7.Carbohydrates

  • Key takeaways from this class
    • What are carbohydrates（什么是碳水化合物）
    • What are the functions for carbohydrates（碳水化合物的功能）
    • What are monosaccharides, Oligosaccharides, and Polysaccharides（什么是单糖、寡糖和多糖）
    • Classification of Monosaccharides（单糖的分类）
    • The Mechanism of Cyclic Structures（环状结构的形成机制）
    • Derivatives of Monosaccharides（单糖的衍生物）
    • Glycosidic Bond（糖苷键）
    • Polysaccharide Types and Functions（多糖的类型与功能）
  • basic
    • definition
      • Carbohydrates are the single most abundant class of organic molecules found in nature.
      • The name carbohydrate arises from the basic molecular formula (CH2O)n, which can be rewritten (C·H2O)n to show that these substances are hydrates of carbon, where n=3 or more.
    • Carbohydrates constitute a versatile多元 class of molecules. examples?
      • Carbohydrates are covalently linked with a variety of other molecules.
      • Carbohydrates linked to lipid molecules, or glycolipids, 醣脂 are common components of biological membranes.
      • Proteins that have covalently linked carbohydrates are called glycoproteins糖蛋白.
    • functions
      • Energy
        • from the sun captured by green plants, algae水藻, and some bacteria during photosynthesis is stored in the form of carbohydrates.
        • In turn, carbohydrates are the metabolic precursors of virtually all other biomolecules.
        • Breakdown of carbohydrates provides the energy that sustains animal life.
      • Glycolipids 醣脂 and glycoproteins 糖蛋白
        • These two classes of biomolecules, together called glycoconjugates 糖缀合物
        • are important components of cell walls and extracellular structures in plants,animals, and bacteria.
      • Recognition
        • serve in a variety of processes involving recognition between cell types or recognition of cellular structures by other molecules.
        • Recognition events are important in normal cell growth, fertilization授精, transformation of cells, and other processes.
    • characteristic chemical features of carbohydrates
      make functions possible
      • The existence of at least one and often two or more asymmetric centers
      • The ability to exist either in linear or ring structures,
      • The capacity to form polymeric structures via glycosidic bonds糖苷键
      • The potential to form multiple hydrogen bonds 氢键 with water or other molecules in their environment.
  • 分类
    • monosaccharides (and their derivatives )
      • The monosaccharides are also called simple sugars and have the formula (CH₂O)ₙ.
      • Monosaccharides cannot be broken down into smaller sugars under mild conditions.
      • 例
    • oligosaccharides
      • Oligosaccharides consist of two to ten simple sugar molecules.
      • Disaccharides 二糖 are common in nature, and trisaccharides also occur frequently.
      • Four- to six-sugar-unit oligosaccharides are usually bound covalently to other molecules, including glycoproteins.
      • 例
    • polysaccharides
      • Polysaccharides are polymers of the simple sugars and their derivatives.
      • They may be either linear or branched polymers and may contain hundreds or even thousands of monosaccharide units.
      • Their molecular weights range up to 1 million or more.
      • 例
  • Monosaccharides
    • Classification
      • 按照官能团分
        • aldoses 醛糖
          • aldehyde group 醛基 -CHO
          • 比如甘油醛
        • ketoses 酮糖
          • ketone group 酮基 -C=O
          • 比如二羟基丙酮
      • 按照原子数分
        • Triose 丙糖
        • Tetrose 丁糖
        • Pentose 戊糖
        • Hexose 己糖
          • the most abundant sugars in nature
          • 比如葡萄糖
    • Stereochemistry
      • Aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers 手性中心
      • D-Fructose 果糖 and L-fructose, an enantiomeric pair 对映 (结构) 体.

