BINDING OF CA2+ IONS TO BOVINE ALPHA-LACTALBUMIN - STUDY BY MEANS OF INTRINSIC PROTEIN FLUORESCENCE CHANGES

  • 1 January 1982
    • journal article
    • research article
    • Vol. 27  (3) , 380-385
Abstract
Binding of Ca2+ ions to bovine .alpha.-lactalbumin molecules causes a conformational change reflected in an almost 2-fold decrease of the fluorescence quantum yield and a rather pronounced spectral shift towards shorter wavelengths (by .apprx. 20 mm). The changes could be interpreted as a result of a transfer of highly exposed tryptophan residue(s) into a rigid internal part of the protein molecule containing effective quenching groups (probably vicinal S-S bridges). The Ca2+ binding constant evaluated from EGTA [ethylene glycol bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid] and pH-titrations is (3-6) .times. 108 M-1. The results of the spectrofluorometric pH-titration of .alpha.-lactalbumin in the presence of various Ca2+ concentrations suggest that the well-known acid conformational change in .alpha.-lactalbumin is due in fact to a competitive replacement of the bound Ca2+ by 3 H+ ions (pK = 5.0 .+-. 0.1) in the Ca2+ binding site.