AFP III-like domain superfamily

Antifreeze proteins (AFPs) are defined by their ability to bind ice and prevent it from growing. In this way they function in both freeze-resistance and freeze-tolerance strategies of organisms that live at sub-zero temperatures and require protection from ice growth. In fish, five AFP types have been described that are remarkably diverse in their 3D structures. They have completely dissimilar folds and no sequence homology. Type III AFPs found in eel pounts are 65-residue proteins with a compact globular fold formed from short beta strands, which presents a flat ice binding surface. These proteins are homologous to the C-terminal region of mammalian and prokaryotic sialic acid synthase (SAS; gene neuB), which has been called AFP-like domain [12171656]. The similarity is greatest in the protein core and the flat ice-binding region. SAS is involved in the condensation of phosphoenolpyruvate with N-acetylmannosamine derivatives to generate N-acetylneuraminic acid, an intermediate used for the sialylation of glycoconjugates. The function of the AFP-like domain, which is a beta-clip fold [15146494], in SAS is not known, but it has been proposed that it could be involved in sugar binding.