ATP synthesis by Ca2+ + Mg2+-ATPase in detergent solution at constant Ca2+ levels

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Physiology

Background:

• The sarcoplasmic reticulum (SR) is crucial for calcium homeostasis in muscle cells.
• Ca2+ + Mg2+-dependent ATPase is the primary protein responsible for calcium transport in the SR.

Purpose of the Study:

• To investigate the ATP synthesis capability of purified SR Ca2+ + Mg2+-dependent ATPase.
• To elucidate the relationship between ATP synthesis and Ca2+ transport mediated by the enzyme.

Main Methods:

• Purification of Ca2+ + Mg2+-dependent ATPase from SR.
• Solubilization using nonionic detergent dodecyloctaoxyethylenglycol-monoether.
• ATP synthesis assay under varying pH and calcium concentrations, including enzymatic digestion with trypsin.

Main Results:

• Optimal ATP synthesis occurred at 0.6 mM CaCl2 and a delta pH of 2.9 +/- 0.2.
• ATP synthesis was directly correlated with Ca2+ uptake into the SR.
• Trypsin digestion affected ATP synthesis and Ca2+ uptake.
• H+ ions did not substitute for Ca2+ but modified Ca2+ binding sites.

Conclusions:

• The purified SR Ca2+ + Mg2+-dependent ATPase is capable of synthesizing ATP.
• The enzyme alone drives active Ca2+ transport in the SR.
• Enzyme integrity between hydrolytic and Ca2+-binding sites is essential for Ca2+ transport, with ion-protein interactions potentially driving ATP synthesis.