Europium Luminescence of EF-Hand Helix−Turn−Helix Chimeras: Impact of pH and DNA-Binding on Europium Coordination

Spectroscopic characterization of europium binding to a calmodulin-EF4 hand peptide-polymer conjugate

The emergence of biological ligand as an alternative to chemical ligands enables a sustainable lanthanide extraction route. In this study, a peptide originating from the loop of domain 4 calmodulin (EF4) was synthesized and the interaction with europium ions was monitored using time resolved laser fluorescence spectroscopy (TRLFS). Despite being retracted from its full protein structure, the twelve amino acids of calmodulin-EF4 showed binding to europium. Europium-peptide complex formation was evident by an increase in decay time from 110 to 187 μs. The spectra of europium bound to peptide can be easily distinguished from the free europium ion as the 5D0 → 7F2 peak intensifies. When europium bound to the peptide-polymer conjugate, the decay time was further increased to 259 μs. This suggests that lanthanide binding can be enhanced by immobilizing the short peptide into a polymer matrix. The europium-peptide/conjugate bond was reversible, triggered by pH, promoting peptide reusability. Due to the fact that the study was conducted exclusively in water, it suggests minimal use of chemicals is possible while maintaining peptide affinity. This makes the calmodulin-EF4 peptide an ideal candidate as biological ligand. This study lays the groundwork for developing a peptide-based filter material for lanthanide separation.

De novo design of Ln(III) coiled coils for imaging applications

A new peptide sequence (MB1) has been designed which, in the presence of a trivalent lanthanide ion, has been programmed to self-assemble to form a three stranded metallo-coiled coil, Ln(III)(MB1)₃. The binding site has been incorporated into the hydrophobic core using natural amino acids, restricting water access to the lanthanide. The resulting terbium coiled coil displays luminescent properties consistent with a lack of first coordination sphere water molecules. Despite this the gadolinium coiled coil, the first to be reported, displays promising magnetic resonance contrast capabilities.

Of Folding and Function: Understanding Active-Site Context through Metalloenzyme Design

In the emerging field of biomolecular design, the introduction of metal-binding sites into loop or turn regions of known protein scaffolds has been utilized to create unique metalloprotein and metallopeptide systems for study. This Forum Article highlights examples of the modular-turn-substitution approach to design and the range of structural and mechanistic questions to which this tool can be applied. Examples from the authors' laboratory are given to show that lanthanide-binding metallopeptides, and now a full metallohomeodomain, can be generated by modular substitution of a Ca-binding EF-hand loop into the unrelated scaffold, the engrailed helix-turn-helix motif. We have previously shown that these peptides bind trivalent Ln(III) ions and promote DNA and phosphate hydrolysis, the targeted function. Here, a series of chimeric peptides are presented that differ only in the ninth loop position [given in parentheses; Peptides P3N (Asn), P3E (Glu), P3A (Ala), and P3W(D) (Asp]. This residue, a putative second-shell ligand stabilizing a coordinated water, was found to influence not only metal affinity but also peptide folding. The affinity for Tb(III) was determined by Trp-Tb fluorescence resonance energy transfer and followed the order Ka = P3W(D) > P3A approximately P3E > P3N. However, circular dichroism (CD) titrations with EuCl3 showed that only P3W(D) and P3N folded to any extent upon metal binding, indicating that the Asp/Asn side chains stabilize the central loop structure and thus propagate folding of the peripheral helices, whereas neither Ala nor Glu appears to be interacting with the metal to organize the loop. Finally, we investigated the longer range context of a given loop substitution by cloning and expressing a lanthanide-binding homeodomain (C2), whose loop insertion sequence is analogous to that of peptide P3W(D). We find by CD that apo-C2 has a significant helical structure (approximately 25% alphahelicity), which increases further upon the addition of Tb(III) (approximately 32% alpha helicity). The protein's Tb(III) affinity is similar to that of the chimeric peptides. However, unlike previously reported metallopeptides, we find that EuC2 does not appreciably promote phosphate or DNA cleavage, which suggests a difference in metal accessibility in the context of the full domain. We have demonstrated that substituting beta turns with metal-binding turns does not necessarily require homologous parental scaffolds or small flexible peptides but rather relies on the structural similarity of the motifs flanking the turn.

Metal complex-DNA interactions: from transcription inhibition to photoactivated cleavage

Metal ions and complexes, because of their cationic character, three-dimensional structural profiles, and propensity for performing hydrolysis, redox, or photoreactions, have a natural aptitude for interacting with DNA. Indeed, the need for cellular regulation of DNA led to the evolution of metallonucleases to catalyze and repair DNA strand breaks. Moreover, inorganic constructs such as cisplatin and bimetallic rhodium acetate exert antitumor activity by inner-sphere coordination to DNA. Because binding and cleavage of DNA is at the heart of cellular transcription and translation, it is an obvious target for therapeutic intervention and the development of diagnostic structural probes. To this end, new metal complexes have been designed that utilize or create open coordination positions for DNA binding and hydrolysis, generate reactive oxygen-containing species or other radicals for DNA oxidation, or perform direct redox reactions with DNA. The recent emerging themes are the development of bifunctional architectures containing multiple metal-binding or reactive sites, specialized ligand implementation, or incorporation of site-specific targeting substructures. This review describes their employment in novel reaction strategies that do not require bimolecular cofactors and as site-specific probes or cleavage agents.