Biochemistry 5 - Carbohydrates And Glycobiology PDF

Carbohydrates and Glycobiology Key principles Carbohydrates can have multiple chiral carbons The configuration of groups around each carbon atom determines how the compound interacts with other biomolecules Monomeric subunits, monosaccharides, serve as the building blocks of large c...

Carbohydrates and Glycobiology Key principles Carbohydrates can have multiple chiral carbons The configuration of groups around each carbon atom determines how the compound interacts with other biomolecules Monomeric subunits, monosaccharides, serve as the building blocks of large carbohydrate polymers Polysaccharides assume 3-D structures with the lowest energy conformations, determined by covalent bonds, hydrogen bonds, charge interactions, and steric factors The sequence of complex polysaccharides are determined by the intrinsic properities of the biosynthetic enzymes that add each monomeric unit to the growing polymer Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers Carbohydrates The most abundant biomolecules on Earth Photosynthesis converts more than 100 billion metric tons of CO2 and H2O into cellulose and other plant products each year Sugar and starch are a dietary staple and the oxidation of carbohydrates is the central energy-yielding pathway in most nonphotosynthetic cells Carbohydrate polymers (also called glycans) serve as structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals Other carbohydrate polymers lubricate skeletal joints and participate in cell-cell recognition and adhesion Complex carbohydrate polymers covalently attached to proteins or lipids act as signals that determine the intracellular destination or metabolic fate of these hybrid molecules, called glycoconjugates Functions of carbs in living systems nutritional (energy storage - starch and glycogen), fuels (glucose), metabolic intermediates (fructose) structural (components of nucleotides - ribose, plant – cellulose and bacterial -peptidoglycan cell walls, arthropod exoskeletons - chitin, animal connective tissue -glycosaminoglycans) informational (cell surface of eukaryotes -- molecular recognition, cell-cell communication) osmotic pressure regulation (bacteria)  Chemistry of carbs Hydrates of carbon Many, but not all, carbohydrates have the empirical formula: (CH2O)n or Cn (H2O) (ratio 1:2:1) n = 3 to 7 (5, 6 in biomat.) Some also contain nitrogen (chitin; C8H13O5N), phosphorus (in common food), or sulphur Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups (at least 2) Carbohydrates are polyhydroxy aldehydes or ketones Functional groups - recap for you Classification of carbs Monosaccharides- One unit of carbohydrate Anywhere from two to ten monosaccharide units, make up an oligosaccharide Disaccharides- Two units of carbohydrates linked together Polysaccharides are much larger, containing hundreds of monosaccharide units Carbohydrates also can combine with lipids to form glycolipids OR With proteins to form glycoproteins Monosaccharides and Disaccharides The simplest of the carbohydrates They are colourless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents; most have a sweet taste Backbone: unbranched carbon chains with single bonds linking all carbons One of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group Other carbon atoms are bonded to a hydroxyl group All common monosaccharides and disaccharides have names ending with the suffix “-ose” They are aldehydes (aldoses) or ketones (ketoses) with two or more hydroxyl groups 3C - Trioses, 4C - tetroses, pentoses, hexoses…  The six-carbon monosaccharides glucose and fructose have five hydroxyl groups Many of the carbon atoms to which the hydroxyl groups are attached are chiral centers, which give rise to the many sugar stereoisomers What are isomers and stereoisomers? What is chiral center? Stereoisomerism in sugars is biologically significant glucose because the enzymes that act on sugars are strictly stereospecific, typically preferring one stereoisomer to another by three or more orders of magnitude  All the monosaccharides (except dihydroxyacetone) contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) dihydroxyacetone and therefore has two different optical isomers (enantiomers) glyceraldehyde  To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas. In these projections, horizontal bonds project out of the plane of the paper, toward the reader; vertical bonds project behind the plane of the paper, away from the reader Examples of isomers 1. Glucose 2. Fructose 3. Galactose 4. Mannose Same chemical formula C6 H12 O6 but different structure due to different position of atoms  D an L isomers In general, a molecule with n chiral centers can have 2n stereoisomers (Glyceraldehyde has 21 = 2 and the aldohexoses, with four chiral centers, have 24 = 16) The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon Monosaccharides examples (a) Two trioses, an aldose and a ketose. The carbonyl group in each is shaded. (b) Two common hexoses (c) The pentose components of nucleic acids. D-Ribose is a component of ribonucleic acid (RNA), and 2-deoxy-D-ribose is a component of deoxyribonucleic acid (DNA) Phosporylated Derivatives Some sugar intermediates are phosphate esters Ex. Glucose 6-phosphate Stable at neutral pH and bear