Crystal structure and kinetic study of dihydrodipicolinate synthase from Mycobacterium tuberculosis

The three-dimensional structure of the enzyme dihydrodipicolinate synthase (KEGG entry Rv2753c, EC 4.2.1.52) from Mycobacterium tuberculosis (Mtb-DHDPS) was determined and refined at 2.28 Å (1 Å=0.1 nm) resolution. The asymmetric unit of the crystal contains two tetramers, each of which we propose to be the functional enzyme unit. This is supported by analytical ultracentrifugation studies, which show the enzyme to be tetrameric in solution. The structure of each subunit consists of an N-terminal (β/α)₈-barrel followed by a C-terminal α-helical domain. The active site comprises residues from two adjacent subunits, across an interface, and is located at the C-terminal side of the (β/α)₈-barrel domain. A comparison with the other known DHDPS structures shows that the overall architecture of the active site is largely conserved, albeit the proton relay motif comprising Tyr¹⁴³, Thr⁵⁴ and Tyr¹¹⁷ appears to be disrupted. The kinetic parameters of the enzyme are reported: K_M^ASA=0.43±0.02 mM, K_M^pyruvate=0.17±0.01 mM and V_max=4.42±0.08 μmol·s⁻¹·mg⁻¹. Interestingly, the V_max of Mtb-DHDPS is 6-fold higher than the corresponding value for Escherichia coli DHDPS, and the enzyme is insensitive to feedback inhibition by (S)-lysine. This can be explained by the three-dimensional structure, which shows that the (S)-lysine-binding site is not conserved in Mtb-DHDPS, when compared with DHDPS enzymes that are known to be inhibited by (S)-lysine. A selection of metabolites from the aspartate family of amino acids do not inhibit this enzyme. A comprehensive understanding of the structure and function of this important enzyme from the (S)-lysine biosynthesis pathway may provide the key for the design of new antibiotics to combat tuberculosis.