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Calcium‐induced conformational change in fragment 1‐86 of factor X
Author(s) -
Häfner A.,
Merola F.,
Duportail G.,
Hutterer R.,
Schneider F. W.,
Hof M.
Publication year - 2000
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/1097-0282(2000)57:4<226::aid-bip4>3.0.co;2-i
Subject(s) - chemistry , fluorescence , calcium , titration , tryptophan , wavelength , analytical chemistry (journal) , synchrotron radiation , crystallography , chromatography , inorganic chemistry , biochemistry , optics , amino acid , physics , organic chemistry
Time‐resolved fluorescence of the single tryptophan residue Trp41 in fragment 1‐86 of factor X (FX F1‐86) is studied using a time‐correlated single photon counting technique with synchrotron radiation as the excitation source. Calcium ions are believed to induce a conformational change in the N‐termini of the activated factor X and other vitamin K dependent proteins, which is accompanied by a decrease in fluorescence intensity. The titration with calcium yields a sigmoidal fluorescence titration curve with a transition midpoint concentration of 0.44 m M . The wavelength‐dependent tryptophan fluorescence decays of the apo‐FX F1‐86 (in the absence of calcium) and Ca‐FX F1‐86 are characterized by conventional multiexponential analysis and fluorescence lifetime distribution analysis. In the absence of calcium there are three significant classes of fluorescence lifetimes (ns) that are nearly wavelength independent: 0.55 ± 0.08 (component A), 2.6 ± 0.1 (component B), and 5.3 ± 0.3 (component C). However, their preexponential amplitudes vary with wavelength. The decay associated emission spectra of the individual components show that components B and C contribute over 85% to the total fluorescence for all examined wavelengths. However, in the presence of calcium , the analysis of the time‐resolved fluorescence data of Ca‐FX F1‐86 yields four wavelength‐independent lifetimes (ns) of 0.30 ± 0.09 (component D), 0.65 ± 0.10 (component A), 2.7 ± 0.2 (component B), and 5.4 ± 0.3 (component C). Calcium addition to the apo‐FX F1‐86 leads to a decrease in the fluorescence intensities of components B and C while their decay times remain unaffected. In Ca‐FX F1‐86 an additional component D arises that has a decay time of 0.30 ns and that contributes up to 35% to the total fluorescence intensity. A comparison with a previous investigation of prothrombin fragment 1 demonstrates the extensive structural and functional homology between the N termini of prothrombin and factor X a .