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Sialic acid is a generic term for the ''N''- or ''O''-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone.〔 〕 It is also the name for the most common member of this group, N-acetylneuraminic acid (Neu5Ac or NANA). Sialic acids are found widely distributed in animal tissues and to a lesser extent in other organisms, ranging from plants and fungi to yeasts and bacteria, mostly in glycoproteins and gangliosides (they occur at the end of sugar chains connected to the surfaces of cells and soluble proteins).〔()〕 That is because it seems to have appeared late in evolution. However, it has been observed in ''Drosophila'' embryos and other insects and in the capsular polysaccharides of certain strains of bacteria.〔()〕 In humans the brain has the highest sialic acid concentration where they have an important role in neural transmission and ganglioside structure in synaptogenesis.〔 In general, the amino group bears either an acetyl or a glycolyl group, but other modifications have been described. These modifications along with linkages have shown to be tissue specific and developmentally regulated expressions, so some of them are only found on certain types of glycoconjugates in specific cells.〔 The hydroxyl substituents may vary considerably; acetyl, lactyl, methyl, sulfate, and phosphate groups have been found. The term "sialic acid" (from the Greek for saliva, ''σίαλον''/''sialon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. ==Structure== The sialic acid family includes 43 derivatives of the nine-carbon sugar neuraminic acid, but these acids unusually appear free in nature. Normally they can be found as components of oligosaccharide chains of mucins, glycoproteins and glycolipids occupying terminal, nonreducing positions of complex carbohydrates on both external and internal membrane areas where they are very exposed and develop important functions.〔 The numbering of the sialic acid structure begins at the carboxylate carbon and continues around the chain. The configuration that places the carboxylate in the axial position is the alpha-anomer. The alpha-anomer is the form that is found when sialic acid is bound to glycans. However, in solution, it is mainly (over 90%) in the beta-anomeric form. A bacterial enzyme with sialic acid mutarotase activity, NanM, that is able to rapidly equilibrate solutions of sialic acid to the resting equilibrium position of around 90% beta/10% alpha has been discovered. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sialic acid」の詳細全文を読む スポンサード リンク
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