Structure-function of IBPs and their interactions with ice

The diversity among the dozen antifreeze proteins (AFPs) and other ice-binding proteins (IBPs) with known or robustly predicted three-dimensional structures is remarkable. Their protein folds range from single short alpha-helices to long beta-solenoids and small globular domains with mixed secondary structure. IBPs differ one from another not only in structure, but also in activity levels, affinity for different ice planes, and ice-binding site size, shape, and amino acid composition. IBPs arose from different evolutionary routes on many different occasions, and even function in different ways to protect the host organism from freeze injury. The only unifying feature of IBPs is their basic function, to bind to ice, and even this is achieved with different orientations and kinetics. This chapter covers the structural diversity of IBPs and their ice-binding sites (IBS). We discuss the correlation between IBS structure and size with activity levels, and how the structural differences are manifested in their binding characteristics. Further we discuss the protein:ice interface at the molecular level and recent mechanisms of ice recognition.