糖苷键Glycosidic bond

按连接的原子分为 O-糖苷键（最常见）、N-糖苷键（核苷中糖与碱基相连）、S-糖苷键和 C-糖苷键。按异头碳构型分为 α 和 β 型，并标明连接位置，如淀粉的 α-1,4、纤维素的 β-1,4、支链的 α-1,6。构型的差异影响巨大：人体淀粉酶能水解 α-1,4 键，却不能水解 β-1,4 键，所以人不能消化纤维素。

By the linking atom glycosidic bonds are O-glycosidic (the commonest), N-glycosidic (sugar to base in nucleosides), S-glycosidic or C-glycosidic. By anomeric configuration they are α or β, with positions given, such as α-1,4 in starch, β-1,4 in cellulose and α-1,6 at branch points. The difference matters enormously: human amylase cleaves α-1,4 bonds but not β-1,4, so people cannot digest cellulose.

化学上，费歇尔糖苷化在酸催化下使糖与醇缩合，生成 α、β 混合物；科尼希斯-克诺尔反应（1901 年）用乙酰溴代糖和银盐控制构型。缩醛对碱稳定、对酸敏感，因此糖苷在稀酸中加热易水解。生物体内由糖基转移酶用核苷酸活化的糖（如 UDP-葡萄糖）高选择性地形成糖苷键，由糖苷酶水解。

Chemically, Fischer glycosidation condenses a sugar with an alcohol under acid catalysis, giving an α/β mixture, while the Koenigs–Knorr reaction (1901) controls configuration using acetylated glycosyl bromides and silver salts. As acetals, glycosides are stable to base but sensitive to acid and hydrolyse on warming in dilute acid. In living cells glycosyltransferases make glycosidic bonds selectively from nucleotide-activated sugars such as UDP-glucose, and glycosidases break them.

糖苷键广泛存在于天然产物中，如强心苷（洋地黄毒苷）、黄酮苷、皂苷和氨基糖苷类抗生素（链霉素）。蛋白质的糖基化修饰（N-连接和 O-连接）影响其折叠和识别，ABO 血型就由红细胞表面不同的寡糖链决定。2022 年诺贝尔化学奖的点击化学和生物正交化学被用来研究细胞表面糖链。

Glycosidic bonds are everywhere in natural products: cardiac glycosides such as digitoxin, flavonoid glycosides, saponins and aminoglycoside antibiotics such as streptomycin. Protein glycosylation (N- and O-linked) affects folding and recognition, and the ABO blood groups are defined by different oligosaccharide chains on red cells. Click and bioorthogonal chemistry, recognised by the 2022 Nobel Prize in Chemistry, are used to study cell-surface glycans.