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Heme binding to calmodulin, troponin C, and parvalbumin, as a probe of calcium-dependent conformational changes
Insights
Heme-CO specifically binds to the active, calcium-bound form of calmodulin (CaM). Other calcium-binding proteins like parvalbumin show different binding affinities and calcium dependencies, highlighting CaM
Area of Science:
- Biochemistry
- Molecular Biology
- Protein-ligand interactions
Background:
- Calmodulin (CaM) is a crucial calcium-binding protein involved in cellular signaling.
- Understanding CaM's interaction with small molecules like Heme-CO provides insights into its conformational changes and function.
- Calcium-binding proteins exhibit diverse structural and functional properties.
Purpose of the Study:
- To investigate the binding characteristics of Heme-CO with calmodulin (CaM) and other calcium-binding proteins.
- To determine the specificity and kinetics of Heme-CO binding to the active versus inactive forms of CaM.
- To compare Heme-CO binding affinities across different calcium-binding proteins, including troponin C and parvalbumin.
Main Methods:
- Spectroscopic analysis of Heme-CO absorption spectra upon binding.
- Stopped-flow kinetics measurements to determine binding rates.
- Comparative binding studies with calmodulin, troponin C, and parvalbumin under varying calcium conditions.
Main Results:
- Heme-CO binds selectively to the active, calcium-bound form of CaM.
- CaM exhibits higher affinity and a greater spectral red-shift for Heme-CO compared to troponin C.
- Parvalbumin shows a higher affinity for Heme-CO than CaM, with increased affinity in the absence of calcium.
- CaM fragments show reduced Heme-CO binding affinity.
- The calcium-binding step and subsequent CaM conformational change are rapid (milliseconds).
Conclusions:
- Heme-CO binding is specific to the calcium-bound state of CaM, discriminating between different calcium-binding protein pockets.
- The kinetics of Heme-CO interaction with active CaM are rapid, with calcium binding and protein conformational changes occurring quickly.
- Comparative analysis reveals distinct Heme-CO binding properties among calcium-binding proteins, underscoring their unique structural and functional roles.
Abstract:
Heme-CO binds to the active (calcium-bound) form of calmodulin (CaM), but not to the inactive form. Despite a similarity in structure of another calcium-binding protein, skeletal muscle troponin C, both the affinity and the spectral red-shift of the absorption of the heme group are greatly decreased for troponin C relative to calmodulin. Parvalbumin, another calcium-binding protein, shows a twofold greater affinity for heme-CO relative to CaM. Unlike calmodulin and troponin C, the affinity of parvalbumin for heme-CO is even greater in the absence of calcium. The affinity of the tryptic and thrombic fragments of CaM for heme-CO are decreased relative to the entire calmodulin. The binding of heme-CO is specific as demonstrated by the discrimination of the calmodulin, troponin C, and parvalbumin pockets. The interaction of heme-CO with active (calcium-bound) CaM is rapid (ms) as determined by stopped flow measurements. No difference in kinetics was observed for mixing inactive (calcium free) CaM with a solution of [heme-CO plus calcium], indicating that the calcium-binding step and subsequent change in protein conformation are rapid.