ALF3 MIMICS THE TRANSITION-STATE OF PROTEIN-PHOSPHORYLATION IN THE CRYSTAL-STRUCTURE OF NUCLEOSIDE DIPHOSPHATE KINASE AND MGADP

Nucleoside diphosphate kinase reversibly transfers the gamma-phosphate of ATP onto its active site histidine. We have investigated the trans ition state of histidine phosphorylation with the high-resolution crys tal structures of the enzyme from Dictyostelium discoideum with MgADP and either aluminium or beryllium fluoride. The bound aluminium fluori de species is the neutral species AlF3 and not the more common AlF4-. AlF3 forms a trigonal bipyramid that makes it an accurate analog of th e transition state of the gamma-phosphate of ATP undergoing transfer t o the catalytic histidine. Its axial ligands are a histidine nitrogen and a beta-phosphate oxygen. Beryllium fluoride also binds at the same position and with the same ligands but in a tetrahedral geometry rese mbling the Michaelis complex rather than the transition state. The two x-ray structures show explicit enzyme-substrate interactions that dis criminate between the ground and the transition states of the reaction . They also illustrate the partially dissociative geometry of the tran sition state of phosphoryl transfer and demonstrate the potential appl ications of metallofluorides for the study of kinase mechanisms.