The successful substructure solution presented here adds to the database of largest selenium substructures that has been determined to date [30]. Although the diffraction

limit of the CaAK crystals was relatively low (3 Å resolution). However, the resolution was compensated by the significant level of non-crystallographic symmetry (NCS) restraints, enabling refinement of the structure. The overall geometry of the model is of good quality, with 86% of the residues in the most favored regions and 14% in allowed regions of the Ramachandran map and model was refined to an R-factor of 20.7% (Rfree of 27.3%). CaAK monomer belongs to the class I type AKs which consists of one catalytic domain and two ACT domains ( Fig. 1a) [25]. The superposition of complete chain of A on the other 11 chains yields root-mean-square deviation (r.m.s.d) ICG-001 mw between 0.68 Å and 1.36 Å, indicating that all 12 chains in the asymmetric unit of the CaAK crystal are similar. The superposition of CaAK dimer AB on the other dimers CD, EF, GH, IJ and KL in the asymmetric unit yield r.m.s.d’s of 1.1 Å, 1.86 Å, 1.5 Å, 1.63 Å and 1.67 Å, respectively. The active biological KU-57788 chemical structure unit of aspartate kinases is homodimeric which is formed between identical ACT domains from two neighboring subunits ( Fig. 1b). ACT1 domains

from chain A and B are arranged side-by-side with the creation of two equivalent effector binding sites at the interface. Similarly, ACT2 of one monomer interacts with the ACT2 of the other monomer. The homodimers are further associates into CaAK tetramer ( Casein kinase 1 Fig. 1c). There were three tetramers of CaAK observed in the asymmetric unit. A simultaneous least-squares superposition of the tetramer ABCD on to EFGH and IJKL tetramers results in alignment with r.m.s.d’s of 2.4 and 2.9 Å, respectively. The three tetramers of CaAK comprise six homodimers which exhibits essentially identical overall dimeric

architecture. The overall fold is similar to the other class I AKs although these shares very low sequence identity. Specifically, Fig. 2 compares E. coli aspartate kinase III (EcAkIII-PDB 2J0X and 2J0W with r.m.s.d 2.2 Å and 3.8 Å, respectively; 25.9% sequence identity) [26], A. thaliana aspartate kinase (AtAK-PDB 2CDQ; rmsd 3.0 Å; 26% sequence identity) [28], and M. jannaschii aspartate kinase (MjAK-PDB 3C1N, 3C20 and 3C1M with rmsd 2.6 Å, 3.0 Å and 4.3 Å, respectively; 27.9% sequence identity) [27]. The N-terminal domain of CaAK is considered to be the catalytic domain (AKα-residues 1–282) and belongs to the amino-acid kinase family [31] with a conserved eight-stranded β-sheet sandwiched between two layers of α-helices. The catalytic domain is further divided into the N-terminal lobe (residues 1–200 shown in purple) and the C-terminal lobe (residues 201–282 shown in brown color) [26], [27] and [28].