Chap2_Carbohydrates.ppt

* 3. Homopolysaccharides (同多糖) Function (功能) Polymer Structure (结构) Glycosidic bond (糖苷键) Size (No. of units) Source (来源) Energy Starch (淀粉) Plants storage ?-Amylose (直链淀粉) Linear (?1?4)Glc 50-5000 贮存多糖 Amylopectin (支链淀粉) Branched (?1?4)Glc (?1?6)Glc Up to 106 Glycogen (糖原) Branched (?1?4)Glc (?1?6)Glc Up to 50000 Animals Structural Cellulose (纤维素) Linear (?1?4)Glc Up to 15000 Plants 结构多糖 Chitin(壳多糖) Linear (?1?4)GlcNAc Very large Animals * Amylose – long, unbranched chains – (?1?4) linkage Amylopectin – highly branched – (?1?4) to join successive glucose residues – (?1?6) linkages at branch points Starch: Amylose amylopectin（直链淀粉 支链淀粉） P.41 ?1?4 A short segment of amylose (直链淀粉) (?1?6) (?1?4) A segment of amylopectin (支链淀粉) * Amylose （直链淀粉） ?1?4 The most stable three-dimensional structure for the (?1?4)-linked chains of starch and glycogen is a tightly coiled helix, stabilized by interchain H bonds. * A cluster of amylose and amylopectin * Amylose amylopectin (直链和支链淀粉) Both have coiled helical structures due to ?1?4 linkages (都是螺旋结构) Both are characterized by reaction with I2 (都与I2发生特征反应) Amylose (直链淀粉) ? dark blue (深蓝色) Amylopectin (支链淀粉) ? purple (紫色) Both have a reducing end (都有一个还原端) Easily hydrated because of –OH groups — ready to form H-bonds with water (都易于水合，因为易与水形成氢键) Ingested by hydrolysis with amylases (淀粉酶水解而被吸收) Amylases are capable of cleaving starch and glycogen: Amylase Cleavage at (?1?4) linkage ?-amylase 内切葡糖苷酶 Random internal cleavage ?-amylase 外切葡糖苷酶 Cleavage from nonreducing end to release ?-maltose P.42 Fig.1-24 * Cellulose（纤维素） IV. Polysaccharides P.45 ?1?4 * Cellulose (?1?4) linkage Flattened sheet structure (片层结构) – very stable Interchain and intrachain H-bonds (链间/链内氢键) Extremely resistant to hydrolysis (不易水解) Insoluble in water (不溶于水) Ingested only by ruminant animals (只能被反刍动物吸收): hydrolysis by cellulases (纤维素酶) IV. Polysaccharides P.46 Fig.1-27 Chitin (壳多糖) A linear homopolysaccharide composed of N-acetylglucosamine residues