Presentation on theme: "Saccharide. Sugar molecules are formed from carbon, hydrogen and oxygen. Although sugars are made from only three elements, some sugar molecules are very."— Presentation transcript:
Sugar molecules are formed from carbon, hydrogen and oxygen. Although sugars are made from only three elements, some sugar molecules are very large and have complicated structures. Several different kinds of sugars exist, and each sugar has its own name. The name used to denote the entire family of sugar molecules is " saccharide.”
SACCHARIDE Under certain conditions, many small sugar molecules combine and form large, complex saccharide molecules. Saccharide molecules are classified according to the number of small, sugar molecules bound together: monosaccharides -simple sugar molecules disaccharides -two simple sugar molecules bound together polysaccharide -three or more sugar molecules bound together into a single molecule Large polysaccharide molecules consist of thousands of small monosaccharide molecules. Pectin and gums are examples of large polysaccharide molecules.
Monosaccharides The monosaccharides are called simple sugars, and many different kinds of simple sugars exist. Each simple sugar molecule contains three, four, five or six carbon atoms. The simple sugars are named according to the number of carbon atoms in the simple sugar molecule: For example, " pentose " sugars contain five carbon atoms, and " hexose " sugars contain six carbon atoms. Winemakers are primarily interested in the two major grape sugars, glucose and fructose, and both glucose and fructose are hexose monosaccharides. Enzymes produced by yeast convert both glucose and fructose into ethyl alcohol.
Glucose is the most common simple sugar Glucose is a part of many different disaccharides and polysaccharides. This is the sugar that provides energy for the human body. Glucose can be produced by splitting (hydrolysis) certain polysaccharides: For example, corn starch is a large polysaccharide molecule, and glucose is produced commercially by hydrolyzing (splitting) corn starch.
Fructose is found in many different kinds of fruit. It is the principal sugar in honey. Fructose is the sweetest tasting common sugar. Because it tastes sweeter than ordinary table sugar (sucrose), fructose is widely used, and it is the "sweetener of choice" in the food and beverage industries. Fructose is sometimes called "levulose ( סוכר פירות)."
Disaccharides Disaccharides are formed when two simple sugar molecules bind together. Sometimes two similar kinds of simple sugars combine. Often, two different kinds of sugar molecules combine to form a disaccharide. Disaccharides are produced commercially by the incomplete hydrolysis of larger polysaccharides. An alternate process combines two monosaccharide sugars by means of a condensation reaction to form disaccharide sugars. Usually, disaccharide sugars must be hydrolyzed and split into their simple sugar components before they can be fermented.
Maltose is a common disaccharide, and it is made up of two glucose sugar molecules. Maltose can be produced in several different ways. Very large quantities of maltose are produced each year from germinated grain( תבואה מונבטת ), and then the maltose is fermented to make beer. Maltose is also produced by the incomplete hydrolysis of starch, glycogen or dextrin.
Sucrose (ordinary white table sugar) is found in many fruits and vegetables, and it also occurs in a variety of grasses including sugar cane. Sucrose is a disaccharide made up of one glucose sugar and one fructose sugar. This sugar is produced commercially in great quantities from both sugar cane and sugar beets. Sugar stored in the roots of grape vines is in the sucrose form. Microorganisms, including wine yeasts, produce enzymes that can hydrolyze sucrose, and when sucrose hydrolyzes, each sucrose molecule splits into one glucose and one fructose molecule. This process produces a 50-50 mixture of glucose and fructose monosaccharides called "invert sugar."
Lactose (milk sugar) is only found in milk from mammals. It is a disaccharide made up of one glucose sugar and one galactose sugar molecule. Lactose is easily hydrolyzed, and it is the basis of many dairy products including cheese. Lactose is an interesting sugar because it has practically no sweet taste.
Polysaccharides Polysaccharides are large, complex carbohydrate molecules containing three or more monosaccharides. Living organisms use polysaccharides to store energy, and polysaccharides also form part of cell structural fibers. Starch consists of many glucose monosaccharides hooked together in both linear and branched forms. Pectin, gums and cellulose are also large polysaccharide molecules.
An outline of some of the types of sugars commonly found in cells.
Carbohydrates - Sugars and Polysaccharides Carbohydrates have the basic composition: (CH 2 O) n H-C-OH Monosaccharides - simple sugars, with multiple hydroxyl groups. Based on the number of carbons (e.g., 3, 4, 5, or 6) a monosaccharide is a triose, tetrose, pentose, or hexose, etc. Disaccharides - two monosaccharides covalently linked Oligosaccharides - a few monosaccharides covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units.
Monosaccharides : Aldoses (e.g., glucose) have an aldehyde at one end. C H-C-OH CH 2 OH HO-C-H OH H-C-OH D-glucose
Ketoses (e.g., fructose) have a keto group, usually at C #2. D-fructose C CH 2 OH HO-C-H O H-C-OH CH 2 OH
Nomenclature for stereoisomers : D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehyde.
For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and L-glucose are shown at right.
Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The six-carbon aldoses have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars).
An aldehyde can react with an alcohol to form a hemiacetal. Similarly a ketone can react with an alcohol to form a hemiketal.
Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons. E.g., glucose forms an intra-molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5, forming a six- member pyranose ring, named after the compound pyran.
Cyclization of glucose produces a new asymmetric center at C1, with the two stereoisomers called anomers, a & b.
Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either: below the ring ( above the ring (
Because of the tetrahedral nature of carbon bonds, the cyclic form of pyranose sugars actually assume a " chair " or " boat " configuration, depending on the sugar.
