Structure of flavoprotein FP390 from a luminescent bacterium Photobacterium phosphoreum refined at 2.7 A resolution.

The three-dimensional structure of a flavoprotein, FP(390), from a luminescent bacterium, Photobacterium phosphoreum, solved by the molecular-replacement method, was refined to an R factor of 24.0% for 17 433 independent reflections, from 6.0 to 2.7 A resolution, collected by synchrotron radiation. The asymmetric unit of the crystal (space group P4(3)22, a = b = 76.8 and c = 242 A) contains two monomer molecules related by a non-crystallographic twofold axis to form a dimer. There are two Q-flavin [flavin mononucleotide (FMN) with myristic acid] molecules in FP(390) monomer. One of them is located at the interface of dimer which is bound to both monomer and the another is at the molecular surface. The electron density of myristic acids of Q-flavins at the dimer interface in both monomer are weak and unclear, showing the possibility that the Q-flavins bound in this site are not a single species but a mixture of two components, 6-(3"-myristic acid)-FMN and 6-(4"- myristic acid)-FMN.