Insiya Fidai

Insight into the Recognition, Binding, and Reactivity of Catalytic Metallodrugs Targeting Stem Loop IIb of Hepatitis C IRES RNA

The complex Cu‐GGHYrFK‐amide (1‐Cu) was previously reported as a novel metallotherapeutic that catalytically inactivates stem loop IIb (SLIIb) of the hepatitis C virus (HCV) internal ribosomal entry site (IRES) RNA and demonstrates significant antiviral activity in a cellular HCV replicon assay. Herein we describe additional studies focused on understanding the cleavage mechanism as well as the relationship of catalyst configuration to structural recognition and site‐selective cleavage of the structured RNA motif. These are advanced by use of a combination of MALDI‐TOF mass spectrometry, melting temperature determinations, and computational analysis to develop a structural model for binding and reactivity toward SLIIb of the IRES RNA. In addition, the binding, reactivity, and structural chemistry of the all‐D‐amino acid form of this metallopeptide, complex 2‐Cu, are reported and compared with those of complex 1‐Cu. In vitro RNA binding and cleavage assays for complex 2‐Cu show a KD value of 76±3 nM, and Michaelis–Menten parameters of kcat=0.14±0.01 min−1 and KM=7.9±1.2 μM, with a turnover number exceeding 40. In a luciferase‐based cellular replicon assay Cu‐GGhyrfk‐amide shows activity similar to that of the 1‐Cu parent peptide, with an IC50 value of 1.9±0.4 μM and cytotoxicity exceeding 100 μM. RT‐PCR experiments confirm a significant decrease in HCV RNA levels in replicon assays for up to nine days when treated with complex 1‐Cu in three‐day dosing increments. This study shows the influence that the α‐carbon stereocenter has for this new class of compounds, while detailed mass spectrometry and computational analyses provide new insight into the mechanisms of recognition, binding, and reactivity.

Iron–sulfur cluster exchange reactions mediated by the human Nfu protein

Human Nfu is an iron–sulfur cluster protein that has recently been implicated in multiple mitochondrial dysfunctional syndrome (MMDS1). The Nfu family of proteins shares a highly homologous domain that contains a conserved active site consisting of a CXXC motif. There is less functional conservation between bacterial and human Nfu proteins, particularly concerning their Iron–sulfur cluster binding and transfer roles. Herein, we characterize the cluster exchange chemistry of human Nfu and its capacity to bind and transfer a [2Fe–2S] cluster. The mechanism of cluster uptake from a physiologically relevant [2Fe–2S](GS)4 cluster complex, and extraction of the Nfu-bound iron–sulfur cluster by glutathione are described. Human holo Nfu shows a dimer-tetramer equilibrium with a protein to cluster ratio of 2:1, reflecting the Nfu-bridging [2Fe–2S] cluster. This cluster can be transferred to apo human ferredoxins at relatively fast rates, demonstrating a direct role for human Nfu in the process of [2Fe–2S] cluster trafficking and delivery.

Glutathione-complexed [2Fe-2S] clusters function in Fe–S cluster storage and trafficking

Glutathione-coordinated [2Fe-2S] complex is a non-protein-bound [2Fe-2S] cluster that is capable of reconstituting the human iron-sulfur cluster scaffold protein IscU. This complex demonstrates physiologically relevant solution chemistry and is a viable substrate for iron-sulfur cluster transport by Atm1p exporter protein. Herein, we report on some of the possible functional and physiological roles for this novel [2Fe-2S](GS4) complex in iron-sulfur cluster biosynthesis and quantitatively characterize its role in the broader network of Fe–S cluster transfer reactions. UV–vis and circular dichroism spectroscopy have been used in kinetic studies to determine second-order rate constants for [2Fe-2S] cluster transfer from [2Fe-2S](GS4) complex to acceptor proteins, such as human IscU, Schizosaccharomyces pombe Isa1, human and yeast glutaredoxins (human Grx2 and Saccharomyces cerevisiae Grx3), and human ferredoxins. Second-order rate constants for cluster extraction from these holo proteins were also determined by varying the concentration of glutathione, and a likely common mechanism for cluster uptake was determined by kinetic analysis. The results indicate that the [2Fe-2S](GS4) complex is stable under physiological conditions, and demonstrates reversible cluster exchange with a wide range of Fe–S cluster proteins, thereby supporting a possible physiological role for such centers.

