Download Model Complexes for the Active Site of Nickel Superoxide

David M. Eichhorn, PhD, Department of Chemistry
Human biology relies on aerobic metabolism, i.e., the decomposition of foods using oxygen. Aerobic
metabolism produces reactive oxygen byproducts, including superoxide, which are toxic to the system.
The human body, and other organisms which obtain energy by aerobic metabolism, contain enzymes
whose purpose is to rapidly degrade these toxic compounds before they can cause damage. One class
of enzymes, Superoxide Dismutases (SODs), facilitate the decomposition of toxic superoxide.
Until recently, the known SODs all belonged to three classes, distinguished by the metals at their active sites: (i) manganese, (ii) iron,
and (iii) copper/zinc. In 1996, Youn, et al., reported a fourth class of SOD containing nickel (NiSOD). The structure of the active site
(shown at left, below) has a nickel atom bound to four atoms from the amino acids which make up the NiSOD protein – an amine
(NH2) nitrogen (green circle), an amide nitrogen (purple circle), and two thiolate S atoms (brown circles). The specific set of atoms
bound to the nickel undoubtedly influences the ability of the nickel to perform its function – decomposition of superoxide. The
approach that we will use is to synthesize small-molecule models for this active site to try to understand this effect. We have recently
made a nickel compound (I, shown at right) which bears some similarities to the NiSOD active site. It also has a Ni atom bound to two
thiolate S atoms (blue circles) and an amine N atom (green circle). The fourth atom bound to Ni is an imine N atom (purple circle),
which is similar, in some aspects, to the amide N in NiSOD. Thus, Compound I has essentially the same types of atoms bound
to the nickel as are present at the active site of the enzyme.
Structure of our
Compound I, which
resembles the NiSOD
structure
Structure of the
active site of the
NiSOD enzyme
There has been a significant interest in recent years in the relationship between oxidative stress – the
buildup of reactive oxygen species - and age-related diseases. In particular, rheumatoid arthritis,
osteoarthritis, osteoporosis, atherosclerosis, familial amyotrophic lateral sclerosis and
Parkinson’s disease, which have been associated with superoxide buildup and deficient SODs.
Studies in mice have also implicated SOD deficiency in the early onset of hearing loss. Although human
SODs have pharmacological issues that prevent their effective clinical use, SOD mimics – chemicals
which reproduce the activity of SODs - have found some clinical utility. We propose to make synthetic
compounds which reproduce the structure of the active site of Nickel SOD – the portion of the enzyme
where the decomposition of superoxide is actually carried out. By synthesizing and studying models
for the active sites of SODs we will gain a greater understanding of how these enzymes work.
The ultimate goal would be to establish methods for treating SOD deficiency. Potentially, the information
gained in this project could be used to develop compounds to be used for the treatment of oxidative
stress and the alleviation of diseases to which it contributes. We would gain an understanding of what
features are necessary in a compound that will be able to metabolize superoxide. It is even possible
that the SOD models to be investigated in this study, if they successfully catalyze the
decomposition of superoxide, can be used as a basis for the development of a new class of
drugs.
The Graduate Research Assistant (GRA) assigned to this project will be a MS or PhD candidate in the
Department of Chemistry. This individual will, in consultation with Dr. Eichhorn, design and carry out the
experiments described, including the synthesis of new compounds; characterization by X-ray
crystallography, spectroscopy, and electrochemistry; and performance of the superoxide decomposition
assays. All of the necessary chemicals will be provided by Dr. Eichhorn or the Department of Chemistry
and all necessary instrumentation is available in the Department of Chemistry.
The GRA will be supervised by Dr. Eichhorn. Thus, Dr. Eichhorn will train the GRA in all areas where
training is needed. Dr. Eichhorn and the GRA will jointly design the experiments and analyze the results.
Weekly meetings will be scheduled with Dr. Eichhorn and all other members of Dr. Eichhorn’s research
group to discuss the results of this and other projects. Publications resulting from the research effort will
be jointly authored, with the GRA responsible for writing the initial draft.
The project will be the subject of the MS thesis or PhD dissertation for the GRA. The GRA will benefit
from learning the various techniques, both of synthesis and characterization, that will be involved in
carrying out the experiments. Additionally, the GRA will learn the process of designing a research project
and changing direction in response to potentially unexpected results. Ultimately, as the project proceeds,
the GRA will be expected to become more independent in terms of project design and making decisions
regarding the directions in which to proceed.
Our approach for this study will consist of three parts:
1) Synthesize a series of Ni complexes related to Compound I, with systematic changes in the groups bound to Ni. This will
allow an analysis of how these groups affect the electronic properties of the Ni atom, and thereby its ability to catalyze the
decomposition of superoxide. The compounds to be synthesized are shown below.
Analogs of Compound I with
different ‘R’ groups to give
different thiolates. Three of
these have already been made.
Analogs of Compound I with
amide N instead of imine N.
Different thiolates will again
be incorporated.
2) Study the synthesized compounds using various physical methods. The first is X-ray crystallography, which will allow us to
determine the exact structures of the compounds that we have made. We will then use various forms of spectroscopy and
electrochemistry to investigate the electronic properties of these compounds. Ultimately, it is the electronic properties of NiSOD
which allow it to specifically catalyze superoxide decomposition. We will want to establish a correlation between the
structures of our compounds and their electronic properties in order to determine the influence of the varied thiolates and the
amide vs. imine substitution. We will also compare these to the properties reported for NiSOD.
3) Test the ability of the synthesized compounds to decompose superoxide. We will react each of the synthesized compounds
with superoxide to determine if, and how well, the compounds can catalyze superoxide decomposition. We will also analyze the
products of the superoxide decomposition in order to gain insight into how it is accomplished (the reaction mechanism). Using
these results and those from part (2), we will be able to correlate a compound’s structure with its electronic properties and its
activity with respect to superoxide decomposition.
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