Screening Using Polymorphs for the Crystallization of Protein–Ligand Complexes

An
efficient crystallization screening method is important in drug design
to yield high resolution crystallographic structures of protein–ligand
complexes to understand inhibitor selectivity and potency for various
members of an enzyme family. The strategy starts with a single condition
for each protein–ligand complex, and more trials encompassing
all polymorph crystallization conditions are included later, eventually
defaulting to a more extensive screening for difficult cases. The
polymorph screening approach relies on an intrinsic positive feedback
mechanism. New polymorphs are constantly discovered since certain
ligands favor variant lattices. The new best diffracting polymorph
is selected for single-conditions testing, ensuring that as more forms
are discovered, the resolution of the structures obtained improves.
Continual optimization of the conditions for all crystal forms yields
new solutions that become increasingly effective in protein–ligand
crystallization trials. More polymorphs imply more lattices suitable
to accommodate ligands of greater diversity. Wider seeding opportunities
combined with optimized enzyme-specific crystallization conditions
improves the outcome and accelerates the screening process so that
a conventional full-range crystallization screening is only rarely
needed. Having tested this approach with a large repertoire of 100
ligands and 4 enzymes, we expect the method to perform equally well
on similar drug-discovery projects.