Project Summary
Heme enzymes are powerhouses for catalyzing a plethora of pivotal reaction pathways in humans, and have
been the focus of intense research interest throughout the past century. The ubiquitous nature of these enzymes
implicates them in a range of physiological functionalities, as well as pathogenic pathways that lead to an array
of disease situations including major neurodegenerative and autoimmune conditions, and cancer. In light of that,
numerous research efforts have focused on targeting heme enzymes for therapeutic purposes, particularly
through meticulously designed selective inhibitors. Nonetheless, a clear mechanistic understanding of a majority
of these crucial enzymes is still in its infancy, impeding the rational design of mechanism-based inhibitors that
could emerge into promising drug targets. Markedly, most of the mechanistic studies thus far have focused on
high-valent heme intermediates, whereby details pertaining to mid-valent heme species (i.e., those containing
Fe(III) centers such as heme superoxo, peroxo, hydroperoxo, peroxynitrite, hyponitrite etc.) have remained
gravely understudied. Importantly, heme enzymes where mid-valent species are active oxidants and/or key
reaction intermediates exhibit a growing significance in human health, making their mechanisms of action of
supreme interest. This research program targets this impactful deficit in current knowledge via the utilization of
carefully designed synthetic mimic compounds as mechanistic probes. The proposed work is predominantly
geared toward gaining a rigorous understanding into heme enzyme mediated nitrogen oxide (NOx/y)
interconversion pathways, which, albeit indispensable for normal physiological functions, could lead to nitrative
and nitrosative stress in humans under certain conditions. Thus, this project will strive to delineate the exact
attributes of heme centers that dictate the geometric and electronic properties of heme-NOx/y intermediates,
thereby fine-tuning the structure-activity relationships that choregraph the precision of their biorelevant reactivity
pathways. In that, a medley of synthetic heme-NOx/y model complexes supported by systematically varied ligand
architectures will be employed (i.e., in relevance to both primary and secondary coordination spheres of the
heme center), and their generation, stabilities, and competencies to engage in biorelevant reactivities will be
evaluated in detail. Mechanistic investigations will be carried out under cryogenic conditions, which will shine
light on the identities and properties of pivotal reaction intermediates; these intermediates will be rigorously
analyzed using a combined approach encompassing both spectroscopic and theoretical elements. Moreover,
the reaction landscapes of interest will also be examined using exhaustive thermodynamic and kinetic studies;
a layer of information that is often exceedingly difficult to obtain solely based on enzymatic systems. This work
will therefore further the frontiers of knowledge relevant to NOx/y interconversions facilitated by heme enzymes,
which will directly benefit ongoing efforts based on such pathways that target the design and implementation of
novel drug candidates with enhanced effectivity and specificity.
