Combination of methods used in the structure solution of pyruvate : ferredoxin oxidoreductase from two crystal forms

The structure of the homodimeric kDa pyruvate:ferredoxin oxidoreductase (PF OR) of Desulfovibrio africanus was solved with data from two crystals forms , both containing two monomers per asymmetric unit. Phases were obtained fr om multiwavelength anomalous dispersion (MAD), solvent flattening (SF), mol ecular replacement (MR) using a 5 Angstrom resolution electron-density sear ch model, multiple isomorphous replacement (MIR) and, finally, electron-den sity averaging (DA) procedures. It is shown how the combination of all thes e techniques was used to overcome problems arising from incompleteness of M AD data and weak phasing power of MIR data. A real-space refinement (RSR) p rocedure is described to improve MR solutions and obtain very accurate prot ein envelopes and non-crystallographic symmetry (NCS) transformations from 5 Angstrom resolution phase information. These were crucial for the phase e xtension to high resolution by DA methods.