Our research focuses on a group of proteins that contain
iron-sulfur clusters. Iron-sulfur
clusters are ubiquitous in biology. Throughout evolution, iron-sulfur
clusters have become integral parts of diverse cellular functions such as
energy conversion, the citrate acid cycle, nitrogen fixation, intracellular
iron homeostasis, heme and biotin biosynthesis, DNA synthesis and DNA repair,
and regulation of gene expression. Research in my laboratory has primarily
focused on two related projects: the oxidative damages of iron-sulfur clusters
and the biogenesis of iron-sulfur proteins.
Oxidative stress is referred to as an excessive production of reactive oxygen
and nitrogen species inside cells. It has been implicated in causing
neurodegenerative disorders, cardiovascular diseases, cancers and other human
diseases. However, specific cellular targets of oxidative stresses still
largely remain elusive. Our research has revealed that iron-sulfur
clusters are readily modified by nitric oxide, a physiological free radical,
and form the protein-bound dinitrosyl iron complex. Such modification of
iron-sulfur clusters by nitric oxide leads to dramatic change of the protein
function. Using the DNA repair enzyme endonuclease III [4Fe-4S] cluster
as an example, we have also demonstrated that the activity of iron-sulfur
proteins can be fully restored when the nitric oxide-modified iron-sulfur
clusters are repaired. This would represent an intriguing recycling
mechanism of iron-sulfur clusters in cells under nitric oxide stress
conditions. Current research is to elucidate the redox reactions by which
iron-sulfur clusters are modified by reactive oxygen species and the cellular
repair mechanism for the modified iron-sulfur clusters.
The
second project in my laboratory is to elucidate the biogenesis of
iron-sulfur proteins. Recently, a highly conserved gene cluster iscSUA-hscBA-fdx
has been identified as critical for the iron-sulfur cluster assembly in
bacteria.
The
iscSUA-hscBA-fdx gene cluster in E. coli
Among
the six proteins (IscS, IscU, IscA, HscB, HscA and Ferredoxin) encoded by the
gene cluster iscSUA-hscBA-fdx, IscS is a cysteine desulfurase that
catalyzes desulfurization of L-cysteine and provides sulfur for the iron-sulfur
cluster assembly in IscU. IscU appears to act as a scaffold and
subsequently transfer the assembled iron-sulfur clusters to target
proteins. However, the iron donor for the iron-sulfur cluster assembly
largely remains elusive. Our research has revealed that IscA from E.
coli is an iron binding protein with an iron association constant of 3x1019M-1
and that the iron-bound IscA can provide iron for the iron-sulfur cluster
assembly in protein. These observations led us to propose that the
primary function of IscA is to recruit intracellular iron and deliver iron for
the iron-sulfur cluster assembly in cells. In collaboration with Dr.
Marcia Newcomer, we have determined the x-ray crystal structure of
IscA. In the crystal structure model,
IscA exists as a tetramer with potential two iron binding sites in a central
channel formed by the association of IscA monomers. The model is
consistent with the hypothesis that IscA is capable of binding iron in the
central channel of tetrameric IscA.
Crystal Structure of E. coli IscA
A Working Model for the
Biogenesis of Iron-Sulfur Clusters
