Shearer J, Kung IY, Lovell S, Kaminsky W, Kovacs JA. 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. J. Am. Chem. Soc. 123 (2001) 463-468.
To determine how a substantially inert metal can play a catalytic role in
the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate CoIII
thiolate complex ([CoIII(S2Me2N3(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
towards 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 thiolate,
and with similar entropies and enthalpies (thiocyanate: dH = -7.5 +- 1.1
kcal/mol. dS = -17.2 +- 3.2 eu; azide: dH = -6.5 +- 1.0 kcal/mol, dS = -12.6
+- 2.4 eu). The rates of azide and thiozyanate displacement from the metal
center are also comparable to one another (kd = (7.22 +- 0.04) x 10-1 s-1
for thiocyanate and kd = (2.14 +- 0.50) x 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 prohibitively slow so as to disallow catalytic action in nonredox
active cobalt metalloenzymes.