Reactivity of μ-hydroxodizinc(II) centers in enzymatic catalysis through model studies

The stable dinuclear complex [Zn-2(BPAN) (mu-OH)(mu-O2PPh2)] (ClO4)(2), where BPAN = 2,7-bis [2-(2-pyridylethyl)-aminomethyl]-1,8-naphthyridine, was chosen as a model to investigate the reactivity of (mu-hydroxo)dizinc(II) centers in metallohydrolases. Two reactions, the hydrolysis of phosphodiesters and the hydrolysis of beta-lactams, were studied. These two processes are catalyzed in vivo by zinc(II)-containing enzymes: P1 nucleases and beta-lactamases, respectively. The former catalyzes the hydrolysis of single-stranded DNA and RNA. beta-Lactamases, expressed in many types of pathogenic bacteria, are responsible for the hydrolytic degradation of beta-lactam antibiotic drugs. In the first step of phosphodiester hydrolysis promoted by the dinuclear model complex, the substrate replaces the bridging diphenylphosphinate. The bridging hydroxide serves as a general base to deprotonate water, which acts asa nucleophile in the ensuing hydrolysis. The dinuclear model complex is only 1.8 times more reactive in hydrolyzing phosphodiesters thana mononuclear analogue, Zn(bpta)(OTf)(2), where bpta = N,N-bis- (2-pyridylmethyl)-tert-butylamine. Hydrolysis of nitrocefin, a beta-lactam antibiotic analogue, catalyzed by [Zn-2(BPAN)(mu-OH)(mu-O2PPh2)](ClO4)(2) involves monodentate coordination of the substrate via its carboxylate group, followed by nucleophilic attack of the zinc(II)-bound terminal hydroxide at the beta-lactam carbonyl carbon atom. Collapse of the tetrahedral intermediate results in product formation. Mononuclear complexes Zn(cyclen)-(NO3)(2) and Zn(bpta)(NO3)(2), where cyclen 1,4,7,10-tetraazacyclododecane, are as reactive in the beta-lactam hydrolysis as the dinuclear complex. Kinetic and mechanistic studies of the phosphodiester and beta-lactam hydrolyses indicate that the bridging hydroxide in [Zn-2(BPAN)(mu-OH)(mu-O2PPh2)] (ClO4)(2) is not very reactive, despite its low pK, value. This low reactivity presumably arises from the two factors. First, the bridging hydroxide and coordinated substrate in [Zn-2(BPAN)(mu-OH)(substrate)](2+) are not aligned properly to favor nucleophilic attack. Second, the nucleophilicity of the bridging hydroxide is diminished because it is simultaneously bound to the two zinc(II) ions.