Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein

Insect antifreeze proteins (AFP) are much more effective than fish AFPs at depressing solution freezing points by ice-growth inhibition(1,2). AFP from the beetle Tenebrio molitor is a small protein (8.4 kDa) composed of tande m 12-residue repeats(3) (TCTxSxxCxxAx). Here we report its 1.4-Angstrom res olution crystal structure, showing that this repetitive sequence translates into an exceptionally regular beta-helix. Not only are the 12-amino-acid l oops almost identical in the backbone, but also the conserved side chains a re positioned in essentially identical orientations, making this AFP perhap s the most regular protein structure yet observed. The protein has almost n o hydrophobic core but is stabilized by numerous disulphide and hydrogen bo nds. On the conserved side of the protein, threonine-cysteine-threonine mot ifs are arrayed to form a flat beta-sheet, the putative ice-binding surface . The threonine side chains have exactly the same rotameric conformation an d the spacing between OH groups is a near-perfect match to the ice lattice. Together with tightly bound co-planar external water, three ranks of oxyge n atoms form a two-dimensional array, mimicking an ice section.