When X-rays modify the protein structure: Radiation damage at work

The majority of 3D structures of macromolecules are
currently determined by macromolecular crystallography,
which employs the diffraction of X-rays on single
crystals. However, during diffraction experiments, the
X-rays can damage the protein crystals by ionization
processes, especially when powerful X-ray sources at
synchrotron facilities are used. This process of radiation
damage generates photo-electrons that can get trapped
in protein moieties. The 3D structure derived from such
experiments can differ remarkably from the structure of
the native molecule. Recently, the crystal structures of
different oxidation states of horseradish peroxidase and
nickel-containing superoxide dismutase were determined
using crystallographic redox titration performed
during the exposure of the crystals to the incident X-ray
beam. Previous crystallographic analyses have not
shown the distinct structures of the active sites
associated with the redox state of the structural features
of these enzymes. These new studies show that, for
protein moieties that are susceptible to radiation
damage and prone to reduction by photo-electrons,
care is required in both the design of the diffraction
experiment and the analysis and interpretation.