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BIO 203 Biochemistry Iby
Seyhun YURDUGÜL,Ph.D.
Lecture 6
Carbohydrates
Content Outline
• Definition
• Monosaccharides
• Disaccharides
• Polysaccharides
Introduction
– Carbohydrates: – carbon compounds that contain large
quantities of hydroxyl groups. – simplest carbohydrates:– also contain either an aldehyde moiety (these
are termed polyhydroxyaldehydes) – or a ketone moiety (polyhydroxyketones).
Carbohydrates:
– All carbohydrates can be classified as: – either monosaccharides, – oligosaccharides, – or polysaccharides.
Monosaccharides:
• Simple carbohydrates:
• consist of a single polyhydroxyaldehyde or ketone unit.
• e.g. Glucose(simplest sugar)
• e.g. Fructose(sugar from honey origin)
• e.g. Galactose(a sugar in milk and yogurt)
Glucose
Two forms of monosaccharides:
• Aldose sugar
• Ketose sugar
Aldose
• If the carbonyl group:
• at an end of the carbon chain:
• monosaccharide is aldehyde and called aldose.
• e.g. glyceraldehyde
D-glyceraldehyde
Ketose
• If the carbonyl group:
• at any other position, monosaccharide is ketone and called ‘ketose’
• e.g. dihydroxyacetone
(2) Dihydroxyacetone
Other monosaccharides:
• Tetroses (4 carbon)
• Pentoses (5 carbon)
• Hexoses (6 carbon)
• Heptoses (7 carbon)
Disaccharides:
• Made up by combination of simple sugars:
• e.g. Maltose: glucose + glucose
(Sugar of malt)
• e.g. Lactose: glucose + galactose
(milk sugar)
• e.g. Sucrose: glucose + fructose
(table sugar)
Oligosaccharides and polysaccharides
– Anywhere from two to ten monosaccharide units, linked by glycosidic bonds:
– make up an oligosaccharide(includes disaccharides).
– Polysaccharides :– much larger, containing hundreds of
monosaccharide units.
Structure of carbohydrates:
• The presence of the hydroxyl groups:
• allows carbohydrates to interact with the aqueous environment,
• and to participate in hydrogen bonding, both within and between chains.
Structure of carbohydrates:
• Derivatives of the carbohydrates can contain:
• nitrogens, • phosphates • and sulfur compounds. • Carbohydrates also can combine with
lipid to form glycolipids; • or with protein to form glycoproteins.
Carbohydrate nomenclature:
• The predominant carbohydrates encountered in the body:
• structurally related to the aldotriose glyceraldehyde;
• and to the ketotriose, dihydroxyacetone. • All carbohydrates contain at least one
asymmetrical (chiral) carbon, • and are, therefore, optically active.
Carbohydrate nomenclature
• In addition, carbohydrates can exist:• in either of two conformations, • as determined by the orientation of the
hydroxyl group about the asymmetric carbon farthest from the carbonyl.
• With a few exceptions, exist in the D-conformation.
• The mirror-image conformations, called enantiomers, are in the L-conformation.
Asymmetry
• All the monosaccharides except dihydroxyacetone:
• contain one or more asymmetric(chiral) C-atoms
• Glyceraldehyde: 2 optical isomers or enantiomers
• A molecule with ‘n’ chiral centers, have 2n
stereoisomers
D- and L- isomers of carbohydrates:
• D: Dextrorotatory L: Levorotatory
• Structures of Glyceraldehyde enantiomers
Structure of carbohydrates:
• The aldehyde and ketone moieties of the carbohydrates with five and six carbons:
• spontaneously react with alcohol groups present in neighboring carbons,
• to produce intramolecular hemiacetals; • or hemiketals, respectively.
Structure of carbohydrates:
• This results in the formation of five-• or six-membered rings. • As five-membered ring structure resembles the
organic molecule furan, • derivatives with this structure are termed
furanoses.
Structure of carbohydrates:
• Those with six-membered rings resemble the organic molecule pyran,
• and are termed pyranoses. • Such structures can be depicted by either
Fischer or Haworth style diagrams.
Structure of carbohydrates:
• The numbering of the carbons in carbohydrates:
• proceeds from the carbonyl carbon, for aldoses;
• or the carbon nearest the carbonyl, for ketoses.
