Cryobaroenzymic studies as a tool for investigating activated complexes: creatine kinase.ADP.Mg.nitrate.creatine as a model.

By combining cryoenzymology with baroenzymology (a technique we term "cryobaroenzymology") one can obtain "stop-action" pictures of the intermediates in an enzyme reaction pathway and then observe their structural and energetic features ("motion features"). We illustrate the potential of this approach by considering the formation of a transient state analogue complex of creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2): enzyme.ADP.nitrate.creatine, where nitrate mimics the transferable gamma-phosphate of ATP. Formation of the analogue complex is accompanied by a conformational change that manifests itself by tryptophan perturbation and thus allows kinetic studies by the stopped-flow method. We studied the formation of the analogue complex under cryoenzymic conditions as a function of pressure and solvent composition. This allowed a detailed description of the structural and energetic features of the activation process of an elementary step in an enzyme pathway.