The S-adenosyl-L-methionine-dependent methyltransferase Rv0560c of Mycobacterium tuberculosis belongs to an orthologous group of heterocyclic toxin methyltransferases (Htm) which likely contribute to resistance of mycobacteria towards antimicrobial natural compounds as well as drugs. Htm_M.t. catalyzes the methylation of the Pseudomonas aeruginosa toxin 2-heptyl-1-hydroxyquinolin-4(1H)-one (also known as 2-heptyl-4-hydroxyquinoline N-oxide), a potent inhibitor of respiratory electron transfer, its 1-hydroxyquinolin-4(1H)-one core (QNO), structurally related (iso)quinolones, and some mycobactericidal compounds. In this study, crystal structures of Htm_M.t. in complex with S-adenosyl-L-homocysteine (SAH) and the methyl-accepting substrates QNO or 4-hydroxyisoquinoline-1(2H)-one, or the methylated product 1-methoxyquinolin-4(1H)-one, were determined at < 1.9 Å resolution. The monomeric protein exhibits the typical Rossmann fold topology and conserved residues of class I methyltransferases. Its SAH binding pocket is connected via a short tunnel to a large solvent-accessible cavity, which accommodates the methyl-accepting substrate. Residues W44, F168, and F208 in connection with F212 form a hydrophobic clamp around the heteroaromatic ring of the methyl-accepting substrate and likely play a major role in substrate positioning. Structural and biochemical data suggest that H139 and T136 are key active site residues, with H139 acting as general base that activates the methyl-accepting hydroxy group. Our structural data may contribute to the design of Htm inhibitors or of antimycobacterial drugs unamenable for methylation.

结核分枝杆菌的S-腺苷-L-甲硫氨酸依赖性甲基转移酶 Rv0560c属于杂环毒素甲基转移酶 (Htm) 的直系同源组，可能有助于分枝杆菌对抗菌天然化合物和药物的耐药性。Htm _{_} 催化绿脓杆菌毒素 2-heptyl-1-hydroxyquinolin-4(1 H )-one（也称为 2-heptyl-4-hydroxyquinoline N - oxide）的甲基化，它是呼吸电子转移的有效抑制剂，其 1- hydroxyquinolin-4(1 H )-one core (QNO)、结构相关的 (iso)quinolones 和一些分枝杆菌化合物。在这项研究中，Htm 的晶体结构_Mt与S-腺苷-L-同型半胱氨酸 (SAH) 和甲基接受底物 QNO 或 4-羟基异喹啉-1(2 H )-one 或甲基化产物 1-甲氧基喹啉-4(1 H ) 复合)-一个，以 < 1.9 Å 的分辨率测定。单体蛋白表现出典型的罗斯曼折叠拓扑结构和 I 类甲基转移酶的保守残基。它的 SAH 结合袋通过一条短隧道连接到一个大的溶剂可接近的空腔，该空腔容纳甲基接受底物。残基 W44、F168 和 F208 与 F212 结合在甲基接受底物的杂芳环周围形成疏水夹，并可能在底物定位中起主要作用。结构和生化数据表明 H139 和 T136 是关键的活性位点残基，而 H139 作为激活甲基接受羟基的通用碱基。我们的结构数据可能有助于设计 Htm 抑制剂或无法进行甲基化的抗分枝杆菌药物。