are interested in NO reduction can enter a new stage of NOR research with this structure,” he says.

About the Researcher: Yoshitsugu Shiro

Yoshitsugu Shiro was born in Nagoya, Japan, in 1956. He graduated from the Faculty of Engineering, Kyoto University, in 1980, and obtained his PhD in 1985 from the same university. After two years of postdoctoral training with the Japan Society for the Promotion of Science at Kyoto University, he moved to RIKEN as a research scientist. In 1990, he was a visiting scholar for one year at the Department of Chemistry, Stanford University. Since then, he has been very interested in the chemistry of metal-containing proteins and enzymes, based on their molecular structures that can be examined using x-ray techniques including crystallography, small angle scattering and spectroscopy. In 1997, when SPring-8 was opened to the public, he moved to the Harima Institute from the Wako campus, and was promoted to chief scientist in 2000. Since then, he has been director of his own research group. In addition to metal-containing proteins, his current research interest focuses on the structures/functions of proteins related to the dynamics of metal elements in biology, e.g. sensing, transport, storage and utilization.

Journal information
[1] Hino, T., Matsumoto, Y., Nagano, S., Sugimoto, H., Fukumori, Y., Murata, T., Iwata, S. & Shiro, Y. Structural basis of biological N2O generation by bacterial nitric oxide reductase. Science 330, 1666–1670 (2010).

gro-pr | Research asia research news
Further information:
http://www.rikenresearch.riken.jp/eng/hom/6547
http://www.researchsea.com

Further reports about: Cox N2O NOR RIKEN amino acid chemical reaction molecular structure nitric oxide

More articles from Life Sciences:

nachricht Building a brain, cell by cell: Researchers make a mini neuron network (of two)
23.05.2018 | Institute of Industrial Science, The University of Tokyo

nachricht Research reveals how order first appears in liquid crystals
23.05.2018 | Brown University

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: LZH showcases laser material processing of tomorrow at the LASYS 2018

At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.

At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...

Im Focus: Self-illuminating pixels for a new display generation

There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?

At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...

Im Focus: Explanation for puzzling quantum oscillations has been found

So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics

Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...

Im Focus: Dozens of binaries from Milky Way's globular clusters could be detectable by LISA

Next-generation gravitational wave detector in space will complement LIGO on Earth

The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A...

Im Focus: Entangled atoms shine in unison

A team led by Austrian experimental physicist Rainer Blatt has succeeded in characterizing the quantum entanglement of two spatially separated atoms by observing their light emission. This fundamental demonstration could lead to the development of highly sensitive optical gradiometers for the precise measurement of the gravitational field or the earth's magnetic field.

The age of quantum technology has long been heralded. Decades of research into the quantum world have led to the development of methods that make it possible...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Save the date: Forum European Neuroscience – 07-11 July 2018 in Berlin, Germany

02.05.2018 | Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

Unique scope of UV LED technologies and applications presented in Berlin: ICULTA-2018

12.04.2018 | Event News

 
Latest News

Research reveals how order first appears in liquid crystals

23.05.2018 | Life Sciences

Space-like gravity weakens biochemical signals in muscle formation

23.05.2018 | Life Sciences

NIST puts the optical microscope under the microscope to achieve atomic accuracy

23.05.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>