Lecture 4 - Carbohydrates.pdf

Carbohydrates & Glycobiology In class exercise Monosaccahrides and Disaccharides Polysaccahrides Glycoconjugates: Glycoproteins, Proteoglycans and Glycolipids Carbohydrates as Informational Molecules: The Sugar Code Techniques in Carbohydrates Function of the carbohyd...

Carbohydrates & Glycobiology In class exercise Monosaccahrides and Disaccharides Polysaccahrides Glycoconjugates: Glycoproteins, Proteoglycans and Glycolipids Carbohydrates as Informational Molecules: The Sugar Code Techniques in Carbohydrates Function of the carbohydrates Glycosaminoglycans GlcUA(1β→3)GalNAc(1β→4))n Hurler disease: glycosaminoglycans cannot be degraded.  Function of the carbohydrates complex coat - frequently attached to lipids and proteins. Secreted proteins are often extensively decorated with carbohydrates essential to a protein’s function. most common post-translational modification of proteins. The extracellular matrix in higher eukaryotes is rich in secreted carbohydrates central to cell survival and cell-to-cell communication. Carbohydrates, carbohydrate-containing proteins, and specific carbohydrate- binding protein interactions that allow cells to form tissues, are the basis of human blood groups, used by a variety of pathogens to gain access to their hosts. A key property of carbohydrates that allows their many functions is the tremendous structural diversity Fundamental constituents of life: DNA -deoxyribose, a cyclic five-carbon sugar monosaccharide. Glycobiology is the study of the synthesis and structure of carbohydrates and how carbohydrates are attached to and recognized by other molecules such as proteins.  Glycoconjugates Glycoconjugates-Covalently joined carbohydrates to lipids or proteins. Three types: – Glycoproteins-One or several oligosaccharides of varying complexity joined covalently to a protein. – Proteoglycans are a subclass of glycoproteins in which the carbohydrate units are polysaccharides that contain amino sugars glycosaminoglycan. – Glycolipids-Carbohydrates are attached to lipid molecules via covalent bonds. – Glycobiology is the study of the synthesis and structure of carbohydrates and how carbohydrates are attached to and recognized by other molecules such as proteins. – new “omics” study in glycobiology are called —glycomics. Glycoproteins Glycoprotein hormone erythropoietin (EPO) Secreted by kidneys - production of red blood cells. N-glycosylated at three asparagine residues and O- glycosylated on a serine residue Glycosylation functions in nutrient sensing Covalent attachment of a single N- acetylglucosamine (GlcNAc) to serine or threonine residues of cytoplasmic, nuclear, and mitochondrial proteins O-GlcNAc transferase More than 4000 proteins are modified by GlcNAcylation Like phosphorylation, GlcNAcylation is reversible, with GlcNAcase catalyzing the removal of the carbohydrate Glycolipid  Significance of proteoglycans Proteoglycans function as structural components and lubricants. he repeating disaccharide unit (GlcUA(1β→3)GalNAc(1β→4))n of chondroit in sulfate Proteoglycans form the extracellular “filler” between cells. Proteoglycans, composed of polysaccharides and protein, have important structural roles anticoagulant Cartilage  Biological significance of glycoconjugates Proteoglycans not only function as lubricants and structural components in connective tissue but also mediate the adhesion of cells to the extracellular matrix and bind factors that regulate cell proliferation. Glycoconjugates as receptors  Fuel: Carbohydrate metabolism in bacteria All bacteria must utilize the energy sources in their environment in order to produce ATP. Heterotrophic bacteria take these sugars from the environment for nutrition and energy while autotrophic bacteria produce their own sugars.  Metabolism of more complex sugars Disaccharides and polysaccharides are composed of simple sugars that are linked by glycosidic bonds. Many bacteria use glucose, a monosaccharide or simple sugar, because many bacteria possess the enzymes required for the degradation and oxidation of this sugar. Fewer bacteria are able to use complex carbohydrates like disaccharides (lactose or sucrose) or polysaccharides (starch).  