Infrared spectroscopic study of the binding of divalent cations to Akazara scallop troponin C: The effect of the methylene side chain of glutamate residue

Akazara scallop striated adductor muscle troponin C (TnC) binds only one Ca2+ because the three EF‐hand motifs are short of critical residues for the coordination of Ca2+. Fourier‐transform infrared spectroscopy was applied to study coordination structures of M2+ (= Mg2+, Ca2+, Sr2+, and Ba2+) bound in an Akazara scallop TnC mutant (E142D) and the wild‐type TnC C‐lobe in D2O solution. The region of the COO− antisymmetric stretch provides information regarding the coordination modes of a COO− group to a metal ion. The side chain COO− group of Asp142 did not bind to Ca2+ in the bidentate coordination mode, suggesting that the absence of a methylene group is critical for the Ca2+ coordination structure of Akazara scallop TnC (Nara et al., Vib Spect 2006, 42, 188–191). The present study has shown that the absence of a methylene group is not compensated for by a larger metal ion such as Sr2+ or Ba2+. CD spectra showed that the secondary structures are conserved between M2+‐free (apo), Mg2+‐loaded, Ca2+‐loaded, Sr2+‐loaded, and Ba2+‐loaded states, which was consistent with the results estimated from their amide I band patterns. The metal‐ligand interaction at position 12 of site IV is discussed in comparison with the coordination mode of the side chain COO− group of the wild‐type TnC C‐lobe. © 2008 Wiley Periodicals, Inc. Biopolymers 89: 595–599, 2008. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com