Effect of Freezing and Thawing on the Conformation of Phosvitin

Phosvitin can undergo a change from unordered conformation to β structure upon freezing and thawing. The transconformation may be identical in terms of its product and of key aspects of its mechanism with the conformational change which can take place, without freezing, if the phosphoprotein has been stripped of most of its anionic character, at pH 2 or less, but the freezing effect can occur with solutions that are more weakly acidic by nearly four orders of magnitude. Nevertheless, the effect is enhanced as the acidity of the solution is increased. It is suppressed by increases in salt or protein concentration. The transconformation appears to be linked with the freezing process. Frozen storage is without appreciable effect. Aggregation is an essential corollary of the transconformation: the monomeric protein species gives no evidence of β structure and the extent of transconformation correlates reasonably well with the extent of disappearance of the monomer. The ordered structure is heat-labile and the effect can also be readily reversed by lowering the acidity, but only if the aggregation process has not been allowed to proceed too far. Otherwise, slow continued aggregation leads to the complete precipitation of fibrous phosvitin even at neutral pH. The essential aspects of this freezing phenomenon are consistent with the hypothesis that, as acid is excluded from the growing ice, the acid tide just ahead of the liquid-ice interface causes the transconformation, followed by the trapping of the ordered protein in the rigid ice meshwork.