Sequence of rubredoxin by x-ray diffraction

A tentative amino acid sequence has been determined for rubredoxin from Clostridium pasteurianum by examining the shapes of the side chains in an electron density map at 2 Å resolution. Superpositions of the appropiate portions of this map and “ball and stick” representations of the amino acids assigned are presented as evidence in support of our identifications. Discrepancies exist between the sequence shown here and that determined by chemical methods. The sequence deduced from the 2Å resolution map, however, represents our best estimate in the absence of chemical data and before refinement of the structure. We emphasize the results at 2 Å resolution because, until recently, the 2 Å resolution map was usually the final result of an X-ray structure determination. It is instructive to show the level of reliability that can be expected at that stage and to illustrate the kinds of mistakes that are likely to occur when making identifications from electrondensity maps.