Carbohydrates - 1st Carb. 203-1 PDF

040817203 The structures and functions of biomolecules: 1) Carbohydrates  OBJECTIVES Introduction and Definition. Functions (in general). Classification: monosaccharides, disaccharides, oligosaccharides and polysacchari...

040817203 The structures and functions of biomolecules: 1) Carbohydrates  OBJECTIVES Introduction and Definition. Functions (in general). Classification: monosaccharides, disaccharides, oligosaccharides and polysaccharides. Properties of monosaccharides. Biologically important carbohydrates. Types and functions of glycosaminoglycans, glycoproteins and proteoglycans.  References 1) Lippincott “ Biochemistry ”. 2) Robert K. Murray “ Harper’s Illustrated Biochemistry ”. 3) Marks “ Essential Medical Biochemistry ”. 4) Lehninger “ Principles of Biochemistry ”. 5) J.L.Jain et al. “ Fundamentals of Biochemistry ”. 6) H.A Harper “ Review of physiological chemistry ”.  http://www.youtube.com/watch?v=aeC7M9 PDjQw&feature=channel. http://www.youtube.com/watch?v=iuW3nk5 EADg  The carbohydrates are widely distributed both in animal and in plant tissues Where do carbohydrates come from? Carbon cycle in nature  carbon photosynthesis. sun dioxide chlorophyll oxygen glucose water photosynthesis. Light energy Photo- Carbon synthesis Glucose Oxygen Water dioxide gas  In plants, they are produced by photosynthesis. HOW……. ? Plants** and photosynthesis Chlorophyll captures light energy from the sun which is transformed into chemical energy (ATP) Chemical energy is used to combine carbon dioxide (CO2) and water (H2O) to form glucose (C6H12O6)  The by-product is oxygen (O2) Extra glucose is stored in plants as starch or used for synthesis of cellulose (a main component of cell wall) 6 CO2 + 6 H2O 6 HCHO + 6 O2 6 HCHO C6H12O6 GLUCOSE  The reactants in photosynthesis are the same as the products of cellular respiration. The equations for the overall process is: C6H12O6 + 6O2 6CO2 + 6H2O 6 CO2 + 6 H2O 6 HCHO + 6 O2 6 HCHO C6H12O6  Some Carbohydrate Functions  Sources of energy.  Intermediates in the biosynthesis of other basic biochemical entities such as fats and proteins.  The chains sticking out of the proteins in the cell membrane are polysaccharides know as cell markers.   Form structural tissues in plants (cellulose, lignin) and murein in microorganisms  Participate in biological transport , cell-cell interactions, developmental processes, regulation of activity, cell-cell recognition and signaling. Definition  The term (carbohydrate) is meaning the carbon containing compound (carbo-) which contains hydrogen and oxygen (hydrate) in a ratio like that of water molecule i.e in ratio of 2 : 1 } Cx (H2O)x {  Also, carbohydrates are polyhydroxylated compounds having at least 3 carbon atoms and a potentially active carbonyl group which may be aldehydic or ketonic group  So, carbohydrates are Also, defined as aldoses (contain aldehyde group) or ketoses (contain ketone group) polyhydroxy (contain more than one hydroxyl group) compounds Classification  Carbohydrates (glycans) have the following basic composition (general formula) I Cx(H2O)x or H - C - OH I and classified according the no. of units to : 1) Monosaccharides (simple sugars): The basic unit for other carbohydrates and have multiple OH groups. They classified based on: 1) no. of carbons (3, 4, 5, 6 & 7) to trioses, tetroses, pentoses, hexoses and heptoses. Also, based on: 2) the type of active or functional group to aldoses (have aldehyde group) and ketoses (have ketone group)  Monosaccharides Aldoses (e.g., glucose) have an aldehyde group at one end. Ketoses (e.g., fructose) have a keto group, usually at C2. H O CH2OH C C O H C OH HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-glucose D-fructose 2) Disaccharides : Two monosaccharide units covalently linked by glycosidic linkage and classified according the type of units to : a) Homo - disaccharides (two similar units) or b) Hetero - disaccharides (two different units). And according the reducing power to : a) Reducing-disaccharides (have free active gp.). b) Non-reducing-disaccharides (have no free active gp. where they used in the formation of glycosidic linkage). 3) Oligosaccharides: A few monosaccharides (3 - 9 or 10 units) covalently linked by glycosidic linkages 4) Polysaccharides : Polymers consisting of chains of monosaccharide units (tens, hundreds or more than thousands). All are non-reducing (has low number of free active group or reducing ends and large molecular weight). They classified to: a) Homo-polysaccharides and b) Heteropolysaccharides.  