a negative charge Functions to trap sugar inside the cell because most cells do not have membrane transporters for phosphorylated sugars The Common Monosaccharides Have Cyclic Structures In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals  Cyclization of monosaccharides Less then 1% of CHO (monosaccharides) exist in an open chain form Predominantly found in a ring form Cyclization involves reaction of C-5 OH group with the C-1 aldehyde group or C-2 of keto group α and β Anomers The carbonyl carbon after cyclization becomes the anomeric carbon (carbon bonded to 2 oxygens) This creates α and β configuration In α configuration the OH is on the same side of the ring in Fischer projection (pointing down) In Haworth's it is on the trans side of CH2OH  Six membered ring structures are called Pyranoses Five membered ring structures are called Furanoses Fischer projection and Haworth perspective Haworth perspective Enzymes can distinguished between these two forms: Glycogen is synthesized from α-D glucopyranose Cellulose is synthesized from β -D glucopyranose  If a carb has a FREE anomeric carbon (not tide-up in a glycosidic bond) it is aka a reducing sugar (has electrons to donate) Most mono and disaccharides have anomeric carbons and are reducing sugars except sucrose Monosaccharides can be oxidized by relatively mild oxidizing agents such as cupric (Cu 2+ ) ion. The carbonyl carbon is oxidized to a carboxyl group. This is the basis of Fehling’s reaction, a semiquantitative test for the presence of reducing sugar that for many years was used to detect and measure elevated glucose levels in people with diabetes mellitus. Disaccharides The ring forms can connect to each other to create dimers, oligomers and polymers, producing disaccharides, oligosaccharides and polysaccharides Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar molecule, typically in its cyclic form, reacts with the anomeric carbon of the other (dehydration condensation reaction – removal of water molecule) The resulting compound is called a glycoside Glycosidic linkage 1,4 glycosidic bonds are formed due to condensation reactions between a hydroxyl oxygen atom on carbon-4 on one sugar and the α-anomeric form of C-1 on the other There are two types of glycosidic bonds - 1,4 alpha and 1,4 beta glycosidic bonds 1,4 alpha glycosidic bonds are formed when the OH on the carbon-1 is below the glucose ring 1,4 beta glycosidic bonds are formed when the OH is above the plane Examples of disaccharides sucrose maltose Lactose https://www.youtube.com/watch?v=KNsFxPgsbrU Polysaccharides 2 types: HOMOpolysaccharides (all 1 type of monomer), e.g., glycogen, starch, cellulose, chitin (serve as storage forms and structural elements) HETEROpolysaccharides (different types of monomers), e.g., peptidoglycans, glycosaminoglycans (provide extracellular support) Polysaccharides Polysaccharides generally do not have defined lengths or molecular weights This distinction between proteins and polysaccharides is a consequence of the mechanisms of assembly There is no template for polysaccharide synthesis The program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of monomer units Functions of polysaccharides: Glucose storage glycogen in animals & and bacteria, starch in plants Both are heavily hydrated because they have many exposed hydroxyl groups available to hydrogen bond Structure (cellulose, chitin, peptidoglycans, glycosaminoglycans) Information (cell surface oligo- and polysaccharides, on proteins/glycoproteins and on lipids/glycolipids) Osmotic regulation  Starches Starches are polymers of glucose Two types are found: Amylose – 15-20 % Amylopectin – 80-85 % Amylose consists of linear, unbranched chains of several hundred glucose residues (units) The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms 6CH OH CH OH 2 CH OH 2CH OH CH OH2 2 2 ( α1---4 linkages) H H O H H 5 H O H H H O H H H O H H H O H OH H 1 4 OH H 1 OH H OH H OH H O O O O OH OH 2 3 H OH H OH H OH H OH H OH amylose  Amylopectin differs from amylose in being highly branched At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (α1---4)(1---6 linkages). CH 2OH CH 2OH H O H H O H amylopectin H H OH H OH H 1 O OH O H OH H OH CH 2OH CH 2OH 6 CH 2 CH 2OH CH 2OH H O H H O H H 5 O H H O H H O H H H H H H OH H OH H 4 OH H 1 4 OH H OH H O O O O OH OH 2 3 H OH H OH H OH H OH H OH Starches The total number of glucose residues in a molecule of amylopectin is several thousands Starches are insoluble in water and thus can serve as storage depots of glucose Plants convert excess glucose into starch for storage Rice, wheat, and corn (maize) are major sources of starch in the human diet Before starches can enter (or leave) cells, they must be digested The hydrolysis of starch is done by amylases With the aid of an amylase (such as pancreatic amylase), water molecules enter at the 1 -> 4 linkages, breaking the chain and eventually producing a mixture of glucose and maltose Glycogen Animals store excess glucose by polymerizing it to form glycogen The structure of glycogen is similar to that of amylopectin, but branches in glycogen tend to be shorter and more frequent Alpha1-4linked subunits, with alpha 1-6 linked branches Glycogen is broken down into glucose when energy is needed (a process called