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pH-Dependent structural changes at Ca(2+)-binding sites of coagulation factor IX-binding protein
Nobuhiro Suzuki1, Zui Fujimoto, Takashi Morita
1Department of Biochemistry, National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8602, Japan.
Journal of Molecular Biology
|September 17, 2005
Summary
Coagulation factor IX-binding protein (IX-bp) from snake venom has distinct calcium-binding sites. Its pH-dependent conformational changes, particularly in subunit A, explain calcium release mechanisms.
Area of Science:
- Biochemistry
- Structural Biology
- Toxicology
Background:
- Coagulation factor IX-binding protein (IX-bp) is a C-type lectin-like anticoagulant from Trimeresurus flavoviridis.
- It possesses homologous subunits A and B, each with a Ca(2+)-binding site exhibiting different affinities.
Purpose of the Study:
- To identify the high-affinity Ca(2+) binding site in IX-bp.
- To elucidate the pH-dependent Ca(2+) release mechanism by analyzing structural changes.
Main Methods:
- X-ray crystallography was used to determine the structures of IX-bp at various pH values (4.6, 6.5, 7.8, 8.0).
- Comparison of apo and Ca(2+)-complexed forms across different pH conditions.
Main Results:
- Subunit A's Glu43 residue shows pH-dependent conformational flexibility, transitioning around pH 6.5.
- Subunit B's Gln43 residue is less sensitive to pH changes.
- At acidic pH (4.6), Ca(2+) binding sites are occupied by water, indicating release.
Conclusions:
- Glu43 in subunit A is crucial for the pH-dependent Ca(2+) release mechanism.
- The differential sensitivity of subunits A and B to protonation underlies the observed Ca(2+) binding and release characteristics.