Sialic acid synthase (SAS) catalyzes the condensation of PEP with ManNAc (bacteria) and ManNAc-6P (mammals) to yield NeuNAc and NeuNAc-9P, respectively. The bacterial and mammalian SASs share 30%–40% sequence identity, and both enzymes are composed of two distinct domains that are joined by an extended linker region. The N-terminal domain (NeuB domain) consists of ~250 amino acid residues, and is considered to bind the sugar substrates. The C-terminal domain consists of ~75 amino acid residues, and is called the antifreeze-like (AFL) domain, since it is similar to those observed in a variety of functional type III antifreeze proteins (AFPs). It has been proposed that the AFL domain is also involved in sugar binding, but the details of its function have remained elusive. The structure of the AFL domain of human sialic acid synthase comprises one alpha- and two single-turn 310-helices and two beta-strands, and is similar to those of the type III antifreeze proteins. In order to understand the correlation between the dynamics of AFL protein and its substrate binding activities, we performed NMR backbone dynamics study on the AFL domain of human SAS. Comparison of its backbone dynamics data with type III AFP provides the information that the class-specific residues in the human AFL domains are important for their substrate binding, while the class-specific residues of the fish antifreeze proteins are gathered on the ice-binding surface.