Lec. 1 Biochemistry of Carbohydrate PDF

Lec.1:Biochemistr of Carbohydrate  What is biochemistry  Biochemistry can be defined as the science concerned with the chemical basis of life. Biochemistry = Chemistry of life  Since cells are the structural units of living systems. Thus, biochemistry can also be described as t...

Lec.1:Biochemistr of Carbohydrate  What is biochemistry  Biochemistry can be defined as the science concerned with the chemical basis of life. Biochemistry = Chemistry of life  Since cells are the structural units of living systems. Thus, biochemistry can also be described as the science concerned with the chemical constituents of living cells and with the reactions they undergo.  Because life depends on biochemical reactions, biochemistry has become the basic language of all biologic sciences.  What dose the Biochemistry discuss? Biochemistry discusses:  Structure and function of cellular components Carbohydrates Lipids Proteins Nucleic acids and other biomolecules  Metabolism and Regulation  Gene expression and modulation  Carbohydrates  The most abundant organic molecules in nature  The general molecular formula of carbohydrate is Cn (H2O)n.  For example, glucose has the molecular formula C6H12O6. Carbohydrates are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis  Functions of Carbohydrates 1. Carbohydrates are the main sources of energy in the body. Brain cells and RBCs are almost wholly dependent on carbohydrates as the energy source. Energy production from carbohydrates will be 4 kcal/g. 2. Storage form of energy (starch and glycogen). 3. Excess carbohydrate is converted to fat. 4. Glycoproteins and glycolipids are components of cell membranes and receptors. 5. Structural basis of many organisms: Cellulose of plants; exoskeleton of insects, cell wall of microorganisms.  Classifications based on number of sugar units in total chain.  Monosaccharides : Single sugar unit  Disaccharides: Two sugar units  Oligosaccharides: 3 to 10 sugar units  Polysaccharides: More than 10 units  PROPERTIES OF MONOSACCHARIDES  Enantiomers : A special type of isomerism is found in the pairs of structures that are mirror images of each other. called enantiomers. Designated as a D- and an L-sugar.  The vast majority of the sugars in humans are D- sugars. CHO CHO H C OH HO C H  In the D isomeric form, CH OH 2 CH OH 2 the –OH group on the D-glyceraldehyde L-glyceraldehyde asymmetric carbon is on the right. CHO CHO  whereas , H C OH HO C H CH OH CH OH in the L-isomer it is on the left. 2 2 D-glyceraldehyde L-glyceraldehyde  Functional Isomers  sugars containing aldehyde group or keto group are called as aldoses or ketoses, respectively. PROPERTIES OF MONOSACCHARIDES  Isomers: Compounds that have the same chemical formula but have different structures are called isomers. For example: fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H12O6.  Epimers: Carbohydrate isomers that differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon) are defined as epimers of each other. Example: Glucose and galactose are C-4 epimers Glucose and mannose are C-2 epimers. Isomers & Epimers  Cyclic Structure for Glucose Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5.  Cyclic Structure for Fructose  Cyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5.  Anomers Cyclization of glucose produces a new chiral carbon at C1. The 2 stereoisomers are called anomers, a & b. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1: 6 CH2OH 6 CH2OH 5 5 α(OH below the ring) H O H H H O OH H 4 H 1 4 H 1 OH β(OH above the ring). OH OH OH OH H 3 2 3 2 H OH H OH -D-glucose -D-glucose  Joining of Monosaccharides  Monosaccharides can be joined to form disaccharides, oligosaccharides, and polysaccharides.  The bonds that link sugars are called glycosidic bonds.  These are formed by enzymes known as glycosyltransferases.  Joining of Monosaccharides  If this anomeric hydroxyl is in the α configuration, the linkage is an αbond. If it is in the β configuration, the linkage is a β-bond.  Disaccharides  Two Joined Monosaccharides, examples:  Sucrose: Glucose + Fructose  Maltose: Glucose + Glucose  Lactose: Glucose + Galactose  Maltose a disaccharide with an (1 4) glycosidic link between C1 - C4 OH of 2 glucoses. 6 CH2OH 6 CH2OH 5 O 5 O H H H H H H 1 4 1 4 OH H OH H OH O OH 3 2 3 2 H OH H OH maltose  Lactose is synthesized by forming a glycosidic bond between carbon 1 of β-galactose and carbon 4 of glucose. The linkage is, therefore, a β(1→4) glycosidic bond.  Sucrose It contains glucose and fructose. common table sugar. glucose is in α-form whereas fructose is in β-form in sucrose. β(1→2). Nonreducing sugar b/c the functional groups of glucose and fructose are involved in glycosidic linkage.  Sweetener Diabetes mellitus patients and people on weight reduction protocols avoid sucrose as sweetener. Most of the artificial sweeteners commonly known as ‘Sugar Free’ contain aspartame, which is a dipeptide L-aspartyl-L phenylalanine methyl ester. Aspartame is also added to the beverages marketed as ‘low caloric’ or ‘Diet drinks’.  Some Sugar Derivatives Organisms contain a variety of sugar derivatives There are a number of sugar derivatives in which a OH group in the parent compound is replaced with another substituent  Sugars frequently contain:  Phosphate groups  Amino groups  Sulfate groups  N-acetyl groups  phosphorylated derivatives of sugars  In the synthesis & metabolism of carbohydrates ,the intermediates are very often not the sugars themselves but their phosphorylated derivatives  Effects of sugar phosphorylation: within cells is to trap the sugar inside the cell ,most cells do not have plasma membrane transporters for phosphorylated sugars also activates sugars for subsequent chemical transformation  Several important phosphorylated derivatives of sugars components of nucleotides  Amino Sugars  In glucosamine, galactosamine, & mannosamine:  The hydroxyl at C-2 of the parent compound is replaced with an amino group  Oligosaccharides  Composed of: Three to ten monosaccharide units. E.g. Fructooligosaccharides.  Polysaccharides  Larger than ten monosaccharide units.  Can reach many thousands of units.  Examples: Glycogen ,Starch ,Fibres ( Cellulose).  Polysaccharides  Important homopolysaccharides are starch, glycogen, cellulose, dextran and inulin and chitin.  All these contain glucose as repeating unit.  Other name for homopolysaccharides are homoglycans.  starch  Structure  1. It consists of two parts. A minor amylose component and a major amylopectin component.  Amylose is a glucose polymer with (14) linkages. CH2OH 6CH OH CH2OH CH2OH CH2OH 2 O 5 O H O H O H H O H H H H H H H H H H H OH H 1 4 OH H 1 OH H OH H OH H O O O O OH OH 2 3 H OH H OH H OH H OH H OH amylose  starch Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than shown above. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur. CH 2OH CH 2OH H O H H O H amylopectin H H OH H OH H 1 O OH O H OH H OH CH 2OH CH2OH 6 CH2 CH2OH CH2OH H O H H O H H 5 O H H O H H O H H H H H H OH H OH H OH H 1 4 OH H OH H O O 4 O O OH OH 2 3 H OH H OH H OH H OH H OH  Glycogen  similar to that of amylopectin of starch. But the number of branches in glycogen molecule is much more than amylopectin.  There is one branch point for 6-7 glucose residues. Function 1. It is the major storage polysaccharide (carbohydrate) in human body. 2. It is mainly present in liver and muscle. 3. It is also called as animal starch.  Glycogen Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more (16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. Carbohydrates present in three forms:  Digestible  Ready-to-absorb  Non-digestible- Cellulose, Hemicellulose, lignin, gums, pectins & pentosans