Carbohydrates of physiological significance
Definition of carbohydrates • Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis
Functions of Carbohydrates • Abundant dietary source of energy • Precursors for many organic compounds • Participate in the structure of cell membrane & cellular functions (as glycoproteins & glycolipids) • Structural components (cell wall in plants, exoskeleton in insects etc) • Storage energy to meet immediate demand
Carbohydrates- widely distributed in plants and animals; having important structural and metabolic roles. In plants- glucose synthesized from carbon dioxide and water by photosynthesis and stored as starch or converted to the cellulose of the plant framework. In animals- can synthesize carbohydrate from lipid glycerol and amino acids, but most animal carbohydrate is derived ultimately from plants.
CLASSIFICATION OF CARBOHYDRATES • Monosaccharides – simpler unit of carbohydrate eg.glucose, fructose, sucrose etc… • (2) Disaccharides - condensation products of two monosaccharide units e.g. maltose and sucrose. • (3) Oligosaccharides - condensation products of three to ten monosaccharides e.g. maltotriose, raffinose • (4) Polysaccharides - condensation products of more than ten monosaccharide units e.g. starch, glycogen, cellulose, dextrin etc, which may be linear or branched polymers.
MONOSACHARIDES • Monosaccharides– those carbohydrates that cannot be hydrolyzed into simpler carbohydrates: aldehyde or ketones that have two or more hydroxyl groups. • Empirical formula-(C-H2O)n .literally ‘ CARBON HYDRATE’
Glucose -the most important carbohydrate: • the major metabolic fuel of mammals • the precursor for synthesis of other carbohydrates in the body, including a universal fuel of the fetus. • glycogen for storage; • ribose and deoxyribose in nucleic acids; • galactose in lactose of milk, • in glycolipids, in combination with protein in glycoproteins Diseases associated with carbohydrate metabolism - diabetes mellitus, glucosuria, glycogen storage diseases, and lactose intolerance.
BIOMEDICALLY, GLUCOSE IS THE MOST IMPORTANT MONOSACCHARIDE A- Fischer projections-H and OH groups attached to the carbon atoms in a straight chain. B- Haworth projections- if the molecule is viewed from the side and above the plane of the ring. By convention, bonds nearest to the viewer are bold and thickened. C- Chair conformation: The six-member ring containing one oxygen atom is in the form of a chair
Sugars Exhibit Various Forms of Isomerism- Glucose, with four asymmetric carbon atoms ( 2n ),can form 16 isomers. D and L isomerism: the D form or of its mirror image L form (enantiomers) is determined by it spatial relationship to the parent compound of the carbohydrates- Glyceraldehyde. Tetroses, pentoses, hexoses having multiple asymetric carbons exist as diastreoisomers- isomers that are not mirror images of each other.
OPTICAL ACTIVITY Optical activity occurs due to asymmetric carbon atoms (chiral carbon): those bonded to four different atoms or groups of atoms When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+); or to the left, levorotatory (−). When equal amounts of D and L isomer - no optical activity – mixture is called racemic mixture .
Pyranose and furanose ring structures:The stable ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a 5-membered ring) For glucose in solution, > 99% is in the pyranose form.
ANOMERIC FORMS: (α and β anomers) hemiacetal - formed by combination of an aldehyde and an alcohol group. Similarly the ring structure of a ketose is a hemiketal. Crystalline glucose is α-D-glucopyranose. The cyclic structure is retained in solution, but isomerism occurs about position 1, the carbonyl or anomeric carbon atom, to give a mixture of α-glucopyranose (38%) and β-glucopyranose (62%).
Epimers: Isomers differing as a result of variations in configuration of the -OH and -H on carbon atoms 2, 3, and 4 of glucose e.g. mannose and galactose, formed by epimerization at carbons 2 and 4, respectively
Oxidation reactions • Aldoses may be oxidized to 3 types of acids • Aldonic acids: aldehyde group is converted to a carboxyl group ( glucose – gluconic acid) • Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH • Glucose --- glucuronic acid • Galactose --- galacturonic acid • Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide) • Glucose ---- saccharic acid • Galactose --- mucic acid • Mannose --- mannaric acid
Reduction reactions • either done catalytically (hydrogen and a catalyst) or enzymatically • the resultant product is a polyol or sugar alcohol • glucose form sorbitol • mannose forms mannitol • fructose forms a mixture of mannitol and sorbitol
Some important carbohydrates Trioses of physiological importance : - both D-glyceraldehyde and dihydroxyacetone (in phosphate esters form)-intermediate in glycolysis Tetroses of physiological importance: - erythrose-4-P ; an intermediate in HMP shunt
Sugars as reducing agents- Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction. The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemiacetal (ring) form, C-1 of glucose cannot be oxidized by Cu2+. However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion.