        • Note that changing the configuration only at C5 would change D-fructose to L-fructose
      • The majority of saccharides in nature have the “D” isomer
    • Cyclic Structures
      • -OH可以与C=O反应形成HO-C-OH, 头尾(身)发生这个反应就可以成环
      • The cyclic pyranose 吡喃糖 and furanose 呋喃糖 forms are the preferred structures for monosaccharides in aqueous solution
      • At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally muchless than 1%).
      • 半缩醛
        • Alcohols react readily with aldehydes to form hemiacetals 半缩醛
        • The reaction is catalyzed by acid (H⁺) or base (OH⁻) and is readily reversible
        • 形成 pyran 吡喃 pi nan
      • 半缩酮
        • ketones can react with alcohols to form hemiketals 半缩酮
        • 形成 furan 呋喃 fu nan
    • Derivatives of Monosaccharides
      • Sugar Acids 糖酸
        • Sugars with free anomeric 异头 carbon atoms are reasonably good reducing agents and will reduce hydrogen peroxide, ferricyanide 铁氰化物, certain metals (Cu²⁺ and Ag⁺, copper and silver), and other oxidizing agents氧化剂.
          异头碳原子(anomeric carbon atoms)是指单糖形成环状结构时，由原来的醛基或酮基碳原子转化而来的、能产生两种不同构型（α- 型和 β- 型）的特定碳原子
        • Such reactions convert the sugar to a sugar acid.
        • For example, addition of alkaline CuSO₄ (copper sulfate, called Fehling’s solution 斐林试剂) to an aldose sugar produces a red cuprous oxide (Cu₂O) precipitate
      • Sugar Alcohols 糖醇
        • can be prepared by the mild reduction (with NaBH₄-sodium tetraborate 硼砂, 硼酸钠 or similar agents) of the carbonyl 羰基 groups of aldoses and ketoses

          • 把C=O还原
      • Deoxy Sugars 脱氧糖
        • Deoxy Sugars are monosaccharides with one or more hydroxyl groups (-OH)replaced by hydrogens
        • Hydrogen atoms highlighted in red are “deoxy” positions
      • Sugar Esters 糖酯
        • Phosphate esters of glucose, fructose, and other monosaccharides are important metabolic intermediates, and the ribose moiety一部分 of nucleotides such as ATP and GTP is phosphorylated at the 5-position.

          • 葡萄糖、果糖及其他单糖的磷酸酯是重要的代谢中间产物，而 ATP和 GTP等核苷酸中的核糖部分，则在 5 位碳原子上发生了磷酸化
        • Several sugar esters are important in metabolism
      • Amino Sugars 氨基糖
        • Amino sugars, including D-glucosamine D - 氨基葡萄糖 and D-galactosamine D - 氨基半乳糖, contain an amino group (instead of a hydroxyl group) at the C-2 position.
        • They are found in many oligo- and polysaccharides, including chitin 几丁质，a polysaccharide in the exoskeletons of crustaceans [krʌ‘steɪʃn] 甲壳类动物 and insects.
  • Oligosaccharides
    • Oligosaccharides consist of two to ten simple sugar molecules.
    • Disaccharides are common in nature, and trisaccharides also occur frequently.
    • Four- to six-sugar-unit oligosaccharides are usually bound covalently to other molecules, including glycoproteins.
    • 二糖
    • 更大分子的寡糖
    • glycosidic bond 糖苷键
      • 生物体内有两种糖苷键
      • 半缩醛羟基与羟基脱水形成的糖苷键称 O - 糖苷键
      • O-Glycosidic Bond - a glycosidic bond between an anomeric carbon & an oxygen atom
      • 与和氮相连的氢（如 - NH₂）脱水形成的糖苷键称 N - 糖苷键
      • N-Glycosidic Bond - a glycosidic bond between an anomeric carbon & a nitrogen atom.
      • 图
  • Polysaccharides
    • 定义
      • Polysaccharides are polymers of the simple sugars and their derivatives.
      • They may be either linear or branched polymers and may contain hundreds or even thousands of monosaccharide units.
      • Their molecular weights range up to 1 million or more.
    • Structure and Nomenclature
      • By far the majority of carbohydrate material in nature occurs in the form of polysaccharides.
        到目前为止，自然界中绝大多数碳水化合物都以多糖的形式存在
      • By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide糖类 structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures.
        根据我们的定义，多糖不仅包括那些仅由糖苷键连接的糖残基构成的物质，还包括含有聚合糖结构的分子 —— 这些聚合糖结构会通过共价键与氨基酸、肽、蛋白质、脂质及其他结构相连
    • Functions
      • Storage materials, structural components, or protective substances.
        储存物质、结构成分或保护物质
      • A variety of cellular recognition and intercellular communication events
        多种细胞识别和细胞间通讯过程
    • 分类
      • Storage Polysaccharides（储存多糖）
        • basic
          • Storage polysaccharides are an important carbohydrate form in plants and animals.
          • It seems likely that organisms store carbohydrates in the form of polysaccharides rather than as monosaccharides to lower the osmotic pressure of the sugar reserves.
            生物体似乎更倾向于将碳水化合物以多糖形式而非单糖形式储存，这样做是为了降低糖类储备的渗透压
          • Because osmotic pressures depend only on numbers of molecules.
        • Starch（淀粉）
          • By far the most common storage polysaccharide in plants is starch
          • two forms
            • α-amylose 直链淀粉
            • amylopectin 支链淀粉
              • Branches in polysaccharides can involve any of the hydroxyl groups on the monosaccharide components
                多糖中的分支结构，可涉及单糖组成单元上的任意一个羟基
              • a highly branched structure, with branches occurring every 12 to 30 residues.
            • Most forms of starch in nature are 10 to 30% α-amylose and 70 to 90% amylopectin
          • detect
            • Suspensions of amylose in water adopt a helical 螺旋的 conformation.
            • Iodine (I₂) 碘 can insert into the middle of the amylose 直链 helix to give a blue color that is characteristic and diagnostic for starch.

