NDLI: Why Is There an “Inert” Metal Center in the Active Site of Nitrile Hydratase? Reactivity and Ligand Dissociation from a Five-Coordinate Co(III) Nitrile Hydratase Model

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Why Is There an “Inert” Metal Center in the Active Site of Nitrile Hydratase? Reactivity and Ligand Dissociation from a Five-Coordinate Co(III) Nitrile Hydratase Model

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Why Is There an “Inert” Metal Center in the Active Site of Nitrile Hydratase? Reactivity and Ligand Dissociation from a Five-Coordinate Co(III) Nitrile Hydratase Model

Content Provider	PubMed Central
Author	Shearer, Jason Kung, Irene Y. Lovell, Scott Kaminsky, Werner Kovacs, Julie A.
Copyright Year	2001
Abstract	To determine how a substitutionally inert metal can play a catalytic role in the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate CoIII thiolate complex ([CoIII(S2 Me2N3(Pr,Pr))](PF6) (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 [CoIII(ADIT2)](PF6) (2). Complex 2 is a six-coordinate CoIII 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: ΔH = −7.5 ± 1.1 kcal/mol, ΔS = −17.2 ± 3.2 eu; azide: ΔH = −6.5 ± 1.0 kcal/mol, ΔS = −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) × 10−1 s−1 for thiocyanate and k d = 2.14 ± 0.50) × 10−2 s−1 for azide), and are considerably faster than one would expect for a low-spin d6 six-coordinate CoIII 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 CoIII center in a ligand environment that resembles NHase is not prohibitivly slow so as to disallow catalytic action in nonredox active cobalt metalloenzymes.
Starting Page	463
File Format	PDF
ISSN	15205126
e-ISSN	15205126
Journal	Journal of the American Chemical Society
Issue Number	3
Volume Number	123
Language	English
Publisher Date	2001-01-24
Access Restriction	Open
Subject Keyword	Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Chemistry Colloid and Surface Chemistry Biochemistry Catalysis

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