glycogenolysis) The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise The liver and skeletal muscle are major depots of glycogen Storage of glucose as polymers avoids high osmolarity Hepatocytes in the fed state store glycogen equivalent to a glucose concentration of 0.4 M That would elevate the osmolarity in the cytosol, which would results in osmotic entry of water, and the cell could burst (lysis) More branching – occurring 10 to 16 glucose units  Cellulose is the single most abundant organic molecule in the biosphere It is the major structural material of which plants are made (Wood is largely cellulose while cotton and paper are almost pure cellulose) Very tough, fibrous, water-insouble substance cellulose differs profoundly from starch in its properties: 1. Made up of repeating units called cellobiose (β 1—4 linkage) 2. This produces a long, straight, rigid molecule 3. There are no side chains in cellulose as there are in starch (many opportunities for hydrogen bonds to form between adjacent chains) The result is a series of stiff, elongated fibrils — the perfect material for building the cell walls of plants) Dietary fibre Cellulose and other non-digestible carbohydrates in food do not supply energy but are an important dietary fibres (found in wheat bran, vegetables, whole grains) Slow digestion Add bulk to stool – prevent constipation Fill stomach – reduce food intake lower risk of heart disease Peptidoglycans Peptidoglycan – rigid component of bacterial cell walls reinforces the bacterial cell wall Heteropolymer of alternating (beta1-4) linked N- acetylglucosamine and N- acetylmuramic acid residues Cross-linked by short peptides Specific antibacterials interfere with the synthesis of the cell wall, weakening the peptidoglycan scaffold within the bacterial wall so that the structural integrity eventually fails Glycoconjugates - information carriers communication between cells and their extracellular surroundings labelling proteins for transport to and localization in specific organelles recognition sites for extracellular signal molecules (growth factors, for example) or extracellular parasites (bacteria and viruses) on almost every eukaryotic cell, specific oligosaccharide chains attached to components of the plasma membrane form a carbohydrate layer (the glycocalyx) several nanometers thick, that serves as an information-rich surface that the cell presents to its surroundings Glycoconjugates Central players in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, wound healing, and other cellular processes In most of these cases the informational carbohydrate is covalently joined to a protein or a lipid to form a glycoconjugate, which is the biologically active molecule Glycobiology is the study of the structure and function of glycoconjugates Types of glycoconjugates Three types of glycoconjugates occur in nature: Glycoproteins Proteoglycans Glycosphingolipids Glycosaminoglycans and Proteoglycans Glycosaminoglycans are a special type of polysaccharide that typically consists of repeating disaccharide units which have been modified to contain an amino group and some sort of negatively charged groups (usually sulphate or carboxylic acid) Chondroitin sulfate Heparin Keratan sulfate Hyaluronate Proteoglycans are proteins that are attached to glycosaminoglycans These modified protein-sugar biomolecules function as: Joint lubricant Structural components of tissue Bind cells to the ECM Regulate movement of molecules through ECM Aggrecan Some proteoglycans can form proteoglycan aggregates enormous supramolecular assemblies of many glycosaminoglycan-decorated core proteins all bound to a single molecule of hyaluronan Aggrecan binds multiple chains of chondroitin sulfate and keratin sulfate via covalent linkages to serine residues When a hundred or more of these decorated core proteins bind a single extended hyaluronan molecule the resulting proteoglycan aggregate has a mass comparable to a bacterial cell Aggrecan interacts with collagen in the ECM of cartilage, contributing to the development, tensile strength, and resilience of this connective tissue Glycoproteins Proteins can be modified by the addition of carbohydrate components to form glycoproteins Protein glycosylation Oligosaccharides may be attached to proteins by: Nitrogen atom on the side chain of aspargine (called the N-linkage) Oxygen atom on the side chain of threonine or serine (O-linkage) Takes place at the ER and Golgi apparatus About half of all proteins of mammals are glycosylated Many of these are plasma membrane proteins and secretory proteins Examples of glycosylated secretory proteins include immunoglobulins (antibodies) and certain hormones such as follicle-stimulating hormone and luteinizing hormone, mucins, EPO The biological advantages of protein glycosylation include improving the solubility of proteins and stability (ex.EPO in plasma) Glycoproteins Glycosaminoglycans, Proteoglycans and Glycoproteins https://www.ncbi.nlm.nih.gov/books/NBK544295/#:~:text=Glycosami noglycans%20(GAGs)%2C%20also%20known,present%20in%20every %20mammalian%20tissue. Physiotherapy students: Focus on chondroitin sulfate https://www.the-scientist.com/sponsored-article/an-introduction-to- glycoproteins-71221 Extra reading from different sources to expand academic reading competencies