Cytosolic iron-sulfur cluster transfer-a proposed kinetic pathway for reconstitution of glutaredoxin 3.

Iron–sulfur (Fe–S) clusters are ubiquitously conserved and play essential cellular roles. The mechanism of Fe–S cluster biogenesis involves multiple proteins in a complex pathway. Cluster biosynthesis primarily occurs in the mitochondria, but key Fe–S proteins also exist in the cytosol. One such protein, glutaredoxin 3 (Grx3), is involved in iron regulation, sensing, and mediating [2Fe‐2S] cluster delivery to cytosolic protein targets, but the cluster donor for cytosolic Grx3 has not been elucidated. Herein, we delineate the kinetic transfer of [2Fe‐2S] clusters into Grx3 from potential cytosolic carrier/scaffold proteins, IscU and Nfu, to evaluate a possible model for Grx3 reconstitution in vivo.

Mapping cellular Fe–S cluster uptake and exchange reactions – divergent pathways for iron–sulfur cluster delivery to human ferredoxins

Ferredoxins are protein mediators of biological electron-transfer reactions and typically contain either [2Fe-2S] or [4Fe-4S] clusters. Two ferredoxin homologues have been identified in the human genome, Fdx1 and Fdx2, that share 43% identity and 69% similarity in protein sequence and both bind [2Fe-2S] clusters. Despite the high similarity, the two ferredoxins play very specific roles in distinct physiological pathways and cannot replace each other in function. Both eukaryotic and prokaryotic ferredoxins and homologues have been reported to receive their Fe-S cluster from scaffold/delivery proteins such as IscU, Isa, glutaredoxins, and Nfu. However, the preferred and physiologically relevant pathway for receiving the [2Fe-2S] cluster by ferredoxins is subject to speculation and is not clearly identified. In this work, we report on in vitro UV-visible (UV-vis) circular dichroism studies of [2Fe-2S] cluster transfer to the ferredoxins from a variety of partners. The results reveal rapid and quantitative transfer to both ferredoxins from several donor proteins (IscU, Isa1, Grx2, and Grx3). Transfer from Isa1 to Fdx2 was also observed to be faster than that of IscU to Fdx2, suggesting that Fdx2 could receive its cluster from Isa1 instead of IscU. Several other transfer combinations were also investigated and the results suggest a complex, but kinetically detailed map for cellular cluster trafficking. This is the first step toward building a network map for all of the possible iron-sulfur cluster transfer pathways in the mitochondria and cytosol, providing insights on the most likely cellular pathways and possible redundancies in these pathways.

Understanding the Molecular Basis of Multiple Mitochondrial Dysfunctions Syndrome 1 (MMDS1)—Impact of a Disease-Causing Gly208Cys Substitution on Structure and Activity of NFU1 in the Fe/S Cluster Biosynthetic Pathway

Iron-sulfur (Fe/S)-cluster-containing proteins constitute one of the largest protein classes, with varied functions that include electron transport, regulation of gene expression, substrate binding and activation, and radical generation. Consequently, the biosynthetic machinery for Fe/S clusters is evolutionarily conserved, and mutations in a variety of putative intermediate Fe/S cluster scaffold proteins can cause disease states, including multiple mitochondrial dysfunctions syndrome (MMDS), sideroblastic anemia, and mitochondrial encephalomyopathy. Herein, we have characterized the impact of defects occurring in the MMDS1 disease state that result from a point mutation (Gly208Cys) near the active site of NFU1, an Fe/S scaffold protein, via an in vitro investigation into the structural and functional consequences. Analysis of protein stability and oligomeric state demonstrates that the mutant increases the propensity to dimerize and perturbs the secondary structure composition. These changes appear to underlie the severely decreased ability of mutant NFU1 to accept an Fe/S cluster from physiologically relevant sources. Therefore, the point mutation on NFU1 impairs downstream cluster trafficking and results in the disease phenotype, because there does not appear to be an alternative in vivo reconstitution path, most likely due to greater protein oligomerization from a minor structural change.

