Conformational stability and crystal packing: polymorphism in Neurospora crassa CAT‐3

Polymorphism is frequently observed from different crystallization conditions. In proteins, the effect on conformational variability is poorly documented, with only a few reported examples. Here, three polymorphic crystal structures determined for a large‐subunit catalase, CAT‐3 from Neurospora crassa , are reported. Two of them belonged to new space groups, P 1 and P 4 3 2 1 2, and a third structure belonged to the same space group, P 2 1 2 1 2 1 , as the previously deposited 2.3 Å resolution structure (PDB entry 3ej6 ), but had a higher resolution (1.95 Å). Comparisons between these polymorphic structures highlight the conformational stability of tetrameric CAT‐3 and reveal a distortion in the tetrameric structure that has not previously been described.