Packing and loop-structure variations in non-isomorphous crystals of FabZ from Plasmodium falciparum

The crystals obtained from various batches of crystallization trials of FabZ from Plasmodium falciparum exhibited non-isomorphism. The c axis of the I222 cell showed a large variation of about 16 A, from c = 81 A to c = 97 A. Complete data sets were collected for three crystal forms with varying lengths of the c axis (form 1, c = 97 A; form 2, c = 92 A; form 3, c = 81 A). The crystal structure of form 1 has been reported previously. Here, the crystal structures of the other two crystal forms are reported and a detailed structural comparison is made of the three crystal forms in order to explore the possible reasons for the existence of non-isomorphism. The conformations of three loops vary between the three crystal forms. The disposition of the loops affects the crystal packing and hence the unit-cell parameter. The crystallization condition and crystallization method employed, which change the evaporation rate, determine the crystal form of the enzyme. The present analysis shows that pH-induced intrinsic conformational changes in the protein play a key role in the observed differences.