title: UV Raman evidence of a tyrosine in apo-human serum transferrin with a low pK(a) that is elevated upon binding of sulphate.
abstract: The binding of sulphate to human serum apo-transferrin has been examined by ultraviolet absorption and ultraviolet resonance Raman difference spectroscopies between pH 6.0 and 9.0. The ultraviolet absorption data reveals a negative feature at 245 nm that increases in magnitude with pH, with an apparent pK(a) of 7.57, which the Raman difference data reveals to be due to tyrosine. The pK(a) of this tyrosine is unusually low and is measured at 7.84 by the Raman difference method and is elevated to greater than 9.0 upon addition of sulphate. Previous studies on the N-lobe imply that Tyr 188 is the tyrosine with a low pK(a) and also that Arg 124 is the primary binding site for the sulphate. The functional relevance may be that with sulphate bound, both carbonate binding and the deprotonation of Tyr will be disfavoured, and as a result so is iron binding.
authors.affiliation: Department of Physics and Astronomy, University of Leeds, LS2 9JT, Leeds, UK. phyjcl@phys-irc.novell.leeds.ac.uk
authors.forename: J
authors.initials: J
authors.lastname: Clarkson
authors.forename: D A
authors.initials: DA
authors.lastname: Smith
mesh_terms: D001059:Apoproteins / Q000737:chemistry*; D006801:Humans; D011994:Recombinant Proteins / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D014168:Transferrin / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D001059:Apoproteins; D011994:Recombinant Proteins; D014168:Transferrin; C028284:apotransferrin; D014443:Tyrosine