Kaminskaia N V, He C, Lippard S J
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Inorg Chem. 2000 Jul 24;39(15):3365-73. doi: 10.1021/ic000169d.
The stable dinuclear complex Zn2(BPAM)(mu-OH)(mu-O2PPh2)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 as a nucleophile in the ensuing hydrolysis. The dinuclear model complex is only 1.8 times more reactive in hydrolyzing phosphodiesters than a 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 Zn2(BPAN)(mu-OH)(mu-O2PPh2)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 Zn2(BPAN)(mu-OH)(mu-O2PPh2)2 is not very reactive, despite its low pKa value. This low reactivity presumably arises from the two factors. First, the briding hydroxide and coordinated substrate in [Zn2(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.
稳定的双核配合物Zn2(BPAM)(μ-OH)(μ-O2PPh2)2（其中BPAN = 2,7-双[2-(2-吡啶基乙基)-氨基甲基]-1,8-萘啶）被选作模型，以研究金属水解酶中(μ-羟基)二锌(II)中心的反应活性。研究了两个反应，即磷酸二酯的水解和β-内酰胺的水解。这两个过程在体内分别由含锌(II)的酶催化：P1核酸酶和β-内酰胺酶。前者催化单链DNA和RNA的水解。在许多类型的致病细菌中表达的β-内酰胺酶负责β-内酰胺类抗生素药物的水解降解。在双核模型配合物促进的磷酸二酯水解的第一步中，底物取代桥连的二苯基次膦酸酯。桥连的氢氧化物作为广义碱使水去质子化，水在随后的水解中作为亲核试剂。双核模型配合物在水解磷酸二酯方面的反应活性仅比单核类似物Zn(bpta)(OTf)2（其中bpta = N,N-双(2-吡啶基甲基)-叔丁胺）高1.8倍。由Zn2(BPAN)(μ-OH)(μ-O2PPh2)2催化的β-内酰胺抗生素类似物头孢硝噻吩的水解涉及底物通过其羧酸根基团的单齿配位，随后锌(II)结合的末端氢氧化物对β-内酰胺羰基碳原子进行亲核攻击。四面体中间体的分解导致产物形成。单核配合物Zn(cyclen)-(NO3)2和Zn(bpta)(NO3)2（其中cyclen = 1,4,7,10-四氮杂环十二烷）在β-内酰胺水解中的反应活性与双核配合物相同。磷酸二酯和β-内酰胺水解的动力学和机理研究表明，Zn2(BPAN)(μ-OH)(μ-O2PPh2)2中的桥连氢氧化物尽管其pKa值较低，但反应活性并不高。这种低反应活性可能源于两个因素。首先，[Zn2(BPAN)(μ-OH)(底物)]2+中的桥连氢氧化物和配位底物没有正确排列以利于亲核攻击。其次，桥连氢氧化物的亲核性降低，因为它同时与两个锌(II)离子结合。
