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.
Nara
Masayuki
M
Laboratory of Chemistry, College of Liberal Arts and Sciences, Tokyo Medical and Dental University, Chiba 272-0827, Japan. nara.las@tmd.ac.jp
Yumoto
Fumiaki
F
Kagi
Hiroyuki
H
Tanokura
Masaru
M
D000473:Alkanes / Q000737:chemistry; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002412:Cations; D002942:Circular Dichroism; D018698:Glutamic Acid / Q000737:chemistry; D008673:Metals, Alkaline Earth / Q000737:chemistry*; D049897:Pectinidae / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D019209:Troponin C / Q000737:chemistry*
D000473:Alkanes; D002412:Cations; D008673:Metals, Alkaline Earth; D019209:Troponin C; D018698:Glutamic Acid