An Antifreeze Protein Folds with an Interior Network of More Than 400 Semi-Clathrate Waters

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Science  14 Feb 2014:
Vol. 343, Issue 6172, pp. 795-798
DOI: 10.1126/science.1247407

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When polypeptide chains fold into a protein, hydrophobic groups are compacted in the center with exclusion of water. We report the crystal structure of an alanine-rich antifreeze protein that retains ~400 waters in its core. The putative ice-binding residues of this dimeric, four-helix bundle protein point inwards and coordinate the interior waters into two intersecting polypentagonal networks. The bundle makes minimal protein contacts between helices, but is stabilized by anchoring to the semi-clathrate water monolayers through backbone carbonyl groups in the protein interior. The ordered waters extend outwards to the protein surface and likely are involved in ice binding. This protein fold supports both the anchored-clathrate water mechanism of antifreeze protein adsorption to ice and the water-expulsion mechanism of protein folding.

Folding When Wet

Most globular proteins release water as they fold to form a dry hydrophobic core. In contrast, Sun et al. (p. 795; see the Perspective by Sharp) report a high-resolution structure showing that the antifreeze protein Maxi retains about 400 water molecules in its core. Maxi is a dimer in which two helical monomers each bend in the middle to form a four-helix bundle. The helices are spaced slightly apart to accommodate two intersecting polypentagonal monolayers of water. The pentagons form cages around inward pointing side chains to stabilize the structure. The ordered waters extend to the protein surface where they are likely to be involved in ice binding.

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