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Biopolymers. 2013 Jul;99(7):472-83. doi: 10.1002/bip.22203.

Coordination structures of Mg2+ and Ca2+ in three types of tobacco calmodulins in solution: Fourier-transform infrared spectroscopic studies of side-chain COO- groups.

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Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba, 263-8522, Japan.


Calmodulin (CaM) is a Ca(2+)-binding protein that regulates a number of fundamental cellular activities. Nicotiana tabacum CaM (NtCaM) comprises 13 genes classified into three types, among which gene expression and target enzyme activation differ. We performed Fourier-transform infrared spectroscopy to compare the secondary and coordination structures of Mg(2+) and Ca(2+) among NtCaM1, NtCaM3, and NtCaM13 as representatives of the three types of NtCaMs. Data suggested that NtCaM13 has a different secondary structure due to the weak β-strand bands and the weak 1661 cm(-1) band. Coordination structures of Mg(2+) of NtCaM3 and NtCaM13 were similar but different from that of NtCaM1, while the Ca(2+)-binding manner was similar among the three CaMs. The amplitude differences of the band at 1554-1550 cm(-1) obtained by second-derivative spectra indicated that the intensity change of the band of NtCaM13 was smaller in response to [Ca(2+)] increases under low [Ca(2+)] conditions than were those of NtCaM1 and NtCaM3, while the intensity reached the same level under high [Ca(2+)]. Therefore, NtCaM13 has a characteristic secondary structure and specific Mg(2+)-binding manner and needs higher [Ca(2+)] for bidentate Ca(2+) coordination of 12th Glu in EF-hand motifs. The Ca(2+)-binding mechanisms of the EF-hand motifs of the three CaMs are similar; however, the cation-dependent conformational change in NtCaM13 is unique among the three NtCaMs.

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