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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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