REFINED SOLUTION STRUCTURE OF TYPE-III ANTIFREEZE PROTEIN - HYDROPHOBIC GROUPS MAY BE INVOLVED IN THE ENERGETICS OF THE PROTEIN-ICE INTERACTION

Background: Antifreeze proteins are found in certain fish inhabiting p olar sea water. These proteins depress the freezing points of blood an d body fluids below that of the surrounding sea water by binding to an d inhibiting the growth of seed ice crystals. The proteins are believe d to bind irreversibly to growing ice crystals in such a way as to cha nge the curvature of the ice-water interface, leading to freezing poin t depression, but the mechanism of high-affinity ice binding is not ye t fully understood. Results: The solution structure of the type III an tifreeze protein was determined by multidimensional NMR spectroscopy. Twenty-two structures converged and display a root mean square differe nce from the mean of 0.26 Angstrom for backbone atoms and 0.62 Angstro m for all non-hydrogen atoms. The protein exhibits a compact fold with a relatively large hydrophobic core, several short and irregular beta sheets and one helical turn. The ice-binding site, which encompasses parts of the C-terminal sheet and a loop, is planar and relatively non polar. The site is further characterized by the low solvent accessibil ities and the specific spatial arrangement of the polar side-chain ato ms of the putative ice-binding residues Gln9, Asn14, Thr15, Thr18 and Gln44. Conclusions: In agreement with the adsorption-inhibition mechan ism of action, interatomic distances between active polar protein resi dues match the spacing of water molecules in the prism planes ({10 (1) over bar 0}) of the hexagonal ice crystal, The particular side-chain conformations, however, limit the number and strength of possible prot ein-ice hydrogen bonds. This suggests that other entropic and enthalpi c contributions, such as those arising from hydrophobic groups, could play a role in the high-affinity protein-ice adsorption. (C) Current B iology Ltd