The 2.1 Å Structure of Torpedo californica Creatine Kinase Complexed with the ADP-Mg2+−NO3-−Creatine Transition-State Analogue Complex†,‡

Creatine kinase (CK) catalyzes the reversible conversion of creatine and ATP to phosphocreatine and ADP, thereby helping maintain energy homeostasis in the cell. Here we report the first X-ray structure of CK bound to a transition-state analogue complex (CK−TSAC). Cocrystallization of the enzyme from Torpedo californica (TcCK) with ADP-Mg2+, nitrate, and creatine yielded a homodimer, one monomer of which was liganded to a TSAC complex while the second monomer was bound to ADP-Mg2+ alone. The structures of both monomers were determined to 2.1 A resolution. The creatine is located with the guanidino nitrogen cis to the methyl group positioned to perform in-line attack at the γ-phosphate of ATP-Mg2+, while the ADP-Mg2+ is in a conformation similar to that found in the TSAC-bound structure of the homologue arginine kinase (AK). Three ligands to Mg2+ are contributed by ADP and nitrate and three by ordered water molecules. The most striking difference between the substrate-bound and TSAC-bound structures is the...