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How molecular bonds strengthen under load

Munich, December 12, 2014

Reversible but stable bonds between biomolecules are key to both development and stability of life. The physics of such bonds is based on a limited variety of fundamental forces combined with a richness of molecular conformations. Numerous strategies to fulfill specific tasks have evolved, amongst them, the ability to strengthen under load.
Certain microbes were selected during evolution for their competence to digest cellulose and being able to hold tightly to cellulose fibrils is key to their success. Since cellulose is rapidly being recognized as a valuable chemical commodity for the production of bio-derived fuels and chemicals, the underlying mechanisms for cellulose digestion have gained growing attention. Cellulose obtained from municipal and agricultural wastes, for example, can be converted into high-value soluble sugars using enzyme catalysts. These sugars are useful for further downstream processing into a variety of products in the chemical and liquid energy markets. Based on its high level importance as a renewable energy source, understanding the fundamental mechanisms by which bacteria degrade and digest cellulose is of prime importance.

Now, an international team of researchers, including Dr. Michael Nash and Prof. Hermann Gaub who holds the LMU Chair for Applied Physics, demonstrate that cellulose-degrading bacteria rely on proteins with extreme mechanical properties to achieve impressive adhesion to cellulose. The researchers first identified a protein complex from the anaerobic bacteria Ruminococcus flavefaciens that is involved in attachment of the bacterial cell wall to cellulose carbon sources within the cow rumen. The researchers produced these proteins and attached one to a glass surface, and the other to a microfabricated silicon cantilever tip. Using a method known as single-molecule force spectroscopy, the researchers were able to measure the amount of force that an individual protein pair could withstand before breakage. Their results demonstrated that, although the protein binding partners do not exhibit abnormally high binding strength when measured in freely-diffusing solutions, when oriented and stretched under applied force they become activated and ultrastable. This behavior is referred to as a “catch bond”, and is analogous to a finger trap that clamps down upon mechanical loading. The single-molecule experimental results were also supported by all-atom molecular dynamics simulations performed by the collaborating group of Prof. Klaus Schulten at the University of Illinois-Urbana Champaign. The group of Prof. Edward Bayer from the Weizmann Institute in Rehovot, Israel, a leading group in cellulose-degrading bacteria, also collaborated on the project.

The article appeared on Dec. 08 in the journal Nature Communications and was supported on the German side by an advanced grant of the European Research Council “Cellufuel” (No. 294438), a German-Israeli Foundation for Scientific Research and Development grant, by DFG-SFB 1032 and the Excellence Cluster Center for Integrated Protein Science Munich. Dr. Nash is supported by the Society in Science - The Branco Weiss Fellowship program from ETH in Zürich, Switzerland.