Molecular Replacement

Abstract

With the isomorphous replacement method or with one of the anomalous diffraction techniques, a preliminary set of protein phase angles and a first model of the protein structure can be obtained. As we will see in Chapter 13, such a model can be refined by minimizing the difference between the observed |F| values and the |F| values calculated from the model. An easier way to obtain a first model can be followed if the structure of a protein with a homologous amino acid sequence has already been established. The structure of this homologous protein is—as it were—borrowed by the protein for which the structure must be determined and serves as a very first model that can subsequently be refined. This procedure is based on the observation that proteins, homologous in their amino acid sequence, have a very similar folding of their polypeptide chain. Also, if, for another reason, two structure can be expected to be similar, one known and the other unknown, the procedure can be applied.