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Bioinorganic Chemistry in the Franz Group
Research in the Franz Group is involved in elucidating the structural and functional consequences of metal ion coordination in biological systems. We are particularly interested in understanding the coordination chemistry utilized by biology to manage essential yet toxic species like copper and iron. Understanding these principles further guides our development of new chemical tools to manipulate biological metal ion location, speciation, and reactivity for potential therapeutic benefit.
Iron and copper are mineral nutrients that are required catalytic cofactors for numerous redox-active enzymes that execute essential life processes. Their utility stems from redox cycling between Fe(II)/Fe(III) and Cu(I)/Cu(II), a property that also leads to metal-promoted oxidative stress and eventually cell death if the metal ions are not properly regulated. Defects in biological regulation of these metal ions are therefore linked to a growing list of human diseases, including neurodegenerative diseases like Alzheimer’s and Parkinson’s. Targets of study aimed at deciphering the role of iron in Parkinson’s Disease include developing medicinal chelating agents and investigating model complexes of the pigment neuromelanin and the protein alpha-synuclein. We are also interested in understanding metal ion homeostasis and trafficking, and are currently investigating the copper-binding properties of proteins involved in cellular copper uptake. In addition, we are exploring how post-translational modifications of proteins, such as phosphorylation, influence their metal binding interactions. We use a combination of synthesis, spectroscopy, and biochemistry in our work.
Ionophores. Iron (Fe) and copper (Cu) play important roles in C. neoformans virulence, but these metals are also potent toxins if misappropriated. Based on the need of C. neoformans to utilize and maintain tight control of these two metals for survival and virulence, we hypothesized that small molecules capable of manipulating Fe and/or Cu bioavailability may have antimicrobial activity. To investigate this possibility, we tested a panel of metal chelating agents for their ability to inhibit C. neoformans growth under a variety of metal growth environments. We also characterized chemical and physical properties of these compounds, allowing us to correlate these properties with metal-dependent antifungal activity.
- Helsel, M. E.; White, E. J.; Razvi, S. Z. A.; Alies, B.; Franz, K. J., Chemical and functional properties of metal chelators that mobilize copper to elicit fungal killing of Cryptococcus neoformans. Metallomics 2017, 9 (1), 69-81.
ROS-Triggered Iron Chelators. A locally elevated iron load in brain regions affected by Parkinson’s disease insinuates iron as a source of cell-damaging hydroxyl radicals. While chelating agents are promising, there are significant challenges in developing agents that sequester deleterious iron without altering the distribution of healthy metal ions. Our strategy is to design pro-chelators that have little to no affinity for metal ions until a protective mask is selectively removed by hydrogen peroxide, a reactive oxygen species (ROS) that combines with iron to form hydroxyl radicals. Disease conditions therefore activate the chelating agent. In our first-generation pro-chelator shown below, a boronic ester conceals a latent phenolic oxygen that is a key donor atom of a high-affinity ligand that scavenges and incapacitates redox-active iron that is the source of OH• generation. These novel reagents will be valuable tools to test hypotheses about the role of iron in neurodegeneration and may have therapeutic potential.
- Charkoudian, L. K.; Pham, D. M.; Franz, K. J. “A Pro-Chelator Triggered by Hydrogen Peroxide Inhibits Iron-Promoted Hydroxyl Radical Formation” J. Am. Chem. Soc.; 2006, 128, 12424–12425.
- Charkoudian, L. K.; Pham, D. M.; Kwan, A.; Vangeloff, A.; Franz, K. J., “Modifications of Boronic Ester Pro-chelators Triggered by Hydrogen Peroxide Tune Reactivity to Inhibit Metal-Promoted Oxidative Stress,” Dalton Trans., 2007, 43, 5031–5042. cover art
- Iron Prochelator BSIH Protects Retinal Pigment Epithelial Cells against Cell Death Induced by Hydrogen Peroxide, J. Inorg. Biochem., 2008, 102, 2130-2135.
