Strain relief at the active site of phosphoserine aminotransferase induced by radiation damage

Abstract

The X‐ray susceptibility of the lysine‐pyridoxal‐5′‐phosphate Schiff base in Bacillus alcalophilus phosphoserine aminotransferase has been investigated using crystallographic data collected at 100 K to 1.3 Å resolution, complemented by on‐line spectroscopic studies. X‐rays induce deprotonation of the internal aldimine, changes in the Schiff base conformation, displacement of the cofactor molecule, and disruption of the Schiff base linkage between pyridoxal‐5′‐phosphate and the Lys residue. Analysis of the “undamaged” structure reveals a significant chemical strain on the internal aldimine bond that leads to a pronounced geometrical distortion of the cofactor. However, upon crystal exposure to the X‐rays, the strain and distortion are relaxed and eventually diminished when the total absorbed dose has exceeded 4.7 × 10 6 Gγ. Our data provide new insights into the enzymatic activation of pyridoxal‐5′‐phosphate and suggest that special care should be taken while using macromolecular crystallography to study details in strained active sites.