Infrared spectroscopic study of the metal-coordination structures of calcium-binding proteins

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Abstract

Carboxylate (COO) groups can coordinate to metal ions in of the following four modes: ‘unidentate’, ‘bidentate’, ‘bridging’ and ‘pseudo-bridging’ modes. COO stretching frequencies provide information about the coordination modes of COO groups to metal ions. We review the Fourier-transform infrared spectroscopy (FTIR) of side-chain COO groups of Ca2+-binding proteins: pike parvalbumin pI 4.10, bovine calmodulin and Akazara scallop troponin C. FTIR spectroscopy of Akazara scallop troponin C has demonstrated that the coordination structure of Mg2+ is distinctly different from that of Ca2+ in the Ca2+-binding site. The assignments of the COO antisymmetric stretch have been ensured on the basis of the spectra of calcium-binding peptide analogues. The downshift of the COO antisymmetric stretching mode from 1565 cm-1 to 1555–1540 cm−1 upon Ca2+ binding is a commonly observed feature of FTIR spectra for EF-hand proteins.

Section snippets

Pike parvalbumin pI 4.10

Parvalbumins, which are ubiquitous in vertebrates, form a group in Ca2+-binding proteins in parallel with calmodulin and troponin C [30]. Although the function of parvalbumins is not yet understood, their involvement in the relaxation process of fast muscles has been proposed [31], [32]. Kretsinger and Nockolds [33] first reported the three-dimensional structure of carp parvalbumin (isoform pI 4.25) in crystal. According to their results, which Moew and Kretsinger later refined [34], this

Calmodulin

Calmodulin regulates the functions of a wide variety of enzymes [31], [32], [39]. It has four Ca2+-binding sites (I–IV). X-ray analyses [40], [41] have revealed that these four sites are similar to the EF-hand motif reported on parvalbumin [33], and that the COO groups of Asp and Glu, the CONH2 groups of Asn and so on are coordinated to Ca2+. Bovine CaM has 17 Asp COO groups and 21 Glu COO groups in a molecule. Of these 38 COO groups, 16 exist in the Ca2+-binding sites, and the COO groups

Akazara scallop troponin C

Muscle contraction of vertebrate skeletal and cardiac muscles is regulated by troponin in a Ca2+-dependent manner [54]. Troponin contains three components: troponin C (TnC), troponin I and troponin T; TnC is the Ca2+-binding component. In general, TnC contains two independent Ca2+-binding domains, each consisting of two EF-hand motifs [55]. Vertebrate TnCs bind three or four Ca2+ ions in a molecule [56], [57], [58] and act as the Ca2+ switch of muscle contraction associated with the binding and

Synthetic peptide analogues of Ca2+-binding site: site IV of Akazara scallop TnC and site III of rabbit skeletal muscle TnC

The use of the synthetic calcium-binding peptide approach has provided valuable results for understanding the calcium-binding properties thus far [72], [73], [74], [75]. Calcium binding to a series of peptides derived from site III of rabbit skeletal muscle TnC has been studied by Reid et al. [72], who found that a 34-residue peptide was required for relatively tight calcium binding. Shorter peptides have decreased calcium affinity, and the isolated 12-residue Ca2+-binding loop binds to Ca2+

Concluding remarks

FTIR spectroscopy is a powerful tool for identifying the coordination structures of M2+ in Ca2+-binding proteins—that is, the coordination structure modes of side-chain COO groups. The downshift of the COO antisymmetric stretching mode from 1565 cm−1 to 1555–1540 cm−1 upon Ca2+ binding is a commonly observed feature of FTIR spectra for EF-hand proteins. Apart from the proteins described here, FTIR spectroscopy has already been successfully applied to other EF-hand proteins: recoverin [78],

Acknowledgments

This work was supported by the 21st Century Center of Excellence Program and by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan.

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