Binding effect of progesterone on the dynamics of α1-acid glycoprotein

Abstract

The fluorescence of the tryptophan residues of asialylated human α1-acid glycoprotein (orosomucoid) was investigated in presence of progesterone. Red-edge excitation spectra did not lead to a shift of the fluorescence emission maximum of the fluorophore, i.e., motions of the Trp residues depend on their microenvironment. This was confirmed by anisotropy studies as a function of temperature in the range of 7–35°C (Perrin plot). These two results identical to those obtained in absence of progesterone [J. Albani, Biochim. Biophys. Acta 1291 (1996) 215–220] indicate that binding of progesterone to orosomucoid does not modify the mean residual motion of the Trp residues. Measurement of the anisotropy in a temperature range of −45° to +6°C in a mixture of 80% glycerol–buffer, allows us to determine the frictional resistance to the local rotations of the tryptophan residues [G. Weber, S.F. Scarlata, M. Rholam, Biochemistry 23 (1984) 6785–6788]. The Y-plot analysis of the anisotropy reveals that the mean motion of the two Trp residues buried in the protein core was different from that of the Trp residue of the surface. The average angles of rotations for buried and surface residues were 16° and 21.5° of arc, respectively, instead of 10° and 14° of arc observed in absence of progesterone [J. Albani, Biochim. Biophys. Acta 1291 (1996) 215–220]. Thus, binding of progesterone to orosomucoid increases the free space of rotation of the two classes of Trp residues.