Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Chapter 7...

Biochemistry 2/e - Garrett & Grisham

Chapter 7

Carbohydratesto accompany

Biochemistry, 2/e

Reginald Garrett and Charles Grisham

All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777

Outline

• 7.1 Carbohydrate Nomenclature

• 7.2 Monosaccharides

• 7.3 Oligosaccharides

• 7.4 Polysaccharides

7.1 Nomenclature

Carbohydrates are hydrates of carbon

• Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions

• Oligo = "a few" - usually 2 to 10

• Polysaccharides are polymers of the simple sugars

7.2 MonsaccharidesAn organic chemistry review

• Aldoses and ketoses contain aldehyde and ketone functions, respectively

• Triose, tetrose, etc. denotes number of carbons

• Aldoses with 3C or more and ketoses with 4C or more are chiral

• Review Fischer projections and D,L system

Stereochemistry ReviewRead text on p. 210-213 carefully!

• D,L designation refers to the configuration of the highest-numbered asymmetric center

• D,L only refers the stereocenter of interest back to D- and L-glyceraldehyde!

• D,L do not specify the sign of rotation of plane-polarized light!

• All structures in Figures 7.2 and 7.3 are D

• D-sugars predominate in nature

More StereochemistryKnow these definitions

• Stereoisomers that are mirror images of each other are enantiomers

• Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers

• Any 2 sugars in a row in 10.2 and 10.3 are diastereomers

• Two sugars that differ in configuration at only one chiral center are epimers

Cyclic monsaccharide structures and anomeric forms

• Glucose (an aldose) can cyclize to form a cyclic hemiacetal

• Fructose (a ketose) can cyclize to form a cyclic hemiketal

• Cyclic form of glucose is a pyranose

• Cyclic form of fructose is a furanose

Cyclic monsaccharide structures and anomeric forms

• Cyclic forms possess anomeric carbons

• For D-sugars, alpha has OH down, beta up

• For L-sugars, the reverse is true

Monosaccharide Derivatives• Reducing sugars: sugars with free

anomeric carbons - they will reduce oxidizing agents, such as peroxide, ferricyanide and some metals (Cu and Ag)

• These redox reactions convert the sugar to a sugar acid

• Glucose is a reducing sugar - so these reactions are the basis for diagnostic tests for blood sugar

More Monosaccharide Derivatives

• Sugar alcohols: mild reduction of sugars

• Deoxy sugars: constituents of DNA, etc.

• Sugar esters: phosphate esters like ATP are important

• Amino sugars contain an amino group in place of a hydroxyl group

• Acetals, ketals and glycosides: basis for oligo- and poly-saccharides

7.3 OligosaccharidesDon't memorize structures, but know the

important features

• Be able to identify anomeric carbons and reducing and nonreducing ends

• Sucrose is NOT a reducing sugar

• Browse the structures in Fig. 7.19 and Figure 7.20

• Note carefully the nomenclature of links! Be able to recognize alpha(1,4), beta(1,4), etc

7.4 PolysaccharidesFunctions: storage, structure, recognition

• Nomenclature: homopolysaccharide vs. heteropolysaccharide

• Starch and glycogen are storage molecules

• Chitin and cellulose are structural molecules

• Cell surface polysaccharides are recognition molecules

Starch A plant storage polysaccharide

• Two forms: amylose and amylopectin

• Most starch is 10-30% amylose and 70-90% amylopectin

• Branches in amylopectin every 12-30 residues

• Amylose has alpha(1,4) links, one reducing end

Starch A plant storage polysaccharide

• Amylose is poorly soluble in water, but forms micellar suspensions

• In these suspensions, amylose is helical– iodine fits into the helices to produce a blue

Why branching in Starch?Consider the phosphorylase reaction...

• Phosphorylase releases glucose-1-P products from the amylose or amylopectin chains

• The more branches, the more sites for phosphorylase attack

• Branches provide a mechanism for quickly releasing (or storing) glucose units for (or from) metabolism

GlycogenThe glucose storage device in animals

• Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass

• Glycogen is stored energy for the organism

• Only difference from starch: number of branches

• Alpha(1,6) branches every 8-12 residues

• Like amylopectin, glycogen gives a red-violet color with iodine

Dextrans A small but significant difference from

starch and glycogen • If you change the main linkages

between glucose from alpha(1,4) to alpha(1,6), you get a new family of polysaccharides - dextrans

• Branches can be (1,2), (1,3), or (1,4)

Dextrans A small but significant difference from

starch and glycogen • Dextrans formed by bacteria are

components of dental plaque

• Cross-linked dextrans are used as "Sephadex" gels in column chromatography

• These gels are up to 98% water!

Structural Polysaccharides Composition similar to storage

polysaccharides, but small structural

differences greatly influence properties • Cellulose is the most abundant natural

polymer on earth • Cellulose is the principal strength and

support of trees and plants

• Cellulose can also be soft and fuzzy - in cotton

Structural Polysaccharides Composition similar to storage

polysaccharides, but small structural

differences greatly influence properties • Beta(1,4) linkages make all the

difference!

• Strands of cellulose form extended ribbons

Other Structural Polysaccharides

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Chapter 7...

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