The conformation and stability of human Zn-alpha 2-glycoprotein in aqueous and methanolic solutions

The stability of Zn-alpha 2-glycoprotein has been studied using UV-spectroscopy and circular dichroism with respect to the influence of temperature, pH, and solvent composition. It has been found that: (1) this protein contains a relatively high proportion of beta-sheet (60%) and a very low amount of other periodic structures as estimated from circular dichroic spectra; (2) at pH 7.4, the circular dichroic spectra change reversibly in the temperature range between 25 and 85 degrees C; small disturbances were observed at 265 nm; (3) with the assumption of the two-state process, the temperature of cooperative denaturation was Tm = 66 degrees C, van't Hoff's enthalpy of this process was 27 kJ mol-1; (4) up to pH 9.5 the dichroic spectrum appeared the same as at pH 7.4; (5) in the presence of methanol (vol. fraction 50%), no isodichroic points on the circular dichroic spectra were found during temperature denaturation; after cooling from 85 degrees C, the alpha-helix content was higher than in the native protein; (6) in the molecule of Zn-alpha 2-glycoprotein, 14 out of 18 tyrosines can dissociate with the instrinsic pK = 11.2; and (7) the temperature perturbation difference spectra yielded nonlinear delta A vs. T curves with temperature transition corresponding to the values found in the circular dichroic spectra; the numbers of chromophores exposed to the solvent as determined by the temperature difference spectra were: 4 tyrosines, 1 tryptophan, and 1 phenylalanine. In several aspects, a parallel has been found between Zn-alpha 2-glycoprotein and orosomucoid (acid alpha 1-glycoprotein), another plasma glycoprotein.