Carbohydates

• Carbohydrates


  Aldehyde and ketone compounds with multiple hydroxyl groups
• Carbohydrates


  • They are one of the most abundant classes of biomolecules in nature
  • They are widely distributed in all life forms
  • They serve many roles
• Roles of carbohydrates


  • Serve as energy stores, fuels and metabolic intermediates
  • They are constituent of RNA and DNA backbones as ribose and deoxyribose sugars
  • Polysaccharides are constituents of cell walls of bacteria and plants
  • Carbohydrates are linked to surfaces of proteins and lipids where they play role as informational materials e.g. in cell-cell interaction and interaction between cells with other elements in the cellular environment
• Classification of carbohydrates


  • Monosaccharides
  • Oligosaccharides
  • Polysaccharides
• Monosaccharides


  The simplest carbohydrates which contain free aldehyde (-CHO) and ketone (>C=O) groups that have two or more hydroxyl (-OH) groups
• Classification of monosaccharides


  • Trioses (C3H6O3)
  • Tetroses (C4H8O4)
  • Pentoses (C5H10O5)
  • Hexoses (C6H12O6)
• Types of monosaccharides


  • Aldoses
  • Ketoses
• Smallest monosaccharides


  • Glyceraldehyde
  • Dihydroxyacetone
• Monosaccharides


  • They exist in both straight chain structure and cyclic structure
  • Cyclic structures are the result of hemiacetal formation by intermolecular reaction between carbonyl group and a hydroxyl group
  • Five membered rings and six membered rings are most stable
• Chiral centre


  All monosaccharides except dihydroxy acetone contain one or more asymmetric (chiral) carbon atoms thus are optically active isomers (enantiomers)
• D and L isomerism

  The orientation of the -OH group that is most distant from the carbonyl carbon determines whether the sugar belongs the D or L sugars
• Most of sugars present in biological system are D sugars
• Anomers


  Isomeric forms of monosaccharide that differ only in their configuration about the hemiacetal or hemiketal carbon atom
• Mutarotation


  The interconversion of α and β anomers in aqueous solution
• Epimers


  Isomers having different configuration of -OH only at one carbon atoms
• Important epimers of glucose

  • Mannose (epimers at C-2)
  • Galactose (epimers at C-4)
• Disaccharides


  Two monosaccharides can join covalently when -OH group of one sugar reacts with the anomeric carbon of other to form a disaccharide
• Glycosidic bond


  The bond formed between the monosaccharides in a disaccharide
• Some common disaccharides


  • Sucrose
  • Lactose
  • Maltose
  • Isomaltase
  • Trehalose
• Reducing and non-reducing disaccharides

  When the anomeric carbon is involved in a glycosidic bond, the easy interconversion of linear and a cyclic form is prevented. Because the carbonyl carbon can be oxidised only when the sugar is in its linear form, formation of a glycosidic bond makes the sugar non-reducing. The end of chain in disaccharide and polysaccharide with free anomeric carbon is called the reducing end.
• Sucrose and trehalose


  • They are non-reducing sugars
  • Sucrose is ideal for storage and transport of energy in plants
  • Trehalose is a major constituent of circulating fluid of insects, serving as energy storage compound
• Polysaccharides


  Polymers composed of ten or more monosaccharide units joined together by glycosidic linkages
• Types of polysaccharides


  • Homopolysaccharides (made up of a single type of monosaccharide units)
  • Heteropolysaccharides (composed of two or more types of monosaccharides)
• Homopolysaccharides


  • Storage polysaccharides (e.g. starch, glycogen, dextran, inulin)
  • Structural polysaccharides (e.g. cellulose, chitin, xylan, pectin)
• Structures and properties of some homopolysaccharides

  • Starch
  • Glycogen
  • Cellulose
  • Chitin
  • Dextran
  • Inulin
  • Pectin
  • Xylan
• Starch


  • It is deposited in the cytoplasm of plant cells as insoluble granules composed of two homopolysaccharides; amylose (15-20%) and amylopectin (80-85%)
  • Amylose is a linear polymer of α-D-glucose monomers linked by α(1→4) bonds
  • Amylopectin has α(1→4) glycosidic linkages with α(1→6) branch points
• Chitin


  1. acetyl-D-glucosamine, β(14) glucoside, very large, structural role: provide rigidity and strength to exoskeleton of insects
• Dextran


  α-D-glucose, α(16) glucoside with α(13) glucoside at branching points, wide range, structural: extracellular adhesive in bacteria
• Inulin


  β-D-fructose, β(21) fructoside, 30-35, energy storage in plants
• Pectin


  α-D-galacturonic acid, α(14) galactoside, structural: holds cellulose fibrils together in plant cell walls
• Xylan


  β-D-xylose, β(14) xylose, 30-100, storage and supporting roles in plants
• Starch


  • Occurs in plants as reserve carbohydrate in tubers, seeds, fruits and roots, deposited in the cytoplasm of plant cells as insoluble granules composed of two homopolysaccharides; amylose (15-20%) and amylopectin (80-85%)
• Amylose


  Linear polymer of α-D-glucose monomers linked by α(14) bonds, molecular weight 10,000 to 50,000 Da
• Amylopectin


  Very high molecular weight (up to 200 million Da) molecule consisting of glucose units linked by α(14) glycoside linkage, highly branched with branch points every 24 to 30 glucose residues and linkage at the branch points is α(16) glycosidic
• Starch


  • Insoluble in water, alcohol and ether at room temperature, highly hydrated, non-reducing polysaccharide, characteristic blue colour with iodine due to amylose
• Glycogen


  Storage homopolysaccharide of animals, consists of glucose units linked by α(14) glycoside linkage like amylopectin, more extensive branching with branch points every 8 to 14 glucose residues and linkage at the branch points is α(16) glycosidic, each molecule contains up to 120,000 glucose units
• Glycogen


  • Highly branched structure permits rapid degradation through simultaneous release of glucose units at the end of every branch, stored in muscle and liver cells
• Cellulose


  Most abundant extracellular structural polysaccharide of plants, linear polymer of up to 15,000 D-glucose units linked by β(14) glucosidic bonds, embedded in and cross-linked by a matrix of other polysaccharides and lignin in plant cell walls
• Cellulose


  • Forms extensive hydrogen bonded structure that gives exceptional strength and water insolubility, not hydrolysed by vertebrates but digested by symbiotic microorganisms in herbivores and wood-rot fungi/bacteria
• Chitin


  Second most abundant polysaccharide after cellulose, linear polysaccharide of β(14) linked N-acetyl-D-glucosamine residues, main structural component of invertebrate exoskeletons and fungal cell walls