The use of pseudocontact shifts to refine solution of paramagnetic metalloproteins: Met80Ala cyano-cytochrome c as an example

The availability of NOE constraints and of the relative solution structure of a paramagnetic protein permits the use of pseudocontact shifts as further structural constraints. We have developed a strategy based on: (1) determination of the chi tensor anisotropy parameters from the starting structure; (2) recalculation of a new structure by using NOE and pseudocontact shift constraints simultaneously; (3) redetermination of the chi tensor anisotropy parameters from the new structure, and so on until self-consistency. The system investigated is the cyanide derivative of a variant of the oxidized Saccharomyces cerevisiae iso-1-cytochrome c containing the Met80Ala mutation. The structure has been substantially refined. It is shown that the analysis of the deviation of the experimental pseudocontact shifts from those calculated using the starting structure may be unsound, as may the simple structure refinement based on the pseudocontact shift constraints only.