ADENYLATE KINASE MOTIONS DURING CATALYSIS - AN ENERGETIC COUNTERWEIGHT BALANCING SUBSTRATE-BINDING

Background: Adenylate kinases undergo large conformational changes dur ing their catalytic cycle. Because these changes have been studied by comparison of structures from different species, which share approxima tely one-third of their residues, only rough descriptions have been po ssible to date. Results: We have solved the structure of unligated ade nylate kinase from Escherichia coli at 2.2 Angstrom resolution and com pared it with the high-resolution structure of the same enzyme ligated with an inhibitor mimicking both substrates, ATP and AMP. This compar ison shows that, upon substrate binding, the enzyme increases its chai n mobility in a region remote from the active center. As this region ' solidifies' again on substrate release, we propose that it serves as a 'counterweight' balancing the substrate binding energy. Conclusions: The comparison of two very different conformations of the same polypep tide chain revealed kinematic details of the catalytic cycle. Moreover , it indicated that there exists an energetic counterweight compensati ng the substrate binding energy required for specificity. This counter weight prevents the enzyme from dropping into a rate-reducing energy w ell along the reaction coordinate.