Reactivity of μ-hydroxodizinc(II) centers in enzymatic catalysis through model studies
Reactivity of μ-hydroxodizinc(II) centers in enzymatic catalysis through model studies
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DOI：
10.1021/ic000169d
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发表时间：
2000-07-24
影响因子：
4.6
通讯作者：
Lippard, SJ
中科院分区：
文献类型：
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作者：
Kaminskaia, NV;He, C;Lippard, SJ
以稳定的双核配合物[Zn-2（BPAN）（mu-OH）（mu-O2 PPh 2）]（ClO 4）（2）（BPAN = 2，7-bis [2-（2-pyridylethyl）-aminomethyl]-1，8-naphthyridine）为模型，研究了金属水解酶中（mu-hydroxy）dizzin（II）中心的反应性.研究了磷酸二酯的水解和β-内酰胺的水解两个反应。这两个过程在体内分别由含锌（II）的酶（P1核酸酶和β-内酰胺酶）催化。前者催化单链DNA和RNA的水解。β-内酰胺酶在许多类型的病原菌中表达，负责β-内酰胺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，底物取代桥接的二苯基次膦酸酯。桥氢氧化物作为一般碱去质子化水，其在随后的水解中阿萨核试剂。双核模型配合物在水解磷酸二酯中的反应性仅为单核类似物Zn（bpta）（OTf）（2）的1.8倍，其中bpta = N，N-双-（2-吡啶甲基）-叔丁胺。在[Zn-2（BPAN）（mu-OH）（mu-O2 PPh 2）]（ClO 4）（2）的催化下，β-内酰胺抗生素类似物头孢硝肟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在β-内酰胺羰基碳原子处的亲核攻击。四面体中间体的坍塌导致产物形成。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-内酰胺水解中与双核配合物一样具有反应性。磷酸二酯和β-内酰胺水解的动力学和机理研究表明，[Zn-2（BPAN）（mu-OH）（mu-O2 PPh 2）]（ClO 4）（2）中的桥连氢氧化物尽管pK值较低，但反应活性不高。这种低反应性大概是由这两个因素引起的。首先，[Zn-2（BPAN）（μ-OH）（底物）]（2+）中的桥接氢氧化物和配位底物没有适当地对齐以有利于亲核攻击。第二，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。
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.