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This article provides an introduction to carbohydrates, focusing on their importance, structure, and classification. Topics covered include monosaccharides, disaccharides, and hexose derivatives in organisms. Prepared by Dr. A. Riasi from Isfahan University of Technology, with reference from Lehninger Biochemistry.
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Isfahan University of Technology Advance Biochemistry Part 1: Carbohydrates Prepared by: Dr A. Riasi ( Isfahan University of Technology) Reference: Lehninger Biochemistry
Introduction • Importance of carbohydrates: • Photosynthesis • Dietary staple: sugar and starch • Oxidation: energy-yielding pathway • Structural and protective elements • Lubricate skeletal joints • Recognition and adhesion between cells
Introduction (Continue) • Many, but not all, carbohydrates have the empirical formula (CH2O)n. • There are three major size classes of carbohydrates: • Monosaccharides • Oligosaccharides • Polysaccharides
Monosaccharides and Disacacharides • Monosaccharide characteristics: • Consist of a single polyhydroxy aldehyde or ketone unit. • Many of the carbon atoms to which hydroxyl groups are attached are chiral centers.
Monosaccharides and Disacacharides • Monosaccharides are colorless, crystalline solids. • In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom.
Monosaccharides and Disacacharides • Monosaccharides are aldose or ketose
Monosaccharides and Disacacharides • Monosaccharides with three, four, five, six, and seven carbon atoms in their backbones: • Triose • Tetroses • Pentoses • Hexoses • Heptoses
Monosaccharides and Disacacharides • The hexoses are the most common monosaccharides in nature.
Monosaccharides and Disacacharides • All the monosaccharides except dihydroxyacetone contain one or more asymmetric carbon atoms.
Monosaccharides and Disacacharides • The simplest aldose, glyceraldehyde, contains one chiral center and therefore has two different optical isomers, or enantiomers.
Monosaccharides and Disacacharides • In general, a molecule with n chiral centers can have 2nstereoisomers. • Glyceraldehyde has 21 = 2; the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers.
Monosaccharides and Disacacharides • Some sugars occur naturally in their L form: • L-arabinose • L isomers of some sugar derivatives that are common components of glycoconjugates.
Monosaccharides and Disacacharides • The formation of ring structures form: • Hemiacetals • hemiketals
Monosaccharides and Disacacharides • The αand βanomers of D-glucose interconvert in aqueous solution by a process called mutarotation.
Monosaccharides and Disacacharides • Ketohexoses also occur in αand β anomeric forms. • D-Fructose readily forms the furanose ring the more common anomer of this sugar in combined forms or in derivatives is β-D-fructofuranose.
Monosaccharides and Disacacharides • Two conformationsof a molecule are interconvertible without the breakage of covalent bonds • Two configurationscan be interconverted only by breaking a covalent bond for example, in the case of αand βconfigurations, the bond involving the ring oxygen atom.
Hexose derivatives in organisms • There are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent or the carbon atom is oxidized to a carboxyl group.
Hexose derivatives in organisms • Different derivatives of hexoses: • Containing an amine group (-NH2) • Containing a N-acetyl group (-NH-CO-CH3) • Containing an acid lactic and an amine group or a N-actyl group • Containing a methyl group (-CH3) • Containing a carboxyl group (-COO-)
Hexose derivatives in organisms • Replacing the hydroxyl group with an amino group.
Hexose derivatives in organisms • Bacterial cell walls contain a derivative of glucosamine. OH OH
Hexose derivatives in organisms • The substitution of a hydrogen hydroxyl group at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively.
Hexose derivatives in organisms • Oxidation of the carbonyl (aldehyde) carbon of glucose, galactose, or mannose forms the corresponding aldonic acids: • Gluconic acid • Galactonic acid • Manonic acid
Hexose derivatives in organisms • Monosaccharides can be oxidized by mild oxidizing agents such as ferric (Fe 3+) or cupric (Cu2+) ion.
Hexose derivatives in organisms • Oxidation of the carbon at the other end of the carbon chain (C-6) of glucose, galactose, or mannose forms the corresponding uronic acid: • Glucuronic • Galacturonic • Mannuronic acid
Hexose derivatives in organisms • The acidic glucose derivatives are:
Hexose derivatives in organisms • In addition to previous mentioned acidic hexose derivatives, there is a nine-carbon acidic sugar:
Hexose derivatives in organisms • In bacterial systems an enzyme uses a mannose derivative as a substrate, inserting three carbons from pyruvate into the resulting sialic acid structure.
Hexose derivatives in organisms • In the synthesis and metabolism of carbohydrates, the intermediates are very often not the sugars themselves but their phosphorylated derivatives.
Disaccharides contain a glycosidic bond • Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond.
Hexose derivatives in organisms • Nonreducing disaccharides are named as glycosides; in this case, the positions joined are the anomeric carbons.
