The binding of divalent cations to myosin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein-ligand interactions

Background:

• Myosin is a crucial motor protein involved in muscle contraction.
• Understanding metal ion binding to myosin is essential for elucidating its enzymatic activity and function.

Purpose of the Study:

• To characterize the binding of manganese (Mn2+) to myosin.
• To identify the number and affinity of Mn2+ binding sites on myosin.
• To investigate the influence of ionic strength, other divalent metal ions, and temperature on Mn2+ binding.

Main Methods:

• Equilibrium dialysis
• Centrifuge transport
• Electron paramagnetic resonance (EPR) spectroscopy
• Enzyme kinetics assays

Main Results:

• Two distinct sets of non-interacting Mn2+ binding sites were identified: Class I (high affinity, 10^6 M^-1) and Class II (low affinity, 10^3 M^-1).
• High affinity sites (2 sites) and low affinity sites (20-25 sites) showed reduced affinity at higher ionic strength (0.6 M KCl).
• Divalent cations (Ca2+, Mg2+, Zn2+, Co2+, Sr2+, Ni2+) competed for high-affinity sites, with Ca2+ being most effective.
• Mg2+ and Ca2+ competitively inhibited Mn2+ binding with inhibitor constants similar to low-affinity sites.
• Heat exposure (37°C) partially inhibited Mn2+ binding to high-affinity sites and ATPase activity.
• ADP and inorganic pyrophosphate (PPi) did not significantly affect Mn2+ binding to high-affinity sites, but Mn2+ enhanced ADP binding affinity at high ionic strength.

Conclusions:

• Myosin possesses multiple Mn2+ binding sites with differential affinities and specificities.
• Mn2+ binding is sensitive to environmental conditions such as ionic strength and temperature.
• These findings provide insights into the regulation of myosin's ATPase activity and interaction with nucleotides.