Summary
Three mutant polypeptides of the type I 37-residue winter flounder antifreeze protein have been synthesized. All four threonine residues in the native peptide have been mutated to serine, valine and glycine respectively and two additional salt bridges were incorporated into the sequences in order to improve aqueous solubility. The peptides have been analyzed by nanoliter osmometry, the ice hemisphere test, the crystal habit test, measurement of ice growth hysteresis and CD spectroscopy. While the valine and serine mutants retain the a-helical structure, only the valine mutant retains antifreeze activity similar to that of the native protein. This data shows that the threonine hydroxyl groups do not play a crucial role in the accumulation of the native antifreeze protein at the ice/water interface and the inhibition of ice growth below the equilibrium melting temperature.