【正文】 Carbohydrates Molecular Biology 大連理工大學(xué) Carbohydrates ? Monosaccharides simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. ? Disaccharides 2 monosaccharides covalently linked. ? Oligosaccharides a few monosaccharides covalently linked. ? Polysaccharides polymers consisting of chains of monosaccharide or disaccharide units. I (CH 2 O ) n or H C OHIStructure: Molecular Biology 大連理工大學(xué) Monosaccharides Aldoses (., glucose) have an aldehyde group at one end. Ketoses (., fructose) have a keto group, usually at C2. CC O HHC HH OC O HHC O HHC H2O HD g l u c o s eOH C HH OC O HHC O HHC H2O HC H2O HC OD f r u c t o s eMolecular Biology 大連理工大學(xué) D and L sugars ?For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group. ?Most naturally occurring sugars are D isomers. D and L sugars are mirror images of one another. O H O H C C H – C – OH H O – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H C H 2 OH C H 2 OH D g luc o s e L g lu c o s e Molecular Biology 大連理工大學(xué) Glycosidic Bonds The hydroxyl group of one sugar and the hydroxyl group of another sugar or some other pound can join together, splitting out water to form a glycosidic bond: ROH + HOR39。 ? ROR39。 + H2O ., methanol reacts with an OH on glucose to form methyl glucoside. OHH OHH OHO HO HHHO H? D g l u c o p y r a n o s eOHH OHH OHO C H3O HHHO Hm e t h y l ? D g l u c o p y r a n o s eC H3 O H+m e t h a n o lH2OMolecular Biology 大連理工大學(xué) Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C1 points up). The b(1?4) glycosidic linkage is represented as a zigzag, but one glucose is actually flipped over relative to the other. HOO HHO HHO HC H 2 O HHO HO HHO HHO HC H 2 O HHOHH123546123456ma lt oseHOO HHO HHO HC H 2 O HHO O HHHO HHO HC H 2 O HHHHO123456123456c e ll obioseDisaccharides: Maltose, a cleavage product of starch, is a disaccharide with an ?(1?4) glycosidic link between the C1 OH C4 OH of 2 glucoses. It is the ? anomer, because the O on C1 points down. Molecular Biology 大連理工大學(xué) Disaccharides Other mon disaccharides include: ?Sucrose, mon table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose fructose. Because the configuration at the anomeric C of glucose is ? (O points down from ring), the linkage is ?(1?2). The full name of sucrose is ?Dglucopyranosyl(1?2)bDfructopyranose.) ?Lactose, milk sugar, is posed of galactose glucose, with b(1?4) linkage from the anomeric OH of galactose. Its full name is bDgalactopyranosyl (1?4)?Dglucopyranose Molecular Biology 大連理工大學(xué) Polysaccharides 多糖 ?They are very plex molecules because sometimes covalent bonds 共價(jià)鍵 occur between many parts of carbon atoms, allowing one sugar unit to be joined to more than two other sugars, which results in the formation of highly branched macromolecules. ? Polysaccharides play various roles, from energy storage (starch, glycogen) to structure (cellulose). Molecular Biology 大連理工大學(xué) Glucose Storage in Plants Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with ?(1?4) linkages. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end. Amylose adopts a helical conformation. H OO HHO HHO HC H 2 O HHO HHO HHO HC H 2 O HHOHH H OOHO HHO HC H 2 O HHH HOHO HHO HC H 2 O HHO HHH OOHO HHO HC H 2 O HHOH1654312a my l o s e Molecular Biology 大連理工大學(xué) Amylopectin is a glucose polymer with mainly ?(1?4) linkages, but it also has branches formed by ?(1?6) linkages. Branches are generally longer than shown above. The branches produce a pact structure provide multiple chain ends at which enzymatic cleavage can occur. Molecular Biology 大連理工大學(xué) Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more ?(1?6) branches. The highly branched structure permits rapid release of glucose from glycogen stores, ., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants. HOO HHO HHO HC H2O HHO HHO HHO HC H2O HHOHH HOOHO HHO HC H2HH HOHO HHO HC H2O HHO HHHOOHO HHO HC H2O HHOHO1 46H OHO HHO HC H2O HHH H OHO HHO HC H2O HHHO1O H3452g ly c og e n Molecular Biology 大連理工大學(xué) Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(1?4) linkages. The role of cellulose is to impart strength and rigidity to plant cell walls. c e l l u l o s e H OO HHO HHO HC H 2 O HHOHO HHO HC H 2 O HHOH H OO HO HHO HC H 2 O HHH OHO HHO HC H 2 O HHHO HH OO HO HHO HC H 2 O