Sugar derivatives. Various derivatives of sugars exist, including: Sugar alcohol - lacks an aldehyde or ketone. An example is ribitol. Sugar acid - the aldehyde at C1, or the hydroxyl on the terminal carbon, is oxidized to a carboxylic acid. Examples are gluconic acid and glucuronic acid. Amino sugar - an amino group substitutes for one of the hydroxyls. An example is glucosamine. The amino group may be acetylated.
Glycosidic bonds: The anomeric hydroxyl group and a hydroxyl group of another sugar or some other compound can join together, splitting out water to form a glycosidic bond. R-OH + HO-R' --> R-O-R' + H 2 O For example, methanol reacts with the anomeric hydroxyl on glucose to form methyl glucoside (methyl-glucopyranose).
Disaccharides : Maltose, a cleavage product of starch, is a disaccharide with an glycosidic linkage between the C1 hydroxyl of one glucose and the C4 hydroxyl of a second glucose. Maltose is the a anomer, because the O at C1 points down from the ring. Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer. The configuration at the anomeric C1 is (O points up from the ring). The glycosidic linkage is represented as a "zig-zag" line, but one glucose residue is actually flipped over relative to the other.
Other disaccharides include: Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose and fructose. Because the configuration at the anomeric carbon of glucose is (O points down from the ring), the linkage is designated The full name is -D-glucopyranosyl-(1 -D- fructopyranose. Lactose, milk sugar, is composed of glucose and galactose with (1->4) linkage from the anomeric hydroxyl of galactose. Its full name is -D-galactopyranosyl-(1->4)-a-D-glucopyranose.
Polysaccharides : 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 a(1->4) glycosidic linkages. The end of the polysaccharide with an anomeric carbon (C1) that is not involved in a glycosidic bond is called the reducing end. Amylose adopts a helical conformation, as is apparent in the structure at right.
Amylopectin is a glucose polymer with mainly (1->4) linkages, but it also has branches formed by (1->6) linkages. The branches are generally longer. The branches produce a compact structure, and provide multiple chain ends at which enzymatic cleavage of the polymer can occur. 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, e.g., in muscle cells during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.
Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose, with (1->4) linkages. Every other glucose in cellulose is flipped over, due to the linkages. This promotes intrachain and interchain hydrogen bonds, as well as van der Waals interactions, that cause cellulose chains to be straight and rigid, and pack with a crystalline arrangement in thick bundles called microfibrils. The regular packing of cellulose strands within a microfibril, stabilized by lateral and above/below strand interactions.
Multisubunit Cellulose Synthase complexes in the plasma membrane spin out from the cell surface. Microfibrils consisting of 36 parallel, interacting cellulose chains. These microfibrils are very strong. The role of cellulose is to impart strength and rigidity to plant cell walls, which can withstand high hydrostatic pressure gradients.
Glycosaminoglycans (mucopolysaccharides) are polymers of repeating disaccharides. Within the disaccharides, the sugars tend to be modified, with acidic groups, amino groups, etc. When covalently linked to specific "core proteins," glycosaminoglycans form large complexes called proteoglycans. Hyaluronate is a glycosaminoglycan with a repeating disaccharide consisting of two glucose derivatives, glucuronate and N-acetylglucosamine. The glycosidic linkages are and Proteoglycans with a hyaluronate backbone are major constituents of the extracellular matrix.
Cell surface heparan sulfate glycosaminoglycans may be covalently linked to core proteins embedded in the plasma membrane. Heparan sulfate may be initially synthesized on a core protein as a polymer of alternating N-acetylglucosamine and glucuronate residues. In segments of this polymer, glucuronate residues may be converted to a sulfated sugar called iduronic acid, while N- acetylglucosamine residues may be deacetylated and/or sulfated. Modified segments remain interspersed with blocks of unmodified polymer, in patterns specific to a cell type. Modified segments may be recognized and bound by specific proteins involved in signaling and recognition at the cell surface.
Oligosaccharides of glycoproteins and glycolipids may be linear or branched chains. They often include modified sugars, e.g., acetylglucosamine, etc. O-linked oligosaccharide chains of glycoproteins vary in complexity. They are linked to a protein via a glycosidic bond between an initial sugar residue and a serine or threonine hydroxyl. O-linked oligosaccharides may have roles in recognition and interaction. A common O-linked glycosylation is the attachment of N- acetylglucosamine to protein serine or threonine residues. This reversible modification may have a regulatory role. N-linked oligosaccharides of glycoproteins are complex branched polymers. N-acetylglucosamine is linked to a protein via the side-chain N of an asparagine residue in a particular 3- amino acid sequence.
Additional monosaccharides are added, and the N-linked oligosaccharide chain is modified by removal and addition of residues, to yield a characteristic branched structure.
Many proteins secreted by cells have attached N- linked oligosaccharide chains. Genetic diseases have been attributed to deficiency of particular enzymes involved in synthesizing or modifying oligosaccharide chains of these glycoproteins. Carbohydrate chains of plasma membrane glycoproteins and glycolipids usually face the outside of the cell. They have roles in cell-cell interaction and signaling, and in forming a protective layer on the surface of some cells.
Lectins are glycoproteins that recognize and bind to specific oligosaccharides. Concanavalin A and wheat germ agglutinin are plant lectins that have been useful research tools. Selectins are integral proteins of the plasma membrane with lectin-like domains that protrude on the outer surface of mammalian cells. Selectins participate in cell-cell recognition and binding.