Electrons travel like urban commuters
After 15 years of research, chemists from Duke University have found that electrons travel through proteins like urban commuters. Like ourselves, these electrons usually find their way through proteins by using the fastest routes, the equivalent of the subway for us. And like us, they also used sometimes alternate routes, like when we take a cab, a bus, or decide to walk. Even if you're not convinced right now, these electron transfers inside proteins are very important because they are part of many processes essential for life, such as harvesting light in photosynthesis in plant cells and generating energy in animal cells.
This research has been conducted by David Beratan, Professor of Chemistry at Duke University and by his colleagues in his research group. Below is a picture of Beratan posing in front of a subway route map to illustrate his research. (Credit: Megan Morr, for Duke University)
Below are some comments from David Beratan about this discovery.
"I think we have discovered the physical framework for thinking about all such protein electron-transfer chemistry," Beratan said. "Having this rule book in place will let scientists pose some hard but interesting questions about evolutionary pressures on protein structures. "Another payoff may be new insight for designing biologically based artificial systems that, for instance, can capture solar energy or make fertilizer from air," he added.
With the help of his colleagues and another research team from the California Institute of Technology, he studied the electron pathways in the electron-transfer protein cytochrome b562.
[Their] analysis uncovered that at seven locations on the protein, electrons took multiple fluctuating pathways. "So there is always a rapid commuter route available, even if the favorite train is out of order," he said. In two other locations, the protein offers only one dominant but slow route. There the electron has no choice but to tunnel through an especially slow bottleneck presented by the protein's structure.
On its current news page, the Department of Chemistry at Duke summarizes the research done. [If you read this post sometimes in the future, you might have to look at the archives.]
David Beratan's group describes a unified theory of protein-mediated electron transfer reactions, resolving a long-standing dispute on the mechanism of these important biological events. Prytkova, Kurnikov and Beratan show that proteins containing many fluctuating coupling pathways produce transport kinetics that depends weakly on details of the protein structure, while proteins with fewer coupling paths have structure-dependent rates. Their analysis explains a large body of kinetic data, collected over the last 25 years, on synthetically modified proteins and natural photosynthetic systems.
This starts to be more complex that was is written in the news release. But wait, there is more.
This research work has been published by Science under the name "Coupling Coherence Distinguishes Structure Sensitivity in Protein Electron Transfer" (Volume 315, Number 5812, Pages 622-625, February 2, 2007). Here is a link to a summary from the journal, "Electron Tunneling on Edge."
In biological electron transfers, the structure of a protein between donor and acceptor sites should exert an effect on the overall transfer rate, but in many cases the data can be fit to simple models where the rate depends on distance. Prytkova et al.calculated rates for electron transfers in cytochrome b562 to surface-bound ruthenium centers, where the measured rates are known to represent electron tunneling. For seven cases where the rates appear to depend only on distance, multiple tunneling pathways through the heme edges are dynamically averaged. For two cases where the rates are much slower than the distance-dependence model predicts, tunneling occurs via a single pathway through an axial ligand.
Here are two links to the abstract and to the accompanying figures. And here is the beginning of the abstract.
Quantum mechanical analysis of electron tunneling in nine thermally fluctuating cytochrome b562 derivatives reveals two distinct protein-mediated coupling limits. A structure-insensitive regime arises for redox partners coupled through dynamically averaged multiple-coupling pathways (in seven of the nine derivatives) where heme-edge coupling leads to the multiple-pathway regime. A structure-dependent limit governs redox partners coupled through a dominant pathway (in two of the nine derivatives) where axial-ligand coupling generates the single-pathway limit and slower rates.
It's hard to understand -- at least for me -- the connection between the contents of the Duke University news release and the scientific paper, but I'm not a theoretical chemist. anyway, if you're interested in this -- difficult -- subject, you also might want to read a previous paper published by The Journal of Physical Chemistry B, "Ab Initio Based Calculations of Electron-Transfer Rates in Metalloproteins" (Volume 109, Issue 4, Pages 1618-1625, February 3, 2005). Here is a link to the abstract.
Sources: Duke University news release, via EurekAlert!, February 1, 2007; and various other websites
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