title: Calcium-induced conformational change in fragment 1-86 of factor X.
abstract: 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 mM. 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).
authors.affiliation: Institute of Physical Chemistry, University of Würzburg, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Häfner
authors.forename: F
authors.initials: F
authors.lastname: Merola
authors.forename: G
authors.initials: G
authors.lastname: Duportail
authors.forename: R
authors.initials: R
authors.lastname: Hutterer
authors.forename: F W
authors.initials: FW
authors.lastname: Schneider
authors.forename: M
authors.initials: M
authors.lastname: Hof
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002417:Cattle; D005170:Factor X / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism; D011487:Protein Conformation / Q000187:drug effects; D013050:Spectrometry, Fluorescence
chemical_list: D010446:Peptide Fragments; D005170:Factor X; D002118:Calcium