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 Ca 2+ because the three EF‐hand motifs are short of critical residues for the coordination of Ca 2+ . Fourier‐transform infrared spectroscopy was applied to study coordination structures of M 2+ (= Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) bound in an Akazara scallop TnC mutant (E142D) and the wild‐type TnC C‐lobe in D 2 O 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 Ca 2+ in the bidentate coordination mode, suggesting that the absence of a methylene group is critical for the Ca 2+ 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 Sr 2+ or Ba 2+ . CD spectra showed that the secondary structures are conserved between M 2+ ‐free (apo), Mg 2+ ‐loaded, Ca 2+ ‐loaded, Sr 2+ ‐loaded, and Ba 2+ ‐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