Chapter 11 Carbohydrates

Carbohydrates are aldehydes or ketone with multiple hydroxyl groups

Energy stores, fuels, and metabolic intermediates Structural framework of DNA and RNA Structural elements in the cell walls of bacteria and plans Linked to many proteins and lipids (cell-cell communication)

Carbohydrates as mediators of cellular interactions Structural diversity in size and stereochemical configuration Built from monosaccharides Attach to lipids and proteins Glycomics: Studies on oligosaccharide structures and functions

11.1 Monosaccharides are aldehydes or ketones with multiple hydroxyl group -Monosaccharides : the simplest carbohydrate. Important fuel, building blocks for nuclearic acid -The smallest monosaccharides: Ketone group Aldehyde group

Stereoisomer - Enantiomer = mirror images of each other

※ Fischer projection -Horizontal bond : in frond of the page -Vertical bond : behind of the page -Clear and simple views of the stereochemistry at each carbon center.

Three carbon triose Four carbon tetrose Five carbon pentose Six carbon hexose D- aldose

D- ketose Three carbon triose Four carbon tetrose Five carbon pentose Six carbon hexose

Pentoses and hexoses cyclize to form furanose and pyranose rings -The predominant forms of ribose, glucose, fructose, and many other sugars are cyclized into rings. -Ring forms are energetically stable. -Aldohexose glucose : C-1 aldehyde reacts with the C-5. -Ketohexose fructose : C-2 keto group reacts with a hydroxyl group of C-5 or C-6.

pyranose

- α-D-glucopyranose: Hydroxyl group is on the opposite side of the ring from the CH 2 OH. - β-D-glucopyranose: Hydroxyl group is on the same side of the ring from the CH 2 OH. - C-1 carbon atom : anomeric carbon atom - α and β form : anomer - In case of glucose : 1/3 α anomer, 2/3 β anomer C-1

furanose

※ Haworth projection : The approximate plane of the ring is perpendicular to the plane of the paper. -Heavy line : projecting toward the reader. -Easy depiction of the stereochemistry of sugars.

- Fructose forms both pyranose and furanose ring.

Pyranose and furanose rings can assume different conformations -The six-membered pyranose ring is not planar. -Pyranose rings adopt two classes of conformations -Chair : substituents on the ring carbon atoms have two orientation ; axial and equatorial. -Boat : glucose boat form disfavored because it is quite sterically hindered.

-The chair form is more stable owing to less steric hindrance because the axial positions are occupied by hydrogen atoms. a : axial e : equatorial

-The five-membered furanose ring is also not planar. -Envelope form : four atoms are nearly coplanar and the fifth is about 0.5Å away from this plane.

Monosaccharides are joined to alcohols and amines through glycosidic bonds -Monosaccharides react with alcohols and amines to form modified monosaccharide. -Ex> D-glucose react with methanol. -O-glucosidic bond : OCH3

-N-glucosidic bond : NR 2 Ex) Nucleotide; ribose+adenine

- Methyl glycopyranoside differ in reactivity from the parent monosaccharides. -Unmodified glucose reacts with oxidizing agents such as Cu 2+ because the open-chain form has a free aldehyde group that is readily oxidized.(=reducing sugar) -Methyl glucopyranoside do not react.(=non-reducing sugar) -Glucose react with hemoglobin to form glycosylated hemoglobin. diabete Fehling’s solution

- Carbohydrates can be modified by the addition of substituents. Such modified carbohydrates are often expressed on cell surfaces.

Phosphorylated sugars are key intermediates in energy generation and biosyntheses - Phosphorylation is a common modification of sugars. Ex> The first step in the breakdown of glucose to obtain energy is its conversion into glucose 6-phosphate. -Phosphorylation makes sugars anionic(negative charge) to prevent sugars from leave the cell spontaneously by crossing membrane -Reactive intermediates in purine and pyrimidine biosyntesis

11.2 Complex carbohydrates are formed by the linkage of monosaccharides -Glycosidic bond : the linkage of two or more monosaccharides by O-glycosidic bond. -Various glycosidic linkage is possible. -Structurally diverse molecules.

Sucrose, lactose, and maltose are the common disaccharides

-A transport form of carbohydrates in plants. -Obtained commercially from cane or beet. -Glucose + fructose -Can be cleaved by sucrase.

-Disaccharide of milk. -Galactose + glucose. By β -1,4-glycosidic linkage -Can be cleaved by lactase in human being. -Can be cleaved by β -galactosidase in bacteria.

-Glucose + glucose. By α -1,4-glycosidic bond. -Can be cleaved by maltase.

-Sucrase, lactase, maltase are located on the outer surfaces of epithelial cells lining the small intestine. Microvillus

Glycogen and starch are mobilizable stores of glucose -Polysaccharides : the linkage of multiple monosaccharides. -Role : energy storage and maintaining the structural integrity. -Homopolymer : all of the monosaccharides are the same. -Glycogen : storage form of glucose. large, branched polymer. -Branch point : α -1,6-glycosidic bond. present about once in 10 unit.

-Starch : nutritional reservoir of carbohydrates in plants. -Two types of starch = amylose and amylopectin. -Amylose : unbranched, α -1,4-linkage. -Amylopectin : branched, one α -1,6-linkage per 30 α -1,4- linkages.

