Section 4.3.7. Engineering crystal contacts to enhance crystallization in a particular crystal form

It is often the case that the structure of some related form of a protein is known, but the protein of interest crystallizes in a different space group. There have been attempts to use this knowledge to obtain crystals in a form that could be readily analysed. However, it may not be necessary to resort to molecular engineering approaches, since molecular replacement methods can often be successfully applied to determine the protein structure.

In one of the first applications of protein engineering to obtain crystals, Lawson et al. (1991) reported the crystal structure of ferritin H. Ferritin has two types of chains, H and L; the structure of rat L ferritin was known. Despite high sequence identity to L ferritin, human recombinant H ferritin did not crystallize satisfactorily. To obtain the structure of a human H ferritin homopolymer, the sequence in the subunit interface was modified to give crystals that were isomorphous with the rat L ferritin. The mutation was introduced, which enabled metal bridge contacts to form, resulting in crystals that diffracted to 1.9 Å. Although the mutant was designed to crystallize from CdSO₄, it did not do so. Rather, CaCl₂ gave large crystals which were isomorphous with rat and horse L ferritin crystals. In these latter crystals, Ca²⁺ is coordinated between Asp84 and Gln86, providing the rationale for the mutation.