Understanding the molecular basis for multiple mitochondrial dysfunctions syndrome 1 (MMDS1): impact of a disease‐causing Gly189Arg substitution on NFU1

Iron–sulfur (Fe/S) cluster‐containing proteins constitute one of the largest protein classes, with highly varied function. Consequently, the biosynthesis of Fe/S clusters is evolutionarily conserved and mutations in intermediate Fe/S cluster scaffold proteins can cause disease, including multiple mitochondrial dysfunctions syndrome (MMDS). Herein, we have characterized the impact of defects occurring in the MMDS1 disease state that result from a point mutation (p.Gly189Arg) near the active site of NFU1, an Fe/S scaffold protein. In vitro investigation into the structure–function relationship of the Gly189Arg derivative, along with two other variants, reveals that substitution at position 189 triggers structural changes that increase flexibility, decrease stability, and alter the monomer–dimer equilibrium toward monomer, thereby impairing the ability of the Gly189X derivatives to receive an Fe/S cluster from physiologically relevant sources.

Analysis of NFU‐1 metallocofactor binding‐site substitutions—impacts on iron–sulfur cluster coordination and protein structure and function

Iron–sulfur (Fe/S) clusters are ancient prosthetic groups found in numerous metalloproteins and are conserved across all kingdoms of life due to their diverse, yet essential functional roles. Genetic mutations to a specific subset of mitochondrial Fe/S cluster delivery proteins are broadly categorized as disease‐related under multiple mitochondrial dysfunction syndrome (MMDS), with symptoms indicative of a general failure of the metabolic system. Multiple mitochondrial dysfunction syndrome 1 (MMDS1) arises as a result of the missense mutation in NFU1, an Fe/S cluster scaffold protein, which substitutes a glycine near the Fe/S cluster‐binding pocket to a cysteine (p.Gly208Cys). This substitution has been shown to promote protein dimerization such that cluster delivery to NFU1 is blocked, preventing downstream cluster trafficking. However, the possibility of this additional cysteine, located adjacent to the cluster‐binding site, serving as an Fe/S cluster ligand has not yet been explored. To fully understand the consequences of this Gly208Cys replacement, complementary substitutions at the Fe/S cluster‐binding pocket for native and Gly208Cys NFU1 were made, along with six other variants. Herein, we report the results of an investigation on the effect of these substitutions on both cluster coordination and NFU1 structure and function. The data suggest that the G208C substitution does not contribute to cluster binding. Rather, replacement of the glycine at position 208 changes the oligomerization state as a result of global structural alterations that result in the downstream effects manifest as MMDS1, but does not perturb the coordination chemistry of the Fe–S cluster.

Analysis of Structure-Activity Relationships Based on the HCV SLIIb IRES RNA-Targeting GGHYRFK-Cu Complex

New therapeutics for targeting the hepatitis C virus (HCV) have been released in recent years. Although they are less prone to resistance, they are still administered in cocktails as a combination of drugs targeting various aspects of the viral life cycle. Herein, we aim to contribute to an arsenal of new HCV therapeutics by targeting the HCV internal ribosomal entry sequence (IRES) RNA through the development of catalytic metallodrugs that function to degrade rather than inhibit the target molecule. Based on a previously characterized HCV IRES stem‐loop IIb RNA‐targeting metallopeptide Cu‐GGHYrFK (1⋅Cu), an all‐l analogue (3⋅Cu) and a series of additional complexes with single alanine substitutions in the targeting domain were prepared and screened to determine the influence each amino acid side chain on RNA localization and recognition, and catalytic reactivity toward the RNA. Additional substitutions of the tyrosine position in complex 3⋅Cu were also investigated. Good agreement between calculated and measured binding affinities provided support for in silico modeling of the SLIIb RNA binding site and correlations with RNA cleavage sites. Examination of the cleavage products from reaction of the Cu complexes with SLIIb provided mechanistic insights, with the first observation of the 5′‐geminal diol and 5′‐phosphopropenal as products through the use of a Cu⋅ATCUN catalytic motif. Together, the data yielded insights into structure–function relationships that will guide future optimization efforts.