              • 碘可以插入链的螺旋中
          • hydrolyze
            • Starch is split into its monosaccharide elements by stepwise phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase 淀粉磷酸化酶
        • Glycogen（糖原）
          • The major form of storage polysaccharide in animals is glycogen.
          • Glycogen is found mainly in the liver (where amount for 10% of liver mass) and skeletal muscle (where account for 1 to 2% of muscle mass)
          • Like amylopectin 支链淀粉, glycogen yields a red-violet color with iodine.
          • hydrolyze
            • Glycogen can be hydrolyzed by both α- and β-amylases 淀粉酶, yielding glucose葡萄糖 and maltose麦芽糖
            • Glycogen can also be hydrolyzed by glycogen phosphorylase 糖原磷酸化酶 (an enzyme present in liver and muscle tissue) to release glucose-1-phosphate 1-磷酸葡萄糖
          • 结构图
        • Dextran（右旋糖苷）
          • Another important family of storage polysaccharides are the dextrans, which are alpha(1→6)-linked polysaccharides of D-glucose with branched chains found in yeast and bacteria
      • Structural Polysaccharides（结构多糖）
        • Cellulose 纤维素
          • the most abundant natural polymer found in the world
          • 结构
            • (a)是amylose直链淀粉, (b)是cellulose纤维素
            • (a) Amylose, composed exclusively of the relatively bent α(1→4) linkages, prefers to adopt a helical conformation
            • (b) cellulose, with β(1→4)-glycosidic linkages, can adopt a fully extended conformation with alternating 180°flips of the glucose units.
          • 强度
            • The hydrogen bonding inherent in such extended structures is responsible for the great strength of tree trunks and other cellulose-based materials.
            • The wood and bark of trees are insoluble, highly organized structures formed from cellulose 纤维素 and also from lignin 木质素
            • 纤维素作为骨架, 木质素把纤维素连在一起, 增加强度
            • The structure of cellulose, showing the hydrogen bonds (blue) between the sheets, which strengthen the structure. Intrachain hydrogen bonds are in red and interchain hydrogen bonds are in green
            • 水解
              • cellulose很难水解, 不论是酸还是消化道里的amylases淀粉酶都难以水解, 所以正常动物不能吃
              • Ruminant animals 反刍动物比如长颈鹿在rumen瘤胃中的细菌可以产生cellulase纤维素酶进行水解
        • Chitin 几丁质，壳聚糖
          • Chitin is present in the cell walls of fungi and is the fundamental material in the exoskeletons of crustaceans 甲壳纲动物，insects, and spiders.
          • Chitin is similar to cellulose, both in its biological function and its primary, secondary, and tertiary structure.
          • The structure of chitin, an extended ribbon, is identical to cellulose, except that the -OH group on each C-2 is replaced by -NHCOCH₃, so that the repeating units are N-acetyl-D-glucosamines in β(1→4) linkage.

