Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560012, India.
J Mol Biol. 2010 Jul 9;400(2):171-85. doi: 10.1016/j.jmb.2010.04.064. Epub 2010 May 6.
Kinetic measurements of enzyme activity indicate that type I pantothenate kinase from Mycobacterium tuberculosis has dual substrate specificity for ATP and GTP, unlike the enzyme from Escherichia coli, which shows a higher specificity for ATP. A molecular explanation for the difference in the specificities of the two homologous enzymes is provided by the crystal structures of the complexes of the M. tuberculosis enzyme with (1) GMPPCP and pantothenate, (2) GDP and phosphopantothenate, (3) GDP, (4) GDP and pantothenate, (5) AMPPCP, and (6) GMPPCP, reported here, and the structures of the complexes of the two enzymes involving coenzyme A and different adenyl nucleotides reported earlier. The explanation is substantially based on two critical substitutions in the amino acid sequence and the local conformational change resulting from them. The structures also provide a rationale for the movement of ligands during the action of the mycobacterial enzyme. Dual specificity of the type exhibited by this enzyme is rare. The change in locations of ligands during action, observed in the case of the M. tuberculosis enzyme, is unusual, so is the striking difference between two homologous enzymes in the geometry of the binding site, locations of ligands, and specificity. Furthermore, the dual specificity of the mycobacterial enzyme appears to have been caused by a biological necessity.
酶活性的动力学测量表明，结核分枝杆菌 I 型泛酸激酶对 ATP 和 GTP 具有双重底物特异性，与来自大肠杆菌的酶不同，后者对 ATP 的特异性更高。两个同源酶特异性差异的分子解释是由与（1）GMPPCP 和泛酸、（2）GDP 和磷酸泛酸、（3）GDP、（4）GDP 和泛酸、（5）AMPPCP 和（6）GMPPCP 形成复合物的结核分枝杆菌酶的晶体结构提供的，这里报道的，以及先前报道的涉及辅酶 A 和不同腺嘌呤核苷酸的两种酶复合物的结构。该解释主要基于氨基酸序列中的两个关键取代以及由此产生的局部构象变化。这些结构还为分枝杆菌酶作用过程中配体的迁移提供了依据。这种酶表现出的双重特异性很少见。在结核分枝杆菌酶的情况下观察到配体在作用过程中位置的变化是不寻常的，两个同源酶在结合位点的几何形状、配体的位置和特异性方面也存在显著差异。此外，分枝杆菌酶的双重特异性似乎是由生物学必要性引起的。
