A comparison of the energetics of annexin I and annexin V

Abstract

The annexins comprise a family of soluble Ca 2+- and phospholipid-binding proteins. Although highly similar in three-dimensional structure, different annexins are likely to exhibit different biochemical and functional properties and to play different roles in various membrane related events. Since it must be expected that these functional differences arise from differences in the characteristic thermodynamic parameters of these proteins, we performed high-sensitivity differential scanning microcalorimetry (DSC) and isothermal guanidinium hydrochloride (GdnHCl)-induced unfolding studies on annexin I and compared its thermodynamic parameters with those of annexin V published previously. The DSC data were analyzed using a model that permits quantitative treatment of the irreversible reaction. It turned out, however, that provided a heating rate of 2 K min −1 is used, unfolding of annexin I can be described satisfactorily in terms of a simple two-state reaction. At pH 6.0 annexin I is characterized by the following thermodynamic parameters: t 1/2 = 61.8°C, Δ H cal = 824 kJ mol −1 and Δ C p = 19 kJ mol −1 K −1. These parameters result in a stability value of Δ G D 0 (20°C) = 51 kJ mol −1. The GdnHCl induced isothermal unfolding of annexin I in Mes buffer (pH 6.0), yielded Δ G D 0 (buffer) values of 48, 60 and 36 kJ mol −1 at 20, 12 and 5°C, respectively. These Δ G D 0 values are in reasonable agreement with the values obtained from the DSC studies. The comparison of annexin I and annexin V under identical conditions (pH 8.0 or pH 6.0) shows that despite the pronounced structural homology of these two members of the annexin familiy, the stability parameters are remarkably different. This difference in stability is consistent with and provides a thermodynamic basis for the potential different in vivo functions proposed for these two annexins.