Cellulose, the major structural polymer of plants, consists of linear chains of glucose units -Cellulose : major polysaccharide of glucose found in plants. structural role rather than nutritional role. most abundant organic compound in the biosphere. β -1,4-linkage of glucose = long, straight chains. Optimal for fiber having a high tense strength α -1,4-linkage (starch and glycogen) = helix conformation. Suitable for accessible store of sugar.

- Proteoglycan : proteins attached to a particular type of polysaccharide called glycosaminoglycan (95% by weight). As joint lubricants and structural components in connective tissue -Many glycosaminoglycans are anionic polysaccharide chains made of repeating disaccharide units containing glucosamine or galactose. -Sugars have a negatively charged carboxylate or sulfate.

※ Aggrecan : shock aborber. - Large protein of 2397 amino acids. - Three globular domain. -The first domain binds to glycosaminoglycanname (=hyaluron). Cartilage key components: aggrecan, collagen - collagen: triple helix provides structure and tensile strength.

Specific enzymes are responsible for oligosaccharide assembly - Glycosyltransferase : catalyze the formation of the glycosidic bonds that link monosaccharide. intermediates

-Carbohydrates are attached to proteins and lipids on the surfaces of the red blood cells. -The A and B antigens are formed by the addition of one extra monosaccharide by type A transferase and type B transferase respectively. -Why are different blood types present in the human population? Arms race! ABO blood type

11.3 Carbohydrates can be attached to proteins to form glycoproteins -A carbohydrate group can be covalently attached to a protein to form a glycoprotein. -Many glycoproteins are components of cell membranes. -Sugars are attached either to the amide nitrogen atom or to the oxygen atom N-linkageO-linkage Attach atomAmide nitrogen atomOxygen atom ResidueAsparagineSerine or threonine sequenceAsn-X-Ser / Asn-X-Thrno

Carbohydrates can be linked to proteins through asparagine (N-linked) or through serine or threonine(O- linked) residues

- All N-linked oligosaccharides have in common a pentasaccharides core consisting of three mannose and two N-acetylglucosamine residue.

※ Erythropoietin (EPO) - Glycoprotein hormone. - Secreted by the kidneys and stimulates the production of red blood cells amino acids. - Three N-glycosylation and one O-glycosylation. (40% carbohydrate by weight) - Glycosylation of EPO enhances the stability of the protein in blood. - Unglycosylated EPO has only about 10% of the bioactivity of the glycosylated. Removed by kidney - recombinant human EPO for anemias, Drug-testing

Protein glycosylation takes place in the lumen of the Endoplasmic Reticulum and in the Golgi complex - Protein glycosylation takes place inside the lumen of the ER and in the Golgi complex. -N-linked glycosylation begins in the ER and continues in the Golgi. -O-linked glycosylation takes place in the Golgi.

20X ※ Dolichol phosphate : lipid molecule located in ER membrane and containing about 20 isoprene units. ※ Phosphate group : the site of attachment of the large activated oligosaccharide. And is transferred to Asn residue of the growing polypeptide chain. Activated sugar and complex enzyme are located on the ER lumen;

- O-inked sugar units are fashioned. - N-linked sugar are modified in many different ways. Golgi is Major Sorting Center

Errors in glycosylation can result in pathological conditions Ex> I-cell disease. (=mucolipidosis) -Lysosomal storage disease. -A carbohydrate marker directs degradative enzymes from Golgi to lysosome. -Patients having I-cell disease cannot attach sugar(mannose) to degradative enzymes. Enzymes cannot be targeted to lysosome but secreted to blood. -Inclusion body of undigested glycosaminoglycans and glyco lipids in lysosome. -Psychomotor retardation and skeletal deformities. -Glycosylation is important for the processing, stability, and targeting of proteins. Lysosome target marker

Oligosaccharides can be “sequenced” Detachment of oligosaccharide from the protein by N-glycosidase F MALDI-TOF or other mass spectrometric techniques Cleavage of the oligosaccharide with enzymes of varying specificities MALDI-TOF or other mass spectrometric techniques Repeat and collect information Glycosylation site?

An enormous number of patterns in the composition and structure of surface sugars are possible because… 1). Different monosaccharides can be joined 2). C-1 linkage can have either α and β configuration 3). Branching Carbohydrate = information rich molecule Lectins are specific carbohydrate-binding proteins

Lectins promote interactions between cells -The diverse carbohydrate structures displayed on cell surface → cell-cell and cell-environment interaction. ※ Lectin(from the Latin “to select”) -Bind to specific carbohydrate. -Function : cell-cell contact. -Contain two or more binding sites for carbohydrate. -C type lectin : Ca 2+ requiring, found in animal.

※ Selectin : member of C type family. Bind to sites of injury in the inflammatory response. L selectin: Lymph-node vessel E selectin: endothelium P selectin: activated blood platelet Embryo implantation: L selection of embryo

-Plants also are rich in lectins. Potent insecticides -The binding specificities of lectins from plants have been well characterized. -E.coli bacteria adhere to the gastrointestinal tract by lectins on the E.coli surface

Influenza virus binds to sialic acid residues -Some viruses gain entry into host cells by adhering to cell-surface carbohydrates. Ex> influenza virus recognizes sialic acid residues through “hamagglutinin” protein. (H) After the virus penetrates the host cell, another viral protein, “neuraminidase”(N), cleaves the glycosidic bond to free the virus. Tamiflu and Relenza are the inhibitor of neuraminidase Ex> H5N1(Avian Influenza), H1N1(Swine Influenza)