Abstract
Twinning is a crystal growth anomaly, which has posed a challenge in macromolecular crystallography (MX) since the earliest days. Many approaches have been used to treat twinned data in order to extract structural information. However, in most cases it is usually simpler to rescreen for new crystallization conditions that yield an untwinned crystal form or, if possible, collect data from non-twinned parts of the crystal. Here, we report 11 structures of engineered variants of the E. coli enzyme N-acetyl-neuraminic lyase which, despite twinning and incommensurate modulation, have been successfully indexed, solved and deposited. These structures span a resolution range of 1.45–2.30 Å, which is unusually high for datasets presenting such lattice disorders in MX and therefore these data provide an excellent test set for improving and challenging MX data processing programs.
Highlights
When there are two twin domains, and the twin operator is a 2-fold rotation, the twinning is called hemihedral twinning
If the twin fraction α approaches 0.5 the diffraction pattern acquires an additional symmetry, imposed by the twinning operator, which may lead to erroneous indexing in a higher symmetry space group
In this report we present 11 diffraction data sets, in multiple space groups, from the E. coli enzyme N-acetyl-neuraminic acid lyase (NAL), which present twin lattices and incommensurate modulation
Summary
All the datasets in crystal form I were indexed with Dirax[21] to determine whether incommensurate modulation was present This was the case for four of the seven datasets, three of which were deposited in the PDB: 2WNN, 2WNQ, 2W05, whilst one, called Y137A, was not, due to unsatisfactory statistics. With the P21 indexing choice, POINTLESS initially assigned the space group C2221 but reflections belonging to one of the 2-fold axes were much stronger than the others (data not shown), which is consistent with pseudo-merohedral twinning in P21, and with this choice the structures could be solved. Whenever only a single lattice with no incommensurate
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