Serendipitous crystallization of E. coli HPII catalase, a sequel to “the tale usually not told”

Protein crystallographers are well aware of the trap crystallizing E. coli proteins instead macromolecule interest if heterologous recombinant protein expression in was part experimental pipeline. Among well-known culprits YodA metal-binding lipocalin (25 kDa) and YadF carbonic anhydrase (a tetramer 25 kDa subunits). We report a novel crystal form another such culprit, HPII catalase, which is tetrameric ~340 molecular weight. likely to contaminate samples, co-purify, then co-crystallize with target proteins, especially their masses size exclusion chromatography ~300–400 kDa. What makes this case more interesting but also parlous, fact that can crystallize from very low concentrations, even below 1 mg/mL.