Metabolism of more complex sugars Bacteria must produce enzymes to cleave these bonds such that the simple sugars that result can be used by the cell. – For example, lactose is a disaccharide consisting of monomeric glucose and monomeric galactose linked by a glycosidic bond. Bacteria that use lactose must produce the enzyme lactase (β-galactosidase) to break the glycosidic bond between these monomers. – Starch is a large polysaccharide consisting of long chains of monomeric glucose linked by glycosidic bonds. Bacteria that use starch produce an exoenzyme, alpha amylase, that break these bonds such that free monomeric glucose is produced. If the bacteria cannot produce these enzymes then the complex carbohydrate is not used.  Identification based on enzymes/carbohydrate metabolism Each bacterium has its own collection of enzymes that enable it to use diverse carbohydrates; this is often exploited in the identification of bacterial species. One can determine if a given bacterial species can utilize a given carbohydrate by checking for the presence of byproducts that are produced by the oxidation of these carbohydrates. Kendall A.I. (1923):Carbohydrate Identification by Bacterial Procedures: Studies in Bacterial Metabolism, LXVII. JIDS, vol 32(5) 362-368.  Different bacteria yield different end products Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly" Simple Classification of Carbohydrates Monosaccharides-Simple sugar consisting 3-9 carbon atoms and a single polyhydroxy aldehyde or ketone unit. eg. Glyceradehyde, Glucose, fructose Disaccharides: Consisting of two monosaccharide units joined by glycosidic linkage or bond. Oligosaccharides: Consisting of several monosaccharide units joined by glycosidic linkage. Polysaccharides: Minimum 20 monosaccharide units joined by glycosidic linkage.  Monosaccharides: Aldose and Ketose Aldose: Aldehyde group containing monosaccharide Ketose: Ketone group containing monosaccharide Asymmetric carbon : carbon which is attached to 4 different groups.  A tetrose-2 asymmetric carbon 4 3 2 1 An aldopentose-3 asymmetric carbon 1 An aldohexose-4 symmetric carbon 1  For example Ketoses have one less asymmetric center than aldoses with the same number of carbon atoms. D-Fructose is the most abundant ketohexose. Aldehyde carbohydrates diastereoisomers Ketone carbohydrates diastereoisomers Counting carbons in different views  Monosaccharide: D or L-isomer D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group also called penultimate carbon (one before last carbon). D & L sugars are isomers and are mirror images of one another. They have the same name. For example, D-glucose and L-glucose.  Isomeric forms of the carbohydrates Constitutional isomers have identical molecular formulas but differ in how the atoms are ordered. eg. Dihydroxyacetone and glyceraldehyde. Stereoisomers are isomers that differ in spatial arrangement. They can have D or L configuration. Eg. D-and L-Glyceraldehyde.  Isomeric forms of the carbohydrates Enantiomers: Stereoisomers which are mirror images of each other. Most vertebrate monosaccharides have the D configuration. Sterioisomers Enantiomers Non superimposable mirror images, chiral D-glucose L-glucose Enantiomers Non superimposable mirror images, chiral  Diastereoisomer Monosaccharides made up of more than three carbon atoms have multiple asymmetric carbons, and so they can exist not only as enantiomers but also as diastereoisomers, The number o possible stereoisomer equals 2n, where n is the number of asymmetric carbon atoms. Thus, a six-carbon aldose with 4 asymmetric carbon atoms can exist as 16 possible diastereoisomers, of which glucose is one such isomer. Non superimposable mirror images, chiral Isomers that are not mirror images  Isomeric forms of the carbohydrates Achiral objects are superimposable with their mirror images. For example, two pieces of paper are achiral.... A Chiral molecule has a mirror image that cannot line up with it perfectly- the mirror images are NOT superimposable. They cannot have a plane of symmetry. Many of our biological molecules such as our DNA, amino acids and sugars, are chiral molecules.  Diastereoisomer Isomers that are not mirror images  Diastereoisomer: Epimers Sugars that are diastereoisomers differing in configuration at only a single asymmetric center are epimers. Thus, D-glucose and D-mannose are epimeric at C- 2; D-glucose and D-galactose are epimeric at C-4.  Anomers Isomers that differs at a new asymmetric carbon atom formed on ring closure. Anomers Ignore this one Chiral What is the difference between D- glucose and L-glucose? L-form of monosaccharides are very rarely found in nature.  Cyclisation of carbohydrates Carbohydrates can freely convert between linear and cyclic forms but are generally cyclic in solution Generally, in solution the equilibrium is at 1/3 α, 2/3 β, and