Shearer J, Kung I Y, Lovell S, Kaminsky W, Kovacs J A
Contribution from the Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
J Am Chem Soc. 2001 Jan 24;123(3):463-8. doi: 10.1021/ja002642s.
To determine how a substitutionally inert metal can play a catalytic role in the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate Co(III) thiolate complex (Co(III)(S(2)(Me2)N(3)(Pr,Pr)) (1)) that resembles the active site of cobalt containing nitrile hydratase (Co NHase) was prepared. This was screened for reactivity, by using low-temperature electronic absorption spectroscopy, toward a number of biologically relevant "substrates". It was determined 1 will react with azide, thiocyanate, and ammonia, but is unreactive toward nitriles, NO, and butyrate. Substrate-bound 1 has similar spectroscopic and structural properties as Co(III)(ADIT(2)) (2). Complex 2 is a six-coordinate Co(III) complex containing cis-thiolates and imine nitrogens, and has properties similar to the cobalt center of Co NHase. Substrate binding to 1 is reversible and temperature-dependent, allowing for the determination of the thermodynamic parameters of azide and thiocyanate binding and the rates of ligand dissociation. Azide and thiocyanate bind trans to a thiolate, and with similar entropies and enthalpies (thiocyanate: DeltaH = -7.5 +/- 1.1 kcal/mol, DeltaS = -17.2 +/- 3.2 eu; azide: DeltaH = -6.5 +/- 1.0 kcal/mol, DeltaS = -12.6 +/- 2.4 eu). The rates of azide and thiocyanate displacement from the metal center are also comparable to one another (k(d) = (7.22 +/- 0.04) x 10(-)(1) s(-)(1) for thiocyanate and k(d) = (2.14 +/- 0.50) x 10(-)(2) s(-)(1) for azide), and are considerably faster than one would expect for a low-spin d(6) six-coordinate Co(III) complex. These rates are comparable to those of an analogous Fe(III) complex, demonstrating that Co(III) and Fe(III) react at comparable rates when in this ligand environment. This study therefore indicates that ligand displacement from a low-spin Co(III) center in a ligand environment that resembles NHase is not prohibitively slow so as to disallow catalytic action in nonredox active cobalt metalloenzymes.
为了确定一种取代惰性金属如何在金属酶腈水合酶（NHase）中发挥催化作用，制备了一种具有反应活性的五配位钴（III）硫醇盐配合物（Co(III)(S(2)(Me2)N(3)(Pr,Pr)) (1)），其类似于含钴腈水合酶（Co NHase）的活性位点。通过低温电子吸收光谱法，对该配合物与多种具有生物学相关性的“底物”的反应活性进行了筛选。结果表明，1会与叠氮化物、硫氰酸盐和氨发生反应，但对腈、NO和丁酸盐无反应。与底物结合的1具有与Co(III)(ADIT(2)) (2)相似的光谱和结构性质。配合物2是一种六配位钴（III）配合物，含有顺式硫醇盐和亚胺氮，其性质与Co NHase的钴中心相似。底物与1的结合是可逆的且依赖于温度，这使得可以测定叠氮化物和硫氰酸盐结合的热力学参数以及配体解离速率。叠氮化物和硫氰酸盐以反式方式与硫醇盐结合，且具有相似的熵和焓（硫氰酸盐：ΔH = -7.5 ± 1.1 kcal/mol，ΔS = -17.2 ± 3.2 eu；叠氮化物：ΔH = -6.5 ± 1.0 kcal/mol，ΔS = -12.6 ± 2.4 eu）。叠氮化物和硫氰酸盐从金属中心的取代速率也彼此相当（硫氰酸盐的k(d) = (7.22 ± 0.04) x 10(-)(1) s(-)(1)，叠氮化物的k(d) = (2.14 ± 0.50) x 10(-)(2) s(-)(1)），并且比人们预期的低自旋d(6)六配位钴（III）配合物要快得多。这些速率与类似的铁（III）配合物相当，表明在这种配体环境中，Co(III)和Fe(III)以相当的速率发生反应。因此，这项研究表明，在类似于NHase的配体环境中，从低自旋Co(III)中心进行的配体取代反应不会慢到禁止非氧化还原活性钴金属酶中的催化作用。
