Carbohydrates PDF

The 4 macromolecules of life  Cells join together small organic molecules (monomers; building blocks) to form large molecules (polymers) or Macromolecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids Carbohydrates  A carbohydrate is an organic compound which has the empirical formula C...

The 4 macromolecules of life  Cells join together small organic molecules (monomers; building blocks) to form large molecules (polymers) or Macromolecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids Carbohydrates  A carbohydrate is an organic compound which has the empirical formula Cn(H2O)n  They are aldehyde or ketone derivatives of polyhydric alcohols Importance of carbohydrates Carbohydrates perform numerous roles: 1. Storage of energy (e.g., starch and glycogen) 2. Structural components (e.g., cellulose in plants and chitin in arthropods). 3. Ribose is an important component of coenzymes (e.g., FAD, and NAD) and the backbone of RNA. 4. Deoxyribose is a component of DNA. 5. Saccharides and their derivatives play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, lubricants, and development. 6. The presence of the hydroxyl groups allows carbohydrates to: - interact with the aqueous environment - participate in hydrogen bonding, both within and between chains. 7. Derivatives of carbohydrates can contain nitrogen, phosphate and sulfur compounds. 8. Carbohydrates also can combine with lipid to form glycolipids or with protein to form glycoproteins Glucose  Is the most important carbohydrate  It is synthesized in plants from CO2 and water by photosynthesis and stored as starch or used to synthesize cellulose for plant cell walls.  Animals derived most of their carbohydrates from plants, but they can synthesize them from amino acids.  Most dietary carbohydrate is absorbed into the blood stream as glucose formed by hydrolysis of dietary polysaccharides and disaccharides, and other sugars are converted to glucose in the liver.  Glucose is the major metabolic fuel of mammals except ruminants, and a universal fuel of the fetus  Glucose is the precursor of all other carbohydrates in the body including: Glycogen for storage / Ribose & deoxy ribose in nucleic acids/ Galactose in lactose of milk/ In combination with lipids in glycolipids / In combination with protein in glycoproteins and proteoglycans  The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. Page 1 of 5  All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active.  Asymmetric carbon atom is the carbon atom attached to four different atoms or chemical groups.  Two mirror images of a chiral molecule are called enantiomers or optical isomers.  Isomers are compounds with the same molecular formula but different structural formulas.  The number of isomers of a molecule depend on the number of the chiral centers in the molecule, and it follows the general formula: Number of isomers = 2n n= the number of asymmetric centers (chiral centers) For example glucose contains 4 asymmetric carbon atoms So Number of isomers of glucose = 24 = 16 that means glucose has 16 isomers The most important types of isomerism exhibited by sugars are: 1. D and L isomerism 2. Pyranose and furanose ring structures 3. Alpha and beta anomers 4. Epimers 5. Aldose and ketose isomerism 1. D and L isomerism  Carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric (chiral) carbon farthest from the carbonyl carbon.  With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation.  The mirror-image conformations, called enantiomers, are in the L-conformation.  The presence of asymmetric carbon atoms in a compound also confers optical activity.  When a beam of plane polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (-).  The direction of rotation of polarized light is independent of the stereochemistry of the sugar, so it may be designated D (-), D (+), or L(-), L(+) 2. Pyranose and furanose ring structures  Because ring structures of monosaccharides resemble the ring structures of either: pyran (six-membered rings) or furan (five- membered rings) derivatives with this structure are termed pyranoses and furanoses, respectively.  for glucose in solution more than 99% is in the pyranose form. Page 2 of 5 3. Alpha and beta anomers  The rings in solution can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) of the hemiacetals and hemiketals.  The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers.  When the hydroxyl attached to the anomeric carbon occur below the plane of the ring it is said to be in the α configuration. When the hydroxyl attached to the anomeric carbon occur above the plane of the ring it is said to be in the β configuration.  Carbohydrates in solution can change spontaneously between α and β configurations-- a process known as mutarotation. 4. Epimers  Isomers differing in the configuration of the –OH and – H on carbon atoms 2,3,4 of glucose are known as epimers.  Biologically the most important epimers of glucose are: mannose at carbon 2 and galactose at carbon 4 5. Aldose and ketose isomerism  Fructose has the same molecular formula as glucose but differs in its structural formula  This is because in fructose there is a keto group in position 2, and C2 in fructose is the anomeric carbon, Whereas in glucose there is an aldehyde group in position one the anomeric carbon of glucose Page 3 of 5  Monosaccharides Classification of Carbohydrates Disaccharides Monosaccharides: Carbohydrates  Monosaccharides are the simplest carbohydrates Oligosaccharides that cannot be hydrolyzed to smaller carbohydrates. Polysaccharides  They are aldehydes or ketones with two or more hydroxyl groups.  They have the general chemical formula (CH2O)n  The smallest monosaccharides, for which n = 3, are dihydroxyacetone and D- and L-glyceraldehyde.  It contains, either an aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones).  And can also be classified as trioses, tetroses, pentoses, hexoses……etc dependening on the number of carbon atom. Carbons Category Name Relevant examples 3 Triose Glyceraldehyde, Dihydroxyacetone 4 Tetrose Erythrose 5 Pentose Ribose, Ribulose, Xylulose 6 Hexose Glucose, Galactose, Mannose, Fructose 7 Heptose Sedoheptulose 9 Nonose Neuraminic acid also called sialic acid  The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce intramolecular hemiacetals or hemiketals, respectively.    This results in the formation of five- or six-membered rings. Page 4 of 5  Because the five-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses.  Those with six-membered rings resemble the organic molecule pyran and are termed pyranoses.  Such structures can be depicted by either Fischer or Haworth style diagrams.  The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon for aldoses, or the carbon nearest the carbonyl, for ketoses.  The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) of the hemiacetals and hemiketals.  The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers.  Carbohydrates can change spontaneously between the α and β configurations -- a process known as mutarotation.  When the hydroxyl group attached to the anomeric carbon occur below the plane of the ring it is said to be in the α configuration.  When the hydroxyl group attached to the anomeric carbon occur above the plane of the ring it is said to be in the β configuration.  The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly described by the two conformations identified as the chair form and the boat form.  The chair form is the more stable of the two.  Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial.  In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose. 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