Carbohydrates PDF

3..3 Carbohydrates Importance of carbohydrates: (4 main functions) 1) Provide energy through their oxidation 2) Supply carbon for the synthesis of cell components 3) Serve as a stored form of chemical energy 4) Form a part of the structural elements of so...

3..3 Carbohydrates Importance of carbohydrates: (4 main functions) 1) Provide energy through their oxidation 2) Supply carbon for the synthesis of cell components 3) Serve as a stored form of chemical energy 4) Form a part of the structural elements of some cells & tissues Chemistry of carbohydrates: Definition:  Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis o Glyceraldehydes, the simplest carbohydrate, can exist in two isomeric forms that are mirror images of each other o Mirror-image isomers, called enantiomers, have the same molecular & structural formulas but different spatial arrangements of their atoms o A carbon atom with 4 different groups attached is called a chiral carbon & its compound is called chiral o The center carbon of glyceraldehydes is chiral because it’s attached to CHO, H, OH & OH groups. o So glyceraldehydes is chiral, has two isomers (D & L) each is mirror image of the other (e.g. enantiomers and are nonsuperimposable o Organic molecules (and specially carbohydrates) may contain more than one chiral carbon  Glucose, for example, has 4 chiral carbon atoms (2,3,4 and 5) because they each have 4 different groups attached o When a molecule contains more than one chiral carbon, the possibility exists for two arrangements of attached groups at each chiral carbon atom o Thus, nearly all molecules having two chiral carbon atoms exists as two pairs of enantiomers, for a total of 4 stereoisomers The general formula is maximum number of possible stereoisomers = where; n is the number of chiral carbon atoms so Glucose has 16 isomers 1  3..3 1 CHO 2 H C OH 3 HO C H 4 H C OH 5 H C OH 6 CH2OH Fischer projections: It’s time consuming to draw molecules in the three dimensional shapes for the 2 enantiomers of glyceraldehyde, but there is a way to represent these mirror images in two dimensions which is called Fischer projection CHO CHO HO H H OH CH2OH CH2OH L-Glyceraldehyde D-Glyceraldehyde  Fischer designated the 2 enantiomers of glyceraldehydes as L & D compounds  Using this system, we can represent the isomers of similar compounds by the direction of bonds about the chiral carbon  L is used to indicate that an -OH group (or another functional group) is on the left of the chiral carbon when the carbonyl is at the top  D means the –OH is on the right of the chiral carbon o The existence of more than 1 chiral carbon in a carbohydrate molecule could lead to confusion of D & L designations o This is avoided by focusing on the hydroxyl group attached to the chiral carbon farthest from the carbonyl group o By convention, the D family of compounds is that in which this hydroxyl group project to the right when the carbonyl is “up” o In the L family of compounds, it projects to the left 2  3..3 CHO CHO HO H H OH H OH HO H HO H H OH on right HO H H OH on left CH2OH CH2OH L-Glucose D-Glucose Physical properties and optical activity:  The physical properties of the two compounds that make up a pair of D & L isomers (enantiomers) are generally the same with the exception of rotation of polarized light o The enantiomer that rotates it to the left is called the Levorotatory (to the left) or ( ) enantiomer o The one that rotates it to the right is the Dextrorotatory (to the right) or (+) enantiomer o The D & L designations don’t represent the words dextrorotatory & levorotatory but only the spatial relationships of a Fischer projection  Thus, for example, some D compounds rotate polarized light to the right (dextrorotatory), and some rotate it to the left (levorotatory) o The spatial relationships D & L and the rotation of polarized light are entirely different concepts and should not be confused with each other o The property of rotating the plane of polarized light is called optical activity, and molecules with this property are said to be optically active o Measurements of optical activity are useful for differentiating between enantiomers Stereoisomers in some living organisms: o Biochemically, stereoisomerism (D, L isomers or the presence of chiral carbons) is so important because living organisms, both plant & animal, consist largely of chiral substances o Most of the compounds in natural systems including biomolecules such as carbohydrates & amino acids are chiral o Although these molecules can in principle exist as a mixture of stereoisomers, quite often only one steroisomer is found in nature o In some instances, 2 enantiomers are found in nature, but they are not found together in the same biological system 3  3..3 Examples:  Lactic acid occurs naturally in both forms  The L-lactic acid in livig muscle, whereas the D-lactic acid is present in sour milk  Glucose  The D form of glucose tastes sweet, is nutritious, and is important component of our diet  The L-isomer, on the other hand, is tasteless, and the body can’t use it  Yeast can ferment only D-glucose to produce alcohol  Amino acids  Most animals are able to utilized only L-amino acids in the synthesis of proteins Classification of carbohydrates:  Carbohydrates can be classified according to the size of the molecules 1) Monosaccharides: consist of a single polyhydroxy aldehyde or ketone unit 2) Disaccharides: are carbohydrates composed of 2 monosaccharide units linked together chemically 3) Oligosaccharides: (less common and of minor importance) contain from 3 to 10 units 4) Polysaccharides: consist of very long chains of linked monosaccharide units Monosaccharide Disaccharide Oligosaccharide (chain containing 3-10 units) Polysaccharide (long chain with possibly hundreds or thousands of units) 4  3..3 Monosacharides o The simplest carbohydrates are the monosaccharides, consisting of a single polyhydroxy aldehyde or ketone unit o Monosaccharidesare further classified according to the number of carbon atoms they contain Classification of monosaccharides according to number of carbon atoms: Monosaccharides having : 1) 3 carbon atoms are named trioses 2) 4 carbon atoms are named tetroses 3) 5 carbon atoms are named pentoses 4) 6 carbon atoms are named hexoses o The presence of an aldehyde group in a monosaccharide is indicated by the prefix aldo- o Similarly, a ketone group is denoted by the prefix keto- o Thus glucose is an aldohexose and ribulose is a ketopentose CH2OH CHO C O H C OH H C OH HO C H H C OH H C OH CH2OH H C OH D-Ribulose CH2OH D-Glucose  Most monosaccharides are aldoses and almost all natural monosaccharides belong to the D series  D-glyceraldehyde, the smallest monosaccharide with a chiral carbon is the standard on which the whole series is based  The formula tells us there must be 2 trioses, 4 tetroses, 8 pentoses & 16 hexoses  Half of those are the D compounds & the other half are the enantiomers or L compounds 5  3..3 CHO H C OH Aldotriose CH2OH D-Glyceraldehyde CHO CHO H C OH HO C H H C OH H C OH Aldotetrose CH2OH CH2OH D-Erythrose D-Threose CHO CHO CHO CHO H C OH HO C H H C OH HO C H H C OH H C OH HO C H HO C H Aldopentose H C OH H C OH H C OH H C OH CH2OH CH2OH CH2OH CH2OH D-Ribose D-Arabinose D-Xylose D-Lyxose CHO CHO CHO CHO CHO CHO CHO CHO H C OH HO C H H C OH HO C H H C OH HO C H H C OH HO C H H C OH H C OH HO C H HO C H H C OH H C OH HO C H HO C H H C OH H C OH H C OH H C OH HO C H HO C H HO C H HO C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH D-Allose D-Altrose D-Glucose D-Mannose D-Gulose D-Idose D-Galactose D-Talose Aldohexose 6