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Calcium and lanthanide binding in the sarcoplasmic reticulum ATPase
T C Squier1, D J Bigelow, F J Fernandez-Belda
1Department of Biological Chemistry, University of Maryland School of Medicine, Baltimore 21201.
The Journal of Biological Chemistry
|August 15, 1990
Summary
Lanthanides and calcium interact with the sarcoplasmic reticulum ATPase enzyme. Lanthanides activate the enzyme but slower than calcium, suggesting a regulatory role in calcium binding.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzyme Kinetics
Background:
- The sarcoplasmic reticulum ATPase (SERCA) is crucial for muscle contraction by pumping calcium ions.
- Understanding cation interactions with SERCA is key to elucidating enzyme regulation.
Purpose of the Study:
- To investigate the binding interactions and activation mechanisms of calcium and lanthanides with SERCA.
- To compare the functional effects of lanthanides and calcium on ATPase activity and conformational changes.
Main Methods:
- Radioactive tracer binding assays.
- Enzyme activity measurements (ATPase partial reactions).
- Fluorescence spectroscopy (tryptophanyl residues and FITC label).
- Förster resonance energy transfer (FRET) for distance measurements.
Main Results:
- Lanthanides displace calcium from SERCA with sequential kinetics.
- Lanthanides activate SERCA, but ATP hydrolysis is slower compared to calcium activation.
- Lanthanides induce conformational changes in the extramembranous region, while calcium affects both transmembrane and extramembranous regions.
- Energy transfer studies indicate lanthanide binding at an intermediate site regulating transmembrane calcium binding.
Conclusions:
- Lanthanides act as calcium analogs, binding to SERCA and influencing its activity.
- A 'midway' cation binding site, potentially occupied by lanthanides, appears to regulate calcium exchange in the transmembrane domain.
- This midway site may play a role in a secondary calcium binding event, modulating enzyme function.