Carbohydrates PDF

CARBOHYDRATES The most abundant organic molecules in nature Definition: Polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis. Functions: Provide a significant fraction of the energy in the die...

CARBOHYDRATES The most abundant organic molecules in nature Definition: Polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis. Functions: Provide a significant fraction of the energy in the diet of most organisms Important source of energy for cells Can act as a storage form of energy Can be structural components of many organisms Can be cell-membrane components mediating intercellular communication Can be cell-surface antigens Can be part of the body’s extracellular ground substance Can be associated with proteins and lipids Part of RNA, DNA, and several coenzymes (NAD+, NADP+, FAD, CoA) Carbohydrate with an aldehyde group: Aldos Carbohydrate with a ketone group: Ketose  Monosaccharides:  They are the simplest units of carbohydrates.  Cannot be broken down into simpler sugars. Monosaccharides classified according to:  The number of carbon atoms  They contain aldhyde or ketone 1. Trioses:  Monosaccharides containing 3 carbon atoms.  When they contain aldhyde group called aldotriose, or ketone group called ketotriose  2. Tetroses: These are monosaccharides containing 4 carbon atoms. when they contain aldhyde group called aldotetroses, or ketone group called ketotetroses  3. Pentoses: These are monosaccharides containing 5 carbon atoms: when they contain aldhyde group called aldopentoses, or ketone group called ketopentoses. Aldopentose Ketopentose s s Ribos deoxyribos Xylos Ribulos Xylulos e e e e e Deoxy suger: is that suger in which there is one oxygen atom is missed. Functions of pentoses: Enter in the structures of nucleic acids: RNA and DNA. Enter in the structures of ATP and GTP. Enter in the structures of coenzymes e.g. NAD and FAD. Enter in the structures of some vitamins: B2 and B12 4. Hexoses:  Monosaccharides containing 6 carbon atoms.  When they contain aldhyde group called aldohexoses, or ketone group called ketohexoses. Aldohexoses Ketohexoses Glucose Mannose Galactose Fructose " The phenomenon in which more than one compounds have the same chemical formula but different chemical structures". Types of Isomerism: There are two types: 1. Structural Isomerism. 2. Stereoisomerism. A. Structural Isomerism: Compounds have the same molecular formula but different structures. 1. Chain Isomerism: Differ in the branching of carbon atoms.  2. Position Isomerism Differ in the position of the functional group or substituent atoms. 3. Functional Isomerism: Differ in the functional group present. 4. Metamerism: This is exhibited by compounds due to the presence of different alkyl chains on either side of the functional group. B. Stereo Isomerism: Compounds have the same molecular formula but differ in the relative positioning or orientation of atoms in space.  Geometric Isomerism: it is shown by molecules in which their spatial positions are locked to each other due to the presence of a ring structure or a double bond.  Optical Isomerism: Two or more compounds that have the same molecular arrangement but differ in the optical activity are optical isomers.  ASYMETRIC (CHIRAL) CARBON ATOM: This is the carbon atom attached to 4 different groups or atoms. Types of isomerism of monosacharides: i. Enantiomers: (D & L configuration). ii. Epimers. iii. Diastomers 1. Enantiomers:  Two isomers which are mirror images.  All monosaccharides are classified into D and L forms according to the position of the –OH group attached to the last chiral carbon in the chain. Glyceraldehye is termed as reference sugar, it has 2 form (D, or L) according to the position of –OH group attached to the asymetric carbon atom.  2. Epimers If two monosaccharides differ in configuration around only one asymtetric carbon atom other than the last chiral carbon in the chain. 3. Diastomers: Non-mirror images. That are not enantiomers. Epimers are diastereomers that differ in configuration of only one stereogenic center  RING (CYCLIC) STRUCTURE OF SUGERS: The open chain formula (Fischer) of sugers fails to explain some aldehydic reactions. Monosaccharides that are 5 or 6 carbons in length have the right bond angles to spontaneously form either a furan or pyran ring structure in aqueous solution. This conversion occurs because of the ability of aldehydes and ketones to react with alcohols.  An aldehyde reacts with a hydroxyl group creates a hemiacetal.  A ketone reacts with a hydroxyl group to form a hemiketal Cyclization steps of monosaccharides: 1. Formation of the cyclic structures occurs as a hydroxyl group (at C4 or C5) attacks the carbonyl carbon (the “anomeric” carbon atom). 2. All groups located on the left side of fisher’s are written upwards in Haworth. 3. All groups located on the right side of fisher’s are written downwards Except in Last C in the ring. Stereoisomers When a straight-chain monosaccharide forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers known as anomers OH group on the first carbon atom projected downward alpha OH group on the first carbon atom pointed upward, beta.