An interdisciplinary research team of the Institutes of Physical Chemistry and Physics of the University of Freiburg and the Max Planck Institute of Biophysics in Frankfurt-am-Main has discovered a new, direction-dependent friction in proteins called anisotropic friction. “Until now, nobody had observed that friction in biomolecules was dependent on direction,” says physicist Dr. Steffen Wolf of the University of Freiburg. The results have been published as cover story in the scientific journal “Nano Letters.”
Experiments on model complex of protein-ligands
Proteins constitute the microscopic machinery of cells. They perform work during their functional cycles. Accordingly, they follow the laws of thermodynamics, exhibit an energy conversion efficiency, and lose energy during their functional cycle due to dissipation. From a macroscopic perspective, the latter effect corresponds to apparent friction. On the microscopic scale of single proteins, a known source of friction is internal friction of proteins that results from the excitation of protein-internal vibrations. A further source is solvent friction, which arises from the acceleration of surrounding solvent molecules. These sources of friction lead to heating of both protein and solvent. Here, the researchers discovered the novel type of friction by carrying out single molecule experiments and simulations on a model complex of a protein and a ligand.
In their single molecule experiments, the team used a new method applying stereographic single molecule force spectroscopy, which is based on atomic force microscopy (AFM). This technique allowed them to study the unbinding of a ligand from a protein bound to a surface not only along a single coordinate, but along all three Cartesian coordinates. During their experiments, Dr. Wanhao Cai, Prof. Dr. Thorsten Hugel and Dr. Bizan N. Balzer of the Institute of Physical Chemistry of the University of Freiburg as well as Dr. Jakob T. Bullerjahn of the Max Planck Institute, made the surprising discovery that friction during ligand unbinding increases with the pulling angle applied.