            • 跟cellulose几乎一样, 除了2上的-OH变成-NHCOCH₃
          • Chitin is the earth’s second most abundant carbohydrate polymer (after cellulose), and its ready availability and abundance offer opportunities for industrial and commercial applications.
        • Alginates 藻酸盐
          • A family of novel extended ribbon丝带 structures that bind metal ions, particularly calcium, they are the alginate polysaccharides

            • 一类具有新颖延伸带状结构的物质，其结构中能结合金属离子, 尤其是钙离子, 这就是藻酸盐多糖
        • Agarose 琼脂糖
          • An important polysaccharide mixture isolated from marine red algae
            从海洋红藻中分离的重要多糖混合物
          • The favored conformation of agarose in water is a double helix with a threefold screw axis.

            • 琼脂糖在水中的优势构象是三重螺旋轴的双螺旋结构
          • The central cavity is large enough to accommodate water molecules.
            中心腔可容纳水分子
          • Agarose and agaropectin readily form gels containing large amounts (up to 99.5%) of water and are widely used in lab as support medium for electrophoresis.
            琼脂糖和琼脂果胶易形成含水量高达 99.5% 的凝胶，广泛用于实验室作为电泳支持介质
          • The ability of agarose to assemble in complex bundles to form gels in aqueous solution makes it useful in numerous chromatographic procedures, including gel exclusion chromatography and electrophoresis.

            • 琼脂糖在水溶液中组装成复杂束形成凝胶的能力，使其可用于多种色谱方法，包括凝胶排阻色谱和电泳
          • Cells grown in culture can be embedded in stable agarose gel “threads” so that their metabolic and physiological properties can be studied.
            培养的细胞可包埋在稳定的琼脂糖凝胶 “丝” 中，以研究其代谢和生理特性
        • Glycosaminoglycans 粘多糖
          • A class of polysaccharides known as glycosaminoglycans is involved in a variety of extracellular (and sometimes intracellular) functions.
            一类被称为黏多糖的多糖参与多种细胞外（有时也在细胞内）功能
          • Glycosaminoglycans consist of linear chains of repeating disaccharides in which one of the monosaccharide units is an amino sugar and one (or both) of the monosaccharide units contains at least one negatively charged sulfate or carboxylate group -COOH
            糖胺聚糖由重复二糖构成的线性链组成，在这些重复二糖中，一个单糖单元为氨基糖，且其中一个（或两个）单糖单元至少含有一个带负电荷的硫酸基团或羧基-COOH
          • The repeating disaccharide structures found commonly in glycosaminoglycans are Heparin肝素
            在糖胺聚糖中常见的重复二糖结构包括肝素（Heparin）
            • with the highest net negative charge of the disaccharides, is a natural anticoagulant substance.
              在所有的二糖中，肝素的净负电荷最高，它是一种天然抗凝物质
            • It binds strongly to antithrombin 抗凝血酶 III (a protein involved in terminating the clotting process) and inhibits blood clotting.
          • Hyaluronate molecules may consist of as many as 25,000 disaccharide units, with molecular weights of up to 10⁷.
            透明质酸Hyaluronate(粘多糖的一种)分子所含的二糖单元数量最多可达 25000 个，分子量最高能达到 10⁷
          • Hyaluronates are important components of the vitreous humor 玻璃体 in the eye and of synovial fluid 滑液，the lubricant fluid of joints in the body.
            透明质酸是眼睛玻璃体（vitreous humor）以及滑液（synovial fluid，即人体内关节的润滑液）的重要组成成分