Carbohydrate Biochemistry Lecture Notes PDF

 Carbohydrate Biochemistry Objectives Recognize the features and functions of different classes of “carbohydrates” Understand the concept of asymmetric carbons and other diastereoisomers, epimers and enantiomers and anomers Recognize the structure of glucose and its rela...

 Carbohydrate Biochemistry Objectives Recognize the features and functions of different classes of “carbohydrates” Understand the concept of asymmetric carbons and other diastereoisomers, epimers and enantiomers and anomers Recognize the structure of glucose and its relationship with other monosaccharides Understand the different reactions of monosaccharide Understand the nature of glycosidic bond and recognize the structure of the common disaccharides Recognize the different types and classes of polysaccharides Understand the basic differences between amylose and cellulose 1 *Functions of Carbohydrates * Classification of CHO - Monosaccharides - aldoses and ketoses * Structure of Monosaccharides -isomers, epimers and enantiomers -cyclic structure, hemiacetal and hemiketal *Reactions of monosaccharides * Derivatives of Hexoses -amino sugars -acidic sugars -Glycosides * Disaccharides -disaccharide formation and hydrolysis -nomenclature of disaccharides -glycosides * Polysaccharides (Glycans) -homo and hetero polysaccharides -Storage polysaccharides; starch & Glycogen 2 -Structural polysaccharides, cellulose & chitin  Biochemistry of Carbohydrate Carbohydrates (CHO) are the most abundant biomolecules in nature CHO are the photosynthesis product nCO2 +H2O (CH2O)n + nO2 light Originally thought to have the formula (CH2O)n. Now known that only simple monosaccharides obey this rule. Carbohydrate- polyhydroxy aldehyde or ketone or a larger molecule which can be hydrolyzed to a polyhydroxy aldehyde or ketone. Functions of Carbohydrates 1. Energy source for plants and animals 2. Source of carbon in metabolic processes 3. Storage form of energy 4. Structural elements of cells and tissues 5. Some CHO participate in recognition and adhesion between cells and mediate some forms of inter cellular communications 3  Classes of Saccharides Monosaccharides = single polyhydroxy aldehyde or ketone unit, (eg. 6-carbon glucose, most abundant in nature) Oligosaccharides = short chain of 2- ~20 monosaccharides joined by “glycosidic bonds” (eg. disaccharide sucrose = glucose-fructose) - oligosaccharides > 3 residues are usually joined to protein or lipid in “glycoconjugates” Polysaccharides - chains > ~ 20 to 1000’s of monosaccharides in length - linear: eg. cellulose (glucose)n or chitin (N-acetylglucosamine)n - branched: eg. glycogen & starch (glucose) - depending on the sugar residues in a polysaccharide and the linkages between them, polysaccharides can have very different biological roles 4  Monosaccharides - Backbone = un-branched carbon chains in which all C atoms are linked by single bonds - Colorless, crystalline, solid, freely soluble in water, insoluble in organic solvents - If it has keto group as the most oxidized functional group = Ketose - If it has aldehyde group as the most oxidized functional group = aldose According to the number of carbon atoms - 3 C = triose, 4 C = tetrose, 5 C = pentose, 6 C = hexose (eg. aldo- or ketohexoses) Simplest monosaccharides are 3 carbon... Common monosaccharides are 6 carbon... Aldotriose ketotriose... 5  Monosaccharides have asymmetric centers All monosaccharides (except dihydroxyacetone) have one or more asymmetric carbons eg. glyceraldehyde: middle C is a chiral center, so thevmolecule has 2 different optical isomers or enantiomers (= stereoisomers that are non-super imposable mirror images of one another) By convention, one enantiomer = the D isomer, the other is L, Configurations of glyceraldehyde: 6  D and L configurations of monosaccharides Stereoisomers of monosaccharides > 3 C are divided into 2 groups: differ in the configuration of the chiral center most distant from the carbonyl C In biochemistry, we use the D-L system (similar principle to the R-S system usually used in organic), but everything is compared with glyceraldehyde In general, a molecule with n chiral centers can have 2n stereoisomers -OH GROUP ON RIGHT, CONFIGURATION = D -OH GROUP ON LEFT, CONFIGURATION = L 1 2 3 IN ALDOHEXOSES, C-2, 4 C-3, C-4, AND C-5 5 = CHIRAL CENTERS, SO 24 = 16 POSSIBLE 6 ALDOHEXOSES: MIRROR 8 D AND 8 L 7 Most of the hexoses in living organisms are D-isomers 9  Series of D-ketoses Have 1 less chiral center than aldoses. C-4 and C-5 ketoses are designated by adding “ul” into the name of their corresponding aldose, eg. D-ribulose = ketopentose corresponding to D-ribose 10