A Selective Na(+) Aptamer Dissected by Sensitized Tb(3+) Luminescence

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysical Chemistry

Background:

• DNAzymes are catalytic DNA molecules with potential applications in molecular biology and medicine.
• Sodium ion (Na+) binding to nucleic acids is a fundamental aspect of molecular recognition with implications for DNA structure and function.
• Previous studies suggested a common Na+ aptamer motif in RNA-cleaving DNAzymes.

Purpose of the Study:

• To investigate the detailed interaction between the Ce13d DNAzyme and sodium ions (Na+).
• To elucidate the structural requirements for Na+ binding within the Ce13d DNAzyme.
• To characterize the binding affinity and specificity of Na+ to Ce13d and explore potential for improved mutants.

Main Methods:

• Utilized sensitized terbium (Tb3+) luminescence spectroscopy to monitor Na+ binding.
• Performed site-directed mutagenesis studies to identify critical nucleotides for Na+ interaction.
• Quantified binding affinity (Kd) for Na+ and other monovalent cations.

Main Results:

• Na+ binding to Ce13d was confirmed by the displacement of Tb3+ and subsequent quenching of luminescence.
• The hairpin structure and a conserved 16-nucleotide loop in the enzyme strand, along with unpaired substrate nucleotides, are crucial for Na+ binding.
• Ce13d exhibited a dissociation constant (Kd) of approximately 20 mM for Na+, with significantly weaker binding to other monovalent cations.
• Mutational analysis revealed that specific Na+ binding was lost with a single nucleotide change, while another mutant showed an improved Kd of 8 mM for Na+.
• A strong correlation was observed between Na+ binding and DNAzyme cleavage activity, highlighting the functional importance of Na+ interaction.

Conclusions:

• The study demonstrates Ce13d as a Na+ aptamer with significant biological and analytical implications.
• Key structural elements governing Na+ binding in DNAzymes have been defined.
• An improved Ce13d mutant with enhanced Na+ binding affinity was developed, opening avenues for novel biosensor applications.