Glycosidic Bond The anomeric hydroxyl and a hydroxyl 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' + H2O Structural representation of sugars Fisher projection: straight chain representation Haworth projection: simple ring in perspective D-ribose and other five-carbon saccharides can form either furanose or pyranose structures D-glucose can cyclize in two ways forming either furanose or pyranose structures  H H C H Ball-and-stick model H Structural formula Space-filling model  Properties of carbohydrates Differences in structures of sugars are responsible for variations in its properties 1) Physical: Shape: Crystalline form, amorphous powder……… Solubility: Soluble in cold water or hot water. Partially soluble (form colloidal solution). Insoluble in water. 2) Chemical: Oxidation - Reduction Reactions Effect of strong acids Isomerization (stereoisomers)  A) Oxidation reactions Aldoses may be oxidized to 3 types of acids 1) Aldonic acids aldehyde group (C1) is converted to a carboxyl group (by Bromine water*) FOR EXAMPLE glucose …. gluconic acid * Weak oxidizing agent.  2) Uronic acids: aldehyde is left and primary alcohol at the other end is oxidized to COOH (by pot. Permenganate**) FOR EXAMPLE Glucose …. glucuronic acid Galactose ….. galacturonic acid ** Strong oxidizing agent.  COOH CHO H C OH H C OH HO C H HO C H H C OH H C OH H C OH H C OH CH2OH COOH D-gluconic acid D-glucuronic acid aldehyde at C1 or OH at C6 is oxidized to a carboxylic acid gluconic acid or glucuronic acid. 3) Saccharic acids (glycaric acids): oxidation at both ends (C1 and C6) of monosaccharide (by nitric acid**) FOR EXAMPLE Glucose …. saccharic acid Galactose …. mucic acid Mannose …. mannaric acid B) Oxidation - Reduction Reactions: Reducing and Non-Reducing Sugars A molecule that donates electrons is called a …… “reducing agent” A sugar that donates electrons is called a …… “reducing sugar” The electron is donated by the carbonyl group Benedict’s or Fehling’s reagent changes its colour when exposed to reducing agent  The carbonyl group is “free” in the straight chain form but not free in the ring form. BUT remember ; both the ring form and the straight chain form are interchangeable. So, all monosaccharides are reducing sugars and all reduce Benedict’s reagent. Benedict’s Test : Benedict’s reagent undergoes a complex colour change when it is reduced. The intensity of the colour change is proportional to the concentration of reducing sugar present. The colour change sequence is: – Blue … – Green … – Yellow … – Orange … – Bloody red … C) Action of strong acids Monosaccharides are normally stable to dilute acids but are dehydrated by strong acids D-ribose (pentoses) when heated with concentrated HCl or H2SO4 yields furfural While, D- glucose (hexoses) under the same conditions yields 5-hydroxymethyl furfural. This is the basis for the (Molisch test) , sensitive or specific test for the detection of carbohydrates. D) Stereochemistry of monosaccharides: 1) D and L isomers: Isomers or isomerisms are the compounds which have the same molecular formula but differ in its structural formula (configuration in the space)  D & L designations CHO CHO are based on the configuration about H C OH HO C H the single CH2OH CH2OH asymmetric C in glyceraldehyde. D-glyceraldehyde L-glyceraldehyde The representations CHO CHO in front are H C OH HO C H CH2OH CH2OH Fischer Projections. D-glyceraldehyde L-glyceraldehyde  For sugars with O H O H more than one C C chiral center, D- or L- refers to the H – C – OH HO – C – H asymmetric C HO – C – H H – C – OH farthest from the H – C – OH HO – C – H aldehyde or keto- group. H – C – OH HO – C – H (pre-final C atom) CH2OH CH2OH D-glucose L-glucose D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose. Other stereoisomers have unique names, e.g. Altrose , Mannose , Galactose , etc. The number of stereoisomers is 2n, where n is the number of asymmetric or chiral centers. The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).  Most naturally occurring sugars are D- isomers and body can metabolize only D-sugars  2) d(+) or l(-) isomers: Optical isomers Also, the presence of asymmetric carbon center in sugar molecule causes a presence of isomers have an optical activity i.e. have the ability of change or rotate the direction (plane) of polarized light  When the plane of polarized light is rotated to the right, the compound is dextrorotatory and is labelled (d) or (+). Likewise, when the plane of polarised light is rotated to the left, the compound is levorotatory (l) or (-).  (d) and (l) isomers are not related with (D) and (L) one i.e D-sugars can be dextrorotatory or levorotatory for example: D-fructose is levorotatory while D-glucose is dextrorotatory. Racemic solution : It is a solution has no optical activity or optical rotatory power because it is a mixture of equal amounts of d and l isomers where, the optical activity of one isomer cancelled by the another one.  Measurement of optical activity in chiral or asymmetric molecules using polarized light by an instrument called a Polarimeter Rotation is either (+) dextrorotatory or (-) levorotatory New polarimeters – usually connected to computer and printer  3) Aldo- and Keto- isomers: Aldoses (e.g., glucose) have an aldehyde group at one end. (at C1) Ketoses (e.g., fructose) have a keto group, (at C2) H O C CH2OH H C OH C O HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-glucose D-fructose  4) Epimers: This kind of isomerism was formed due to the internal distribution of hydroxyl group and hydrogen atom around carbon atom no. 2,3,4 for hexoses or 2,3 for pentoses. FOR EXAMPLE: Glucose and mannose are epimers at carbon no. 2 while, glucose and galactose are epimers at carbon no. 4  EPIMERS Monosaccharides which differ in configuration around one specific C-atom are called epimers of one another C-2 epimers: glucose and mannose C-4 epimers: glucose and galactose C-3 epimers: ????????  CARBON-2 EPIMERS H C O H C O H C OH HO C H HO C H HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH D-GLUCOSE D-MANNOSE  CARBON-4 EPIMERS H C O H C O H C OH H C OH HO C H HO C H H C OH HO C H H C OH H C OH CH2OH CH2OH D-GLUCOSE D-GALACTOSE  5) α and β anomers: An aldehyde can H H react with an C O + R' OH R' O C OH alcohol to form R R a hemiacetal. aldehyde alcohol hemiacetal A ketone can R R react with an alcohol to form C O + "R OH "R O C OH a hemiketal. R' R' ketone alcohol hemiketal Pentoses and hexoses 1 CHO can cyclize as the H C OH 2 ketone or aldehyde HO C H 3 D-glucose reacts with a distal H C OH (linear form) OH. 4 H C OH Glucose forms an 5 intra-molecular 6 CH2OH hemiacetal, as the C1 6 CH2OH 6 CH2OH aldehyde & C5 OH 5 5 H O H H O OH react, to form a 6- H H member pyranose 4 OH H 1 4 OH H 1 ring, named after OH 2 OH OH 3 2 H 3 pyran. H OH H OH -D-glucose -D-glucose These representations of the cyclic sugars are called Haworth projections.  ANOMERIC CARBON ATOM The carbon atom which is part of the carbonyl group Alpha(α) and Beta(β) anomers differ from each other only in respect to configuration around anomeric carbon atom. CYCLIC STRUCTURES OF MONOSACCHARIDES D-(+)-Glucose, an aldohexose which forms a six-membered ring hemiacetal  Alpha and Beta anomers of glucose H OH HO H C C H C OH H C OH HO C H O HO C H O H C OH H C OH H C H C H C OH H C OH H H α-D-GLUCOPYRANOSE β-D-GLUCOPYRANOSE  D-Glucose 6 6 HOCH2 HOCH2 5 H 5 O H H O OH H 4 H 1 4 OH H 1 OH H HO 3 2 OH HO 3 2 H H OH H OH -D-Glucopyranose -D-Glucopyranose  6 CH2OH 6 CH2OH 5 O 5 O H H H OH H H 4 H 1 4 H 1 OH OH OH OH OH H 3 2 3 2 H OH H OH -D-glucose -D-glucose Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers,  & . Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:  (OH below the ring) and  (OH above the ring). D-(-)-Fructose is an ketohexose which forms a five-membered ring hemiacetal  1 CH2OH 2C O HO C H 1 CH2OH 3 HOH2C 6 O H C OH 4 5 H HO 2 H C OH H 4 3 OH 5 OH H 6 CH2OH D-fructose (linear) -D-fructofuranose Fructose forms either  a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or  a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.  Fructose (levulose) HOCH2 O CH2 OH HOCH2 O OH HO HO CH2 OH OH OH OH -D-Fructofuranose -D-Fructofuranose CH2OH CH2OH OH H C O C O CH2OH HO C H HO C H OH H C OH H C OH O OH OH H C OH H C OH H C OH CH2OH H  Naturally-occurring free form (fructopyranose).  Mutarotation Mutarotation is a term given to the change in the observed optical rotation of a substance with time. Glucose, for example, can be obtained in either its  or -pyranose form. The two forms have different physical properties such as melting point and optical rotation. When either form is dissolved in water, its initial rotation changes with time. Eventually both solutions have the same rotation.  Mutarotation of D-Glucose  -D-Glucopyranose Initial: +112.2° -D-Glucopyranose Initial: +18.7°  Mutarotation The optical rotation of glucose in water solution changes to constant value. 25D = +520  - D - glucose -> D - glucose