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Helix variants of troponin C with tailored calcium affinities
G Trigo-Gonzalez1, G Awang, K Racher
1Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada.
Abstract:
Muscle fiber contraction is regulated through calcium-induced changes in the conformation of troponin C. In this study, we explored the relationship between the stability of a specific helix in the protein and the metal ion affinity of associated binding sites. Serial replacement of the amino acid at position 130 caused the calcium affinity of the paired Ca2+/Mg2+ sites to be attenuated. In the crystal structures of chicken and turkey troponin C, position 130 is the N-cap residue of the G-helix. The ion affinities of variant proteins were shifted in the order Ile < Gly < Asp < Asn < Thr < Ser. Although differing in ion affinities, the variant proteins all exhibited high cooperativity. The results of this study point to a specific relationship between alpha-helix stability and ion affinity in troponin C and suggest that troponin C may be a paradigm for protein folding problems.
Insights
Altering a key amino acid in troponin C (a muscle protein) affects its calcium binding affinity. This reveals a link between helix stability and ion binding, offering insights into protein folding.
Area of Science:
- Biochemistry
- Molecular Biology
- Structural Biology
Background:
- Muscle contraction relies on calcium-regulated troponin C.
- Troponin C's function involves conformational changes driven by calcium binding.
- Understanding these changes is crucial for muscle physiology.
Purpose of the Study:
- To investigate the relationship between G-helix stability in troponin C and its metal ion binding affinity.
- To determine how amino acid substitutions at position 130 impact calcium and magnesium ion binding.
- To explore the implications for protein folding mechanisms.
Main Methods:
- Site-directed mutagenesis to create troponin C variants with amino acid substitutions at position 130.
- Analysis of crystal structures of chicken and turkey troponin C to identify position 130.
- Measurement of ion affinities for Ca2+ and Mg2+ in variant proteins.
Main Results:
- Amino acid substitutions at position 130 significantly altered the calcium affinity of troponin C's binding sites.
- Ion affinities varied in the order Ile < Gly < Asp < Asn < Thr < Ser.
- Despite altered ion affinities, all variant proteins maintained high cooperativity.
Conclusions:
- A direct correlation exists between alpha-helix stability and metal ion affinity in troponin C.
- Troponin C serves as a model system for studying protein folding and stability.
- The N-cap residue of the G-helix plays a critical role in modulating ion binding properties.