- Dickens, M. G.; Franz, K. J., A Prochelator Activated by Hydrogen Peroxide Prevents Metal-Induced Amyloid beta Aggregation. ChemBioChem 2009, doi: 10.1002/cbic.200900597
- “A Prochelator Activated by β-Secretase Inhibits Aβ Aggregation and Suppresses Copper-Induced Reactive Oxygen Species Formation,” Folk, D. S.; Franz, K. J., J. Am. Chem. Soc. 2010, 132, 4994-4995. 10.1021/ja100943r
Caged Metal Complexes. In addition to protecting cells from oxidative stress, we are also interested in promoting such reactivity. For this strategy we are incorporating photolabile groups into the backbone of ligands to “cage” stable metal chelates. The inspiration for these compounds comes from the very successful caged calcium compounds that have found widespread use in studying Ca(II) in neurotransmission, muscle contraction, and other biological processes. We are interested in caging transition metal ions and taking advantage of their change in reactivity post-photolysis. Activation with UV light induces bond cleavage and release of metal complexes that are both more bioavailable and reactive toward ROS, as shown in cartoon form in the adjacent figure. These new reagents will be valuable tools for on-demand delivery of metal ions to study mechanisms of metal ion trafficking, as well as applications such as chemotherapy where toxic metal ions can be released to induce cell death.
- “A Photolabile Ligand for Light-Activated Release of Caged Copper,” Ciesienski, K. L.; Haas, K. L.; Dickens, M. G.; Tesema, Y. T. Franz, K. J. J. Am. Chem. Soc.
- “A Caged Platinum(II) Complex that Increases Cytotoxicity upon Light Activation,“ Ciesienski, K. L.; Hyman, L. M.; Yang, D. T.; Haas, K. L.; Dickens, M. G.; R. J. Holbrook; and Franz, K. J. Eur. J. Inorg. Chem. DOI: 10.1002/ejic.201000098
- “Toward the Detection of Cellular Copper(II) by a Light-Activated Fluorescence Increase,” Ciesienski, K. L.; Hyman, L. M.; Derisavifard, S.; Franz, K. J., Inorg. Chem. 2010, 49, 6808-6810.
Copper Uptake. Copper is required in virtually all cell types from bacteria to humans, and cells have elaborate systems for its uptake, transport, and distribution. In order to elucidate the molecular mechanisms by which cells acquire copper, we are investigating the copper binding interactions of peptide sequences found on the extracellular region of high-affinity copper transport proteins in the Ctr family. The unique feature of the extracellular region is the presence of methionine-rich domains arranged as MXXM or MXM motifs containing 3–5 methionine residues per “Mets” motif. We use peptide synthesis to create a series of MX2MX2M peptides and investigate their copper-binding properties by mass spectrometry, UV-vis spectroscopy and cyclic voltammetry. The Mets peptides bind selectively to Cu(I) with impressive affinity. Removing even one of the methionine residues abolishes this capability and establishes the MX2MX2M motif as a Cu(I) binding domain.
- Jiang, J.; Nadas, I. A.; Kim, M. A.; Franz, K. J. “A Mets Motif Peptide Found in Copper Transport Proteins Selectively Binds Cu(I) with Methionine-Only Coordination” Inorg. Chem., 2005, 44, 9787–9794.
- Crider, S. E.; Holbrook, R. J.; Franz, K. J. “Coordination of Platinum therapeutic Agents to Met-Rich Motifs of hCtr1” Metallomics, DOI: 10.1039/b916899k
- Rubino, J. T.; Riggs-Gelasco, P.; Franz, K. J. “Methionine motifs of copper transport proteins provide general and flexible thioether-only binding sites for Cu(I) and Ag(I)” J. Biol. Inorg. Chem., DOI: 10.1007/s00775-010-0663-9
Phosphorylation-Dependent Metal Binding. Proteins often bind metal ions by using the oxygen, nitrogen, or sulfur-containing functional groups of the amino acids that make up the protein. A protein’s complexity increases, however, by the action of enzymes that chemically modify specific amino acids, for example by adding a phosphate group. We are interested in understanding how protein phosphorylation changes a protein’s affinity for metal ions and how such phosphoprotein-metal complexes consequently alter protein conformation, as shown schematically below.
By using Tb3+ as a luminescent probe, we demonstrated that the phosphorylation state of a 14-residue peptide fragment of a-synuclein, a protein implicated in Parkinson’s disease, dramatically affects the metal ion affinity of the peptide. Whereas the unphosphorylated peptide and its phosphoserine analogue show weak Tb3+ binding, the phosphotyrosine analogue shows tight 1:1 binding as well as 2:1 and 3:1 Tb:peptide adducts. Our data suggest that the phosphorylated amino acid must be appropriately positioned among additional ligating residues to establish this phosphorylation-dependent metal binding.
- “Phosphorylation of an a-Synuclein Peptide Fragment Enhances Metal Binding” Liu, L. L.; Franz, K. J. J. Am. Chem. Soc. 2005, 127, 9662–9663.
- “Metal Binding Studies of Phosphorylated a-Synuclein Peptide Fragments” Liu, L. L.; Franz, K. J.; J. Biol. Inorg. Chem., 2007, 12, 234–247 .