An Approach to More Accurate Model Systems for Purple Acid Phosphatases (PAPs)

The
active site of mammalian purple acid phosphatases (PAPs) have a dinuclear
iron site in two accessible oxidation states (FeIII2 and FeIIIFeII), and the heterovalent
is the active form, involved in the regulation of phosphate and phosphorylated
metabolite levels in a wide range of organisms. Therefore, two sites
with different coordination geometries to stabilize the heterovalent
active form and, in addition, with hydrogen bond donors to enable
the fixation of the substrate and release of the product, are believed
to be required for catalytically competent model systems. Two ligands
and their dinuclear iron complexes have been studied in detail. The
solid-state structures and properties, studied by X-ray crystallography,
magnetism, and Mössbauer spectroscopy, and the solution structural
and electronic properties, investigated by mass spectrometry, electronic,
nuclear magnetic resonance (NMR), electron paramagnetic resonance
(EPR), and Mössbauer spectroscopies and electrochemistry, are
discussed in detail in order to understand the structures and relative
stabilities in solution. In particular, with one of the ligands, a
heterovalent FeIIIFeII species has been produced
by chemical oxidation of the FeII2 precursor.
The phosphatase reactivities of the complexes, in particular, also
of the heterovalent complex, are reported. These studies include pH-dependent
as well as substrate concentration dependent studies, leading to pH
profiles, catalytic efficiencies and turnover numbers, and indicate
that the heterovalent diiron complex discussed here is an accurate
PAP model system.