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Carbohydrates and Glycobiology
Lehninger 3rd ed. Chapter 9
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Carbohydrates: • The most abundant bio-molecules on earth:
Photosynthesis converts 1014 Kg of CO2 and H2O to carbohydrates per year.
• Energy stores and fuels. • Serve as strucutral and protective elements. • Lubricate joints in the body. • Participate in recognition events. • Adhesions between cells.
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What are Carbohydrates?
• Three major classes: – Monosaccharides. – Oligosaccharides (mostly
disaccharides). – Polysaccharides.
• Glycoconjugates.
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What are Carbohydrates? cont. • Polyhydroxy aldehydes or
ketones. • Or substances that yield
such compounds upon hydrolysis.
• Most have the empirical formula: (CH2O)n
• May contain other atoms (e.g. N, P or S).
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Aldoses Ketoses
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Monosaccharides are chiral:���n chiral centers => 2n stereoisomers
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D versus L • The designation of D
or L is solely due to the chiral carbon furthest away from the carbonyl carbon.
• OH on the right => D. • OH on the left => L. • Most monosaccharides
are D.
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Epimers differ from one another by the configuration of 1 chiral center
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Most Monosaccharides are Cyclic • Aldoses with more than 3 carbons form cyclic
hemiacetals. • Ketoses with more than 4 carbons form cyclic
hemiketals.
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• The “old” carbonyl carbon is now a chiral carbon called the anomeric carbon.
• It can be either α or β. • Inter-conversion
between α and β is called mutarotation.
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Pyranoses and Furanoses
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Pyranoses Chair Forms
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Hexose derivatives
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Monosaccharides are reducing agents
Glucose���oxidase
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Polymerization of Monosaccharides • Tremendous variety: • Anomeric carbon can
be α or β. • Many alcohol groups
can attack.
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Polysaccharides
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Starch and glycogen are stored fuels
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Starch 2D Structure
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Starch 3D Structure
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Cellulose and Chitin are Structural Homopolysaccharides
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Chitin: Arthropod exoskeleton
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Peptidoglycan: Bacterial cell wall
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Extra cellular Matrix: Glycosaminoglycans
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Glycoconjugates: Proteoglycans, Glycoproteins & Glycolipids
• Proteoglycans: One or more glycosaminoglycans covalently bound to a membrane or secreted protein in the extracelular matrix
• Glycoproteins: One or several oligosaccharides covalently bound to a membrane or secreted protein.
• Glycolipids: Oligosaccharide covalently bound to a lipid.
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Proteoglycan
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Proteoglycan aggregates
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O- and N-linked Glycoproteins
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Glycolipids
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Lectins • Lectins through their ability to specifically
bind Oligosaccharide mediate many important cellular phenomena.
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