New molecular packing in a crystal of pseudoazurin fromAlcaligenes faecalis: a double-helical arrangement of blue copper

Abstract

Pseudoazurin from the denitrifying bacteriumAlcaligenes faecalis(AfPAz) is a blue copper protein and functions as an electron donor to copper-containing nitrite reductase (CuNIR). Conventionally,AfPAz has been crystallized using highly concentrated ammonium sulfate as a precipitant. Here, a needle-like crystal ofAfPAz grown in a solution containing a macromolecular precipitant, polyethylene glycol 8000 (PEG 8000), is reported. The crystal belonged to space groupP61, with unit-cell parametersa=b= 68.7,c= 94.2 Å. The structure has been determined and refined at 2.6 Å resolution. The asymmetric unit contained twoAfPAz molecules contacting each other on negatively charged surfaces. The molecular packing of the crystal showed a right-handed double-helical arrangement ofAfPAz molecules and hence of blue copper sites. This structure provides insight into the excluded-volume effect of PEG and the manner of assembly ofAfPAz.