Exo-α-sialidase (EC 3.2.1.18, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkages of terminal sialic acid residues in oligosaccharides, glycoproteins, glycolipids, colominic acid and synthetic substrates Neuraminidase enzymes are a large family, found in a range of organisms. The best-known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread of influenza infection. The viral neuraminidases are frequently used as antigenic determinants found on the surface of the influenza virus. Some variants of the influenza neuraminidase confer more virulence to the virus than others. Other homologues are found in mammalian cells, which have a range of functions. At least four mammalian sialidase homologues have been described in the human genome (see NEU1, NEU2, NEU3, NEU4). Sialidases may act as pathogenic factors in microbial infections. Reaction There are two major classes of Neuraminidase that cleave exo or endo poly-sialic acids: Exo hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)-glycosidic linkages of terminal sialic acid residues Endo hydrolysis of (2→8)-α-sialosyl linkages in oligo- or poly(sialic) acids (see endo-α-sialidase.) Function Sialidases, also called neuraminidases, catalyze the hydrolysis of terminal sialic acid residues from the newly formed virions and from the host cell receptors. Sialidase activities include assistance in the mobility of virus particles through the respiratory tract mucus and in the elution of virion progeny from the infected cell. Subtypes Swiss-Prot lists 137 types of neuraminidase from various species as of October 18, 2006. Nine subtypes of influenza neuraminidase are known; many occur only in various species of duck and chicken. Subtypes N1 and N2 have been positively linked to epidemics in humans, and strains with N3 or N7 subtypes have been identified in a number of isolated deaths. CAZy defines a total of 85 glycosyl hydrolase families, of which families GH34 (viral), GH33 (cellular organisms), GH58 (viral and bacterial), GH83 (viral) are major families that contain this enzyme. GH58 is the only endo-acting family. The following is a list of major classes of neuraminidase enzymes: Viral neuraminidase Bacterial neuraminidase Mammalian neuraminidases: Structure Influenza neuraminidase is a mushroom-shaped projection on the surface of the influenza virus. It has a head consisting of four co-planar and roughly spherical subunits, and a hydrophobic region that is embedded within the interior of the virus' membrane. It comprises a single polypeptide chain that is oriented in the opposite direction to