Disacharides • Disacharides are the molecules which consist of two monosacharaides units held together by a glycosidic bond • These are of two types: • Reducing disacharides (having free aldehyde or keto group) e.g. maltose, lactose • Non-reducing disacharides (having no free aldehyde or keto group) e.g. sucrose
An -OH (alcohol) of one glucose (right) condenses with intramolecular hemiacetal of the other glucose (left), with elimination of H2O and formation of an O-glycosidic bond. The reversal of this reaction is hydrolysis—attack by H2O on the glycosidic bond. The maltose molecule retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond.
MALTOSE, SUCROSE, & LACTOSE ARE IMPORTANT DISACCHARIDES Lactase and sucrase deficiencies- malabsorption leads to diarrhea and flatulence.
Sugars Form Glycosides With Other Compounds & With Each Other • Glycosides – • formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. • If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an -OH group) and an-other -OH group. • If the hemiacetal portion is glucose, the resulting compound is a glucoside; if galactose, a galactoside.
Sugars Form Glycosides With Other Compounds & each Other • If the second group is an amine, an N-glycosidic bond is formed, e.g. between adenine and ribose in nucleotides such as ATP . • Glycosides are widely distributed in nature; the aglycone may be methanol, glycerol, a sterol, a phenol, or a base such as adenine. • The glycosides that are important in medicine because of their action on the heart (cardiac glycosides) all contain steroids as the aglycone. • These include derivatives of digitalis and strophanthus such as ouabain, an inhibitor of the Na+-K+ ATPase of cell membranes and antibiotics such as streptomycin.
Deoxy Sugars Lack an Oxygen Atom a hydroxyl group has been replaced by hydrogen--deoxyribose in DNA.
Oligosaccharides • Trisaccharide: raffinose (glucose, galactose and fructose) • Tetrasaccharide: stachyose (2 galactoses, glucose and fructose) • Pentasaccharide: verbascose (3 galactoses, glucose and fructose) • Hexasaccharide: ajugose (4 galactoses, glucose and fructose)
Oligosaccharides occur widely as components of antibiotics derived from various sources
Definition & Classification • Polysacharides are linear as well as branched chain polymers of monosacharides or their derivatives, held together by glycosidic bonds • Polysacharides are of two types • Homopolysacharides: which on hydrolysis yield only a single type of monosacharide • Heteropolysacharides: which on hydrolysis yield a mixture of a few monosacharides or their derivatives
Polysaccharides or glycans • homoglycans (starch, cellulose, glycogen, inulin) • heteroglycans (gums, mucopolysaccharides) • functions: serve storage and structural function • characteristics: • polymers (MW from 200,000) • White and amorphous products (glassy) • not sweet • not reducing; do not give the typical aldose or ketose reactions) • form colloidal solutions or suspensions
HOMOPOLYSACHARIDES • When the polysacharides are composed of same types of monosacharides or their derivatives, they are referred to as homopolysacharides or homoglycans
Starch • most common storage polysaccharide in plants • composed of 10 – 30% a-amylose and 70-90% amylopectin depending on the source • the chains are of varying length, having molecular weights from several thousands to half a million
Amylose Amylosehas a non-branching helical structure composed of glucose residues
Amylopectin amylopectin consists of branched chains composed of 24–30 glucose residues united by 1 → 4 linkages in the chains and by 1 → 6 linkages at the branch points.
Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues
suspensions of amylose in water adopt a helical conformation iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch
Glycogen • also known as animal starch • stored in muscle and liver • present in cells as granules (high MW) • contains both a(1,4) links and a(1,6) branches at every 8 to 12 glucose unit • complete hydrolysis yields glucose • glycogen and iodine gives a red-violet color • hydrolyzed by both a and b-amylases and by glycogen phosphorylase
Glycogen is highly branched structure with chains of 12–14 α-D-glucopyranose residues (in α[1 → 4]-glucosidic linkage), with branching by α(1 → 6)-glucosidic bonds. A: General structure. B: Enlargement of structure at a branch point.
Inulin • b-(1,2) linked fructofuranoses • linear only; no branching • lower molecular weight than starch • colors yellow with iodine • hydrolysis yields fructose • sources include onions, garlic, dandelions and jerusalem artichokes • used as diagnostic agent for the evaluation of glomerular filtration rate (renal function test) Jerusalem artichokes
Chitin • Chitin is the second most abundant carbohydrate polymer • consists of N-acetyl-D-glucosamine units joined by β (1 →4)-glycosidic linkages • Present in the cell wall of fungi and in the exoskeletons of crustaceans, insects and spiders • Chitin is used commercially in coatings (extends the shelf life of fruits and meats)
Dextrins • produced by the partial hydrolysis of starch along with maltose and glucose • dextrins are often referred to as either amylodextrins, erythrodextrins or achrodextrins • used as mucilages (glues) • also used in infant formulas (prevent the curdling of milk in baby’s stomach)