An ab initio model system investigation of the proposed mechanism for activation of peroxidases: Cooperative catalytic contributions from the ion andmicrosolvent water
De. Woon et Gh. Loew, An ab initio model system investigation of the proposed mechanism for activation of peroxidases: Cooperative catalytic contributions from the ion andmicrosolvent water, J PHYS CH A, 102(50), 1998, pp. 10380-10384
In the enzymatic cycle of the peroxidase family of heme proteins, hydrogen
peroxide is transformed into the catalytically active species, an Fe=O heme
species known as compound I. The postulated mechanism involves the formati
on of a transient HOOH-Fe heme complex that is transformed via proton trans
fer to the oxywater isomer H2OO-Fe heme species, which is followed by facil
e cleavage of the O-O bond to yield compound I and water. The proton transf
er process is thought to be aided by a highly conserved distal histidine th
at serves as a catalyst. The plausibility of this step has been assessed in
this work by characterizing the isomerization of HOOH to H2OO in complexes
with Fe+, Fe2+, Fe3+, Na+, Mg2+ and Al3+ using second- order perturbation
theory and the coupled-cluster method in conjunction with various basis set
s. The putative catalytic role of a proton acceptor was investigated by det
ermining the influence of one microsolvent water on the Mg2+ system. The re
sults demonstrate that although the gas-phase isomerization is highly endot
hermic and possesses a large activation energy, the metal ion significantly
stabilizes the oxywater isomer. The Na+ - Mg2+ - Al3+ sequence of complexe
s reveals that the stabilization effect increases sharply with the charge o
n the ion. Fe+ and Fe2+ calculations found a small amount of additional sta
bilization with respect to the corresponding Na+ and Mg2+ systems. While th
e barrier to isomerization was not significantly reduced by binding peroxid
e to a cation alone, it was reduced dramatically when a single water was ad
ded to the Mg2+ system. The ion contributes to this effect by increasing th
e protonicity of the H that is being transferred, allowing it to interact m
ore strongly with the water. The proton transfer is thus strongly enhanced
by cooperative contributions by the metal ion and microsolvent water.