The mechanism of aconitase: 1.8 angstrom resolution crystal structure of the S642A : citrate complex

The crystal structure of the S642A mutant of mitochondrial aconitase (mAc) with citrate bound has been determined at 1.8 Angstrom resolution and 100 K to capture this binding mode of substrates to the native enzyme. The 2.0 A ngstrom resolution, 100 K crystal structure of the S642A mutant with isocit rate binding provides a control, showing that the Ser --> Ala replacement d oes not alter the binding of substrates in the active site. The aconitase m echanism requires that the intermediate product, cis-aconitate, flip over b y 180 degrees about the C alpha-C beta double bond. Only one of these two a lternative modes of binding, that of the isocitrate mode, has been previous ly visualized. Now, however, the structure revealing the citrate mode of bi nding provides direct support for the proposed enzyme mechanism.