, 2008). Two other residues, selleck chemicals Gln 157 and His 256, located in the active site cleft are essential for catalysis (Singh et al., 2008). From homology modelling studies, Cys 130 and His 256 have been proposed as two important residues for selective inhibitor development against Mt-Asd (Singh et al., 2008). Mt-DapA (Rv 2753c) catalyses an aldol condensation between l-aspartate-β-semialdehyde and pyruvate to form 2,3-dihydrodipicolinic acid (Kefala et al., 2008). Mt-dapA

has been expressed in E. coli, purified, crystallized and solved to 2.28 Å (Kefala & Weiss, 2006; Kefala et al., 2008). A ribbon model of Mt-DapA is shown in Fig. 2. The protein structure reveals a classical α8β8 ‘TIM barrel’, with an active site architecture similar to homologues from other bacteria (Kefala et al., 2008). DapA exists as a tetramer with an apparent molecular weight of approximately 120 kDa, with

two independent tetramers in the asymmetric unit (Kefala & Weiss, 2006). A recent study with a A204R variant (obligate dimer) revealed tetramerization to be nonessential for activity (Evans et al., 2011). Mt-DapB (Rv2773c) reduces the α,β-unsaturated cyclic imine 2,3-dihydrodipicolinic acid to yield 2,3,4,5-tetrahydrodipicolinic acid using NADH or NADPH with nearly PS-341 mouse equal efficiency with Km values of 3.2 ± 0.4 and 11.8 ± 1.5 μM, respectively (Cirilli et al., 2003). Mt-DapB occurs as a 100-kDa homotetramer (Kefala et al., 2005). The first reported Sitaxentan structures for Mt-DapB were ternary complexes with NADH/NADPH and the inhibitor pyridine-2,6-dicarboxylic acid (2,6-PDC) (Cirilli et al., 2003). In both structures, the enzyme was observed in a proposed closed conformation (Cirilli et al., 2003). Subsequent structures

of Mt-DapB have been solved in an apo form and also as a binary complex with its cofactor NADH (Janowski et al., 2009). The fold of Mt-DapB consists of an N-terminal Rossmann-like catalytic domain and C-terminal αβ sandwich tetramerization domain, which exhibit significant interdomain flexibility (Kefala et al., 2005; Janowski et al., 2009). A ribbon model of Mt-DapB is depicted in Fig. 2. Inhibitors of DapB have been identified by molecular modelling as well as from a conventional screening of a Merck library and screened against the Mt-DapB enzyme (Paiva et al., 2001). A number of sulphonamide inhibitors of DapB were identified by the molecular modelling approach. The Ki values of the inhibitors ranged from 7 to 48 μM, and the compounds inhibited competitively with respect to the substrate 2,3-dihydrodipicolinic acid; however, the sulphonamide compounds lacked good antimicrobial activity (Paiva et al., 2001). Compared to the E. coli enzyme, Mt-DapB has a larger substrate or inhibitor binding site because of differences in the shape of the pocket at the N-terminal end of β8 (β9 in E. coli enzyme) and the nearby hinge region (Cirilli et al., 2003).