Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase

Merohedral twinning is a packing anomaly that seriously impairs the determination of macromolecular crystal ­structures. Crystals of deacetoxycephalosporin C synthase (DAOCS), an enzyme involved in the expansion of the penicillin nucleus to form the core structure of the cephalosporin antibiotics, were found to be merohedrally twinned by many diagnostic criteria. Here, the structure determination of DAOCS from twinned crystals based on a combination of isomorphous replacement and the use of a multiple-wavelength diffraction data set is described.