Flavins Perform Electron Magic

Researchers discover the secret behind the third way living organisms extract energy from their environment.

Electron bifurcation
Electron bifurcation reaction catalyzes two simultaneous reactions. The bifurcating flavin cofactor (center) accepts two electrons and divides their energy into two separate and energetically distinct one-electron pathways. The departure of the low-energy electron (left) creates a high-energy electron (right) capable of reducing ferredoxin. Both reduced compounds, NADH and ferredoxin, perform crucial downstream processes of their own, but the ferredoxin has higher energy and can be used for more difficult chemical reactions. Image courtesy of David Mulder, National Renewable Energy Laboratory

The Science

Electron bifurcation is a clever means that living things use to better extract energy from their metabolic processes. Electrons in metabolism can be thought of as high energy, low energy, or somewhere in between. Bifurcation takes two intermediate electrons and creates one high- and one low-energy electron. High-energy electrons are needed to perform some difficult chemical reactions. How bifurcation works was a mystery—until now. Scientists studied what happens after the flavin cofactor, a compound containing nitrogen and several six-membered rings which is closely related to the vitamin riboflavin, accepts two intermediate-energy electrons. They found a highly energetic, short-lived flavin intermediate, created after the first electron is sent away. The flavin rapidly channels its energy to the remaining electron, giving it a bump up in energy.

The Impact

Knowing these details of how flavin-based electron bifurcation works provides important new insights into living organisms. Specifically, the research shows how organisms effectively and efficiently manage the energetic content of chemical bonds, positioning electrons at energies to match the reactions they need to grow and survive. It would be a waste of energy to use high-energy electrons in a reaction that does not require them. Conversely, low-energy electrons won’t work in a reaction that demands significant energy input. These results may someday inspire the design of a new class of catalysts for certain industrial processes. Also, they may help guide the re-engineering of microbes to more efficiently produce renewable fuels and chemicals.

Summary

There are currently only three known ways that living things can capture energy from their environment to enable life to exist. The most recently discovered of the three, electron bifurcation, is especially critical to the biochemistry that drives microbial life at the thermodynamic limits observed for anaerobic processes such as methane formation, acetate formation and hydrogen metabolism. Electron bifurcation catalyzes two simultaneous reactions: the first is spontaneous (“downhill” in free energy), the second is not spontaneous (“uphill” in free energy). The mechanistic details of how the first drives the second, however, have been elusive. Researchers in the Biological Electron Transfer and Catalysis (BETCy) Energy Frontier Research Center uncovered these details by applying an integrated suite of biophysical and biochemical tools. They discovered that a central flavin cofactor bifurcates the energy of a donated pair of electrons—the low-energy electron drives the spontaneous reaction, and the high-energy electron drives the non-spontaneous reaction. The energy difference between the two bifurcated electrons, an unprecedented 1 volt, enables catalysis of even relatively “difficult” reaction chemistries. The difference is made possible by a highly energetic and short-lived flavin intermediate. The high-energy electron from this intermediate is transferred through a special iron–sulfur cluster and, ultimately, provides the energy for the non-spontaneous reaction. The kinetic, thermodynamic and structural principles revealed in this work provide a foundation for understanding how bifurcating enzymes merge unique cofactors with catalytic sites to accomplish a diverse array of biochemical reactions.

Source : U.S. Department of Energy