Analysis of affinity and specificity in an EF-hand site using double mutant cycles

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• The EF-hand Ca(2+)/Mg(2+) binding site in smooth muscle myosin regulatory light chain (RLC) is crucial for muscle contraction.
• Understanding how mutations affect ion binding is key to elucidating RLC function.

Purpose of the Study:

• To investigate the impact of specific mutations (D5S, D9E, D12E) and their combinations on the Ca(2+) and Mg(2+) binding properties of the RLC EF-hand site.
• To determine the effects of these mutations on ion affinity and binding free energy.

Main Methods:

• Site-directed mutagenesis was used to create single, double, and triple mutants of the RLC EF-hand site.
• Ca(2+) and Mg(2+) binding affinities and coupling energies were analyzed for all generated mutants.

Main Results:

• Mutants D5S and D9E, and D5S/D9E, exhibited low Ca(2+) affinity.
• Mutants with D12E showed Ca(2+)-specificity and higher Ca(2+) affinities compared to wild type.
• All mutants displayed reduced Mg(2+) affinity compared to wild type.
• Coupling energies for Mg(2+) binding indicated an interaction between positions 5 and 9, which is dependent on the residue at position 12.

Conclusions:

• The D12E mutation confers Ca(2+)-specificity to the RLC EF-hand site.
• Interactions between residues at positions 5 and 9 are critical for Mg(2+) binding and are modulated by the residue at position 12.
• These findings provide insights into the structural and functional roles of the EF-hand site in RLC.