A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change. A macromolecular conformational change may be induced by many factors such as a change in temperature, pH, voltage, ion concentration, phosphorylation, or the binding of a ligand.
Many biophysical techniques such as crystallography, NMR, electron paramagnetic resonance (EPR) using Spin label techniques, Circular Dichroism (CD), Hydrogen Exchange, and FRET can be used to study macromolecular conformational change. Dual Polarisation Interferometry is a benchtop technique capable of measuring conformational changes in biomolecules in real time at very high resolution.
A specific nonlinear optical technique called second-harmonic generation (SHG) has been recently applied to the study of conformational change in proteins. In this method, a second-harmonic-active probe is placed at a site that undergoes motion in the protein by mutagenesis or non-site-specific attachment, and the protein is adsorbed or specifically immobilized to a surface. A change in protein conformation produces a change in the net orientation of the dye relative to the surface plane and therefore the intensity of the second harmonic beam. In a protein sample with a well-defined orientation, the tilt angle of the probe can be quantitatively determined, in real space and real time. Second-harmonic-active unnatural amino acids can also be used as probes.
- Book Chapter on Protein Motions from website of RH Austin  at Princeton University
- Frauenfelder, H. New looks at protein motions Nature 338, 623 - 624 (20 April 1989).
- Detection of Protein Conformational Change by Optical Second-Harmonic Generation
- The Database of Macromolecular Motions (molmovdb)
- Protein dynamics
- Database of protein conformational diversity
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