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Research Interests:
Iron-sulfur clusters are ubiquitous redox cofactors. They exist in the forms
of [4Fe-4S] cluster, [2Fe-2S] cluster, [3Fe-4S] cluster, and other more complicated
clusters with other metals. Throughout evolution, iron-sulfur clusters have
become integral parts of diverse cellular functions including energy conversion,
the citrate acid cycle, nitrogen fixation, amino acid metabolism, intracellular
iron homeostasis, biogenesis of heme, biotin and lipoic acid, DNA repair, RNA
modification, and the regulation of gene expression. Our research has primarily
focused on two closely related projects: biogenesis of iron-sulfur clusters
and modification of iron-sulfur clusters by nitric oxide.
Biogenesis of iron-sulfur clusters: At least six
highly conserved proteins, IscS, IscU, IscA, HscB, HscA and Ferredoxin encoded
by a gene cluster iscSUA-hscBA-fdx, have been identified as essential for general
biogenesis of iron-sulfur clusters in E. coli. Among the six proteins, IscS
is a cysteine desulfurase that catalyzes desulfurization of L-cysteine and transfers
sulfane sulfur to a proposed scaffold IscU for the iron-sulfur cluster assembly.
IscU acts as a scaffold protein that transfers the assembled clusters to target
proteins. HscB and HscA are two heat shock cognate proteins that modulate the
transfer of the assembled clusters from IscU to target proteins. The function
of IscA, however, remains elusive. Our research has demonstrated that IscA is
a novel iron binding protein with an iron association constant of 2x1019M-1,
and that the iron-loaded IscA can provide the iron for the iron-sulfur cluster
assembly in IscU under physiologically relevant conditions. The results led
us to propose that the primary function of IscA is to recruit the intracellular
“free” iron and deliver the iron for the iron-sulfur cluster assembly
in IscU. The current goals of this project are to define the structure of the
iron binding site in IscA, to determine the iron transfer process from IscA
to IscU, and to explore the iron acquisition mechanism of IscA under different
physiological conditions.
Modification of iron-sulfur clusters by nitric oxide:
Excessive production of nitric oxide has been implicated in causing neurodegenerative
disorders, cardiovascular diseases, cancers, and aging. The specific cellular
targets of nitric oxide cytotoxicity, however, are not fully understood. We
have found that iron-sulfur clusters can be readily modified by nitric oxide,
forming the protein-bound dinitrosyl iron complexes (DNIC) both in vitro and
in vivo. Modification of iron-sulfur clusters will not only inactivate the proteins
that contain iron-sulfur clusters, the iron released from the modified iron-sulfur
clusters will further promote the production of reactive free radicals via Fenton
reaction. The current goals of this project are to understand the redox reactions
underlying the modification of iron-sulfur clusters by nitric oxide and to elucidate
the cellular repair mechanism for the modified iron-sulfur clusters. Two iron-sulfur
proteins, ferredoxin [2Fe-2S] cluster and the DNA repair enzyme endonuclease
III [4Fe-4S] cluster, are used as models. In addition, we are in the process
of developing proteomic approaches to investigate the modifications of cellular
iron-sulfur proteins when cells are subjected to nitric oxide stresses. .
1. Yang, J., Bitoun, J. & Ding, H. (2006) Interplay of IscA and IscU in Biogenesis of Iron-Sulfur Clusters. J. Biol. Chem. 281, 27956-27963.
2. Ding, H., Harrison, K. & Lu, J. (2005) Thioredoxin
Reductase System Mediates Iron Binding in IscA and Iron Delivery for the Iron-Sulfur
Cluster Assembly in IscU. J. Biol. Chem. 280, 30432-30437.
3. Ding, B., Smith, E. & Ding, H. (2005) Mobilization of
the iron center in IscA for the iron-sulfur cluster assembly in IscU. Biochem.
J. 389, 797-802.
4. Ding, H., Clark, R. J. & Ding, B. (2004) IscA mediates
iron delivery for assembly of iron-sulfur clusters in IscU under the limited
accessible free iron conditions. J. Biol. Chem. 279, 37499-37504.
5. Ding, H. & Clark, R. J. (2004) Characterization of Iron
Binding in IscA, An Ancient Iron-Sulfur Cluster Assembly Protein. Biochem.
J. 379, 433-440.
6. Bilder, P. W., Ding, H. & Newcomer, M. E (2004) Crystal
Structure of the Ancient, Fe-S Scaffold IscA Reveals a Novel Protein Fold. Biochemistry
43, 133-139.
7. Rogers, P. A., Eide, L., Klungland, A. & Ding, H. (2003)
Reversible inactivation of endonuclease III by nitric oxide via modification
of its [4Fe-4S] cluster. DNA Repair, 2, 809-817.
8. Yang, W., Rogers, P. A. & Ding, H. (2002) Repair of
Nitric Oxide-modified Ferredoxin [2Fe-2S] Cluster by Cysteine Desulfurase (IscS).
J. Biol. Chem. 277, 12868-12873.
9. Rogers, P. A. & Ding, H. (2001) L-cysteine-mediated
destabilization of initrosyl iron complexes in proteins. J. Biol. Chem.
276, 30980-30986.
10. Ding, H. & Demple, B. (2000) Direct nitric oxide signal
transduction via nitrosylation of iron-sulfur centers in the SoxR transcription
activator. Proc. Natl. Acad. Sci. USA. 97, 5146-5150.