Affinity for the active site of the ground state MEDChem Express Ganetespib enzyme whereas slow Enzastaurin cost binding inhibitors show a high affinity for an intermediate state of the enzyme. Slow binding inhibition is characterized by an initial weak binding to the ground state enzyme, followed by tighter binding to the transition state structure. In general, this type of inhibition is considered more physiologically relevant since upstream accumulation of the substrate cannot relieve the inhibition brought about by this form of inhibition. The results presented here suggest that PLP is a slow tight binding inhibitor of ePL kinase. The mechanism of inhibition consists in the formation of a Schiff base between PLP and an active site lysine residue. The inactivation of the enzyme is faster when both PLP and MgADP are present, compared to when PLP is present alone or together with MgATP. Therefore, the inhibition occurs more rapidly during the catalytic turnover of the enzyme, in which the enzyme may go through an intermediate state whose conformation favors the covalent binding of PLP. It appears that during the catalytic cycle, or when both PLP and MgADP are bound, the active site of ePL kinase is in a conformation that places the e-amino group of K229 in a favorable position to form a covalent bond with C49 of PLP. The position of K229 in the active site structure of the unliganded ePL kinase is shown in Fig. 6B. Formation of an aldimine between PLP and the e-amino moiety of K229 is suggested by the absorption maximum at 420 nm and by the failure of PLP to bind tightly to K229Q ePL kinase and to inhibit its activity. The 336 nm absorbing band of the tightly bound PLP can be accounted for by several possible structures. One of the most probable is a carbinolamine intermediate, which occurs during the formation of the aldimine. In the carbinolamine structure, the C49 carbon of PLP is tetrahedral because of the addition of the e-amino moiety of K229 across the double bond to oxygen. Another possible structure is the enolimine tautomer of the PLP protonated aldimine also found at the active site of PLP-dependent enzymes. The rate of dissociation of PLP from the ePL kinaseNPLP complex is very slow, as shown by the CD studies in the presence of specific and non-specific PLP phosphatases. This slow rat
