Neutron crystallographic study of heterotrimeric glutamine amidotransferase CAB

Heterotrimeric glutamine amidotransferase CAB (GatCAB) possesses an ammonia-self-sufficient mechanism in which ammonia is produced and used in the inner complex by GatA and GatB, respectively. The X-ray structure of GatCAB revealed that the two identified active sites of GatA and GatB are markedly distant, but are connected in the complex by a channel of 30 Å in length. In order to clarify whether ammonia is transferred through this channel in GatCAB by visualizing ammonia, neutron diffraction studies are indispensable. Here, GatCAB crystals were grown to approximate dimensions of 2.8 × 0.8 × 0.8 mm (a volume of 1.8 mm³) with the aid of a polymer using microseeding and macroseeding processes. Monochromatic neutron diffraction data were collected using the neutron single-crystal diffractometer BIODIFF at the Heinz Maier-Leibnitz Zentrum, Germany. The GatCAB crystals belonged to space group P2₁2₁2₁, with unit-cell parameters a = 74.6, b = 94.5, c = 182.5 Å and with one GatCAB complex (molecular mass 119 kDa) in the asymmetric unit. This study represented a challenge in current neutron diffraction technology.