• Cyclic Fischer projection of alpha-D-Glucose
Haworth projection of α-D-glucose:
♨The rings can open and re-close,
♨allowing rotation to occur about the carbon bearing the reactive carbonyl yielding:
Epimers
• When two sugars differ only in the configuration around one carbon atom:
• called ‘epimers’ or ‘diastereoisomers’ of each other:
• e.g. D-glucose and D- mannose,
• or D-erythrose and D-threose.
Examples of epimers
Anomers
• The carbon about which this rotation in the carbonyl carbon occurs:
• is the anomeric carbon,
• and the two forms: termed anomers.
Anomers:
• Carbohydrates can change spontaneously between the α and β configurations:
• a process known as mutarotation.
Anomers
• When drawn in the Fischer projection;
• the α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring.
• When drawn in the Haworth projection, the α configuration:
• places the hydroxyl downward.
Conformations of carbohydrates in the space:
• The spatial relationships of the atoms of the furanose and pyranose ring structures,
• more correctly described by the two conformations identified:
• as the ‘chair’ form
• and the ‘boat’ form.
Conformations of carbohydrates in the space
• The chair form:
• the more stable of the two.
• Constituents of the ring that project above or below the plane of the ring are axial
• and those that project parallel to the plane are equatorial.
Conformations of carbohydrates in the space:
• In the chair conformation:
• the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose: axial
• and equatorial in β-D-glucose.
• Chair form of α-D-glucose
Disaccharides:
• Covalent bonds between the anomeric hydroxyl of a cyclic sugar;
• and the hydroxyl of a second sugar (or another alcohol containing compound):
• termed as ‘glycosidic’ bonds, • and the resultant molecules: ‘glycosides’.
Disaccharides:
• The linkage of two monosaccharides to form disaccharides:
• involves a glycosidic bond.• Several physiologically important disaccharide
examples:• sucrose, • lactose • and maltose.
Structure of sucrose:
prevalent in sugar cane
and sugar beets,
is composed of glucose and fructose
by linkage through an α-(1,2)-β-glycosidic bond.
Demonstration of sucrose:
• Lactose: • found exclusively in
the milk of mammals. • consists of galactose
and glucose in a β-(1,4) glycosidic bond
• Maltose:
• the major degradation product of starch,
• composed of two glucose monomers in an α-(1,4) glycosidic bond.
Polysaccharides
• Most of the carbohydrates found in nature: • occur in the form of high molecular weight
polymers; • called polysaccharides. • The monomeric building blocks used to generate
polysaccharides:• varied; in all cases, • however, the predominant monosaccharide
found in polysaccharides: • D-glucose.
Polysaccharides
• When composed of a single monosaccharide building block:
• they are termed ‘homopolysaccharides’.
• Polysaccharides composed of more than one type of monosaccharide:
• termed as ‘heteropolysaccharides’.
Glycogen
• the major form of stored carbohydrate in animals.
• a homopolymer of glucose in α-(1,4) linkage; it is also highly branched, with α-(1,6) branch linkages occurring every 8-10 residues.
Glycogen
• has a very compact structure;
• that results from the coiling of the polymer chains.
• This compactness allows large amounts of carbon energy to be stored in a small volume;
• with little effect on cellular osmolarity.
Starch
• the major form of stored carbohydrate in plant cells.
• Structure: identical to glycogen, • except for a much lower degree of
branching (about every 20-30 residues). • Unbranched starch: amylose; • branched starch: called amylopectin.
Structure of amylose
Structure of amylopectin
Structural polysaccharides in living organisms:
• e.g. Cellulose:
• present in the cell walls of plants, stems, stalks, trunks
• linear, unbranched homopolysaccharide(10000 to 15000 D-glucose units)
• resembles amylose and the main chains of glycogen
Structure
Structural polysaccharides in living organisms:
• Chitin
• Principal component of the hard exoskeletons of nearly a million species of arthropods(insects, lobsters and crabs)
Chitin structure:
LITERATURE CITED
• Devlin,T.M. Textbook of Biochemistry with Clinical Correlations,Fifth Edition,Wiley-Liss Publications,New York, USA, 2002.
• Lehninger, A. Principles of Biochemistry, Second edition, Worth Publishers Co., New York, USA, 1993.
• Matthews, C.K. and van Holde, K.E., Biochemistry, Second edition, Benjamin / Cummings Publishing Company Inc., San Francisco, 1996.