Previously, the ACT domain has been identified as modular regulatory unit associated with the control of variety of metabolic processes [9], [32], [33] and [34]. ACT1 (residues 295–372) has a βαββαβ topology similar to the typical ACT domain and was first identified in the structure of 3-phosphoglycerate dehydrogenase (PGDH; PDB 1YGY) [35]. ACT2 made up of C-terminal residues 372–437 has the topology βαββα and another β strand (residues 283–295) located before the ACT1

to complete the ACT domain architecture. This arrangement is first NVP-BKM120 identified in the AtAK [28]. The allosteric mechanisms associated with the ACT domains are generally linked to ligand binding to these domains elicits structural changes that alter the catalytic function at the active site located at the other region of the enzyme [36]. The active

biological unit of aspartate kinases is homodimeric which is formed between identical ACT domains from two neighboring subunits. ACT1 domains from chain A and B are arranged side-by-side with the creation of two equivalent effector Selleckchem AZD2281 binding sites at the interface. Similarly, ACT2 of one monomer interacts with the ACT2 of the other monomer. Thus, the entire regulatory domain consists of the four ACT domains making the core of 16 strands with eight-stranded antiparallel β-sheet with four helices on each side. The homodimeric arrangement of CaAK closely resembles the T-state conformation of the AK structures ( Fig. 3A and B). It was hypothesized from the crystal structures of EcAKIII (PDB Ids 2J0X and 2J0W) that binding of lysine to the enzyme induces the conformational

transition from the R-state to T-state ( Fig. 3B and C). Close inspection of the electron density map reveals that two Lys molecules are bound at the ACT1 dimer interface of CaAK ( Fig. 7A) similar to the other lysine bound AK crystal structures further supporting a T-state conformation of our Ca AK structure. Further, the mean solvent accessible surface area (SASA) for the isolated Ca AK monomers and dimers are calculated to be 20,227 and 36,571 Å2, respectively. The mean SASA between monomers and dimers is approximately 3880.6 and 7761 Å2, respectively. These values are about 3% less when compared to the other structures of class Nintedanib (BIBF 1120) I AKs ( Table 3). The dimer interface present in the CaAK is noteworthy for hydrophobic interactions that stabilize the homodimer including the interactions with the lysine bound between the ACT domains. The residues which are involved in dimeric interactions are shown in blue letters at the top of the sequence ( Fig. 1). Dimerization of AK in solution has been reported [26], [27], [28] and [37] and has been also identified in the crystal state by X-ray crystallography. Further, nearly all class I AK crystal structures bound to effector molecules have been crystallized as a dimer of dimers.