Structural knowledge or X-ray damage? A case study on xylose isomerase illustrating both

Area of Science:

• Structural biology
• Biochemistry
• Crystallography

Background:

• Xylose isomerase (XI) is a key metalloprotein with a complex reaction mechanism.
• Understanding XI's mechanism is vital for industrial applications.
• X-ray crystallography is a primary tool for structural determination, but radiation damage poses a significant challenge.

Purpose of the Study:

• To investigate the effects of radiation damage on xylose isomerase (XI) structure.
• To determine how radiation dose influences structural changes and interpretations of XI's reaction mechanism.
• To differentiate between true structural changes and radiation-induced artifacts.

Main Methods:

• Collected consecutive high-resolution X-ray diffraction datasets from a single XI crystal at 100 K.
• Tracked progression of radiation damage over an increasing dose range (0.13-3.88 MGy).
• Analyzed structural perturbations, including metal cofactor position and active site residue movements.

Main Results:

• Observed build-up of free radicals in the catalytic metal and surrounding amino acid environment.
• Documented radiation-damage-induced structural changes, including metal positional shifts and specific residue motions.
• Determined that apparent metal movement is an artifact of global damage and unit-cell expansion, while residue motion is dose-dependent.

Conclusions:

• Accurate interpretation of XI's reaction mechanism from X-ray structures requires careful consideration of radiation damage.
• Distinguishing between radiation-induced artifacts and biologically relevant conformational changes is critical.
• Understanding radiation damage effects enhances the reliability of structural biology studies.