pH and temperature resolve the kinetics of two pools of calcium bound to the sarcoplasmic reticulum Ca2+ -ATPase

Area of Science:

• Biochemistry
• Molecular Biology
• Enzyme Kinetics

Background:

• Sarcoplasmic reticulum Ca2+-ATPase (SERCA) is crucial for muscle contraction.
• Understanding calcium binding kinetics is vital for SERCA function.

Purpose of the Study:

• To investigate the kinetics of calcium binding to Ca2+-ATPase.
• To determine the influence of pH and temperature on calcium binding rates.

Main Methods:

• Calcium was removed from Ca2+-ATPase using EGTA.
• Kinetic calcium binding was studied across a pH range (6.0-8.8) and temperature range (0-20°C).
• Calcium concentrations were saturating (50-100 µM).

Main Results:

• At pH 6.0 and 0°C, calcium binding had a slow rate (t1/2 ≈ 10s).
• Increasing pH from 6.0 to 8.8 shifted approximately 50% of calcium sites to a rapid binding state (<2s), peaking at pH 7.4.
• At pH 7.0 and 20°C, ~90% of sites exhibited rapid calcium binding.

Conclusions:

• pH and temperature differentially affect calcium binding kinetics to Ca2+-ATPase.
• Results suggest the existence of two distinct pools of calcium-binding sites with different kinetic properties.
• These findings provide insights into the regulation of Ca2+-ATPase activity.