Crystallization and avoiding the problem of hemihedral twinning in crystals of Δ¹-pyrroline-5-carboxylate dehydrogenase from Thermus thermophilus

Δ¹-Pyrroline-5-carboxylate dehydrogenase from Thermus thermophilus (TtP5CDh) has been crystallized in a citrate-bound form (TtP5CDh-cit). The crystals diffracted to well beyond 2 Å resolution, but exhibited perfect or near-perfect hemihedral twinning. Variation of crystallization conditions resulted in the growth of larger untwinned crystals or crystals with significantly reduced twin content, all with similar unit-cell parameters. The soaking of TtP5CDh-cit crystals in citrate-free solution produced crystals of the apo form (TtP5CDh-apo). The TtP5CDh-apo crystals belong to space group R3, with unit-cell parameters a = b = 102.29, c = 279.28 Å, and diffract to 1.08 Å. Crystals soaked in solution with NAD⁺ (TtP5CDh-NAD), NADH (TtP5CDh-NADH) and glutamate (TtP5CDh-Glu) were also prepared and characterized.