Download - Chapter 7 Carbohydrates. Carbohydrates are the most abundant biomolecule in nature Chapter 7: Overview Functions of carbohydrates 1.Provide energy through

Chapter 7

Carbohydrates

Carbohydrates are the most abundant biomolecule in nature

Chapter 7: Overview

Functions of carbohydrates

1. Provide energy through their oxidation

2. Supply carbon for synthesis of cell components

3. Serve as stored form of chemical energy

4. Form structural elements of some cells and tissues

Classes of Carbohydrates (saccharides)

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm

Hydrates of carbon Cm(H2O)n characterized by having multiple functional groups • Hydroxyl (alcohols)• Carbonyl (aldehydes or ketones)

Mono- and disaccharides are simple sugars

glucosefructose

galactoseribose

deoxyribose

sucroselactose

Fructo-oligosaccharidesgalactooligosaccharides

starchglycogencellulose

chitin

Stereochemistry of Carbohydrates

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://web.fccj.org/~ethall/stereo/stereo.htm

Stereoisomers: • Same molecular formula• Same sequence of bonded atoms (constitution), • Different 3-D orientations of their atoms in space.Enantiomers: Stereoisomers that are mirror images of each other.Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image.-Defined relative to central, chiral atom (carbon)

enantiomers

Fischer Projections

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

When carbonyl is up: D-family: OH (or NH2) group of chiral C most distant from anomeric center projects to right L-family: OH (or NH2) group of chiral C most distant from anomeric center projects to left Anomeric center: Carbonyl (aldehyde or ketone) carbon

Biological systems can only utilize the D- isomers.

*

* *

*

*

*

*

*

*

*

*

*

Monosaccharides


Contain single polyhydroxy aldehyde or ketone unit Sugars with an aldehyde functional group are aldoses Sugars with an ketone functional group are ketoses

Further classified based on number of C atoms Aldehydes contain prefix aldo-, ketones have prefix keto-

p. 528

chiral carbons in glucose


Max # of possible stereoisomers = 2n where n = number of chiral carbon atoms

Monosaccharides: D- aldoses


Chiral C’s?

1

2

3

4

stereoisomers

2

4

8

16

Section 7.1: Monosaccharides

Cyclic Structure of Monosaccharides Sugars with four or more carbons exist primarily

in cyclic forms Ring formation occurs because aldehyde and

ketone groups react reversibly with hydroxyl groups in an aqueous solution to form hemiacetals and hemiketals

Figure 7.5 Formation of Hemiacetals and Hemiketals

Monosaccharides: Chemical Properties


Depicted using Haworth structures Ring is drawn with oxygen to the back, and anomeric carbon to the right.

Furanose ring: A 5-member ring containing an oxygen atom. Pyranose ring: A 6-member ring containing an oxygen atom.

Glucose (Blood Sugar)


C

C

C

C

CH2

C

OH

OH

OH

OH

HO H

H

H

H

H O

1

2

3

4

5

6

Beta-hydroxy group: OH attached to anomeric carbon above ring. Alpha-hydroxy group: OH attached to anomeric carbon below ring.

Hemiacetal (on C1)

64%

0.02%

36%


Five-membered rings are called furanoses and six-membered rings are pyranoses

Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose

Figure 7.8 Furan and Pyran

Figure 7.9 Fischer and Haworth Representations of D-Fructose


Reaction of Monosaccharides The carbonyl and hydroxyl groups can undergo

several chemical reactions Most important include oxidation, reduction,

isomerization, esterification, glycoside formation, and glycosylation reactions

Joining Monosaccharides


Monosaccharide units can be joined together by glycosidic linkages to form glycosides. Glycosidic linkages are same as adding alcohol to hemi- intermediates

O

R H

+ H

R

O

RO

HO

R H

H+, RO-H

RO

RO

R H + H-O-H

Hemiacetal intermediate

acetal

O

R H

+ H

R

O

RO

HO

R H

H+, RO-H

RO

RO

R H + H-O-H

O

OH

CH2OH

OH

O

O

CH2OH

OHH

+O

CH2OH

OH

O

CH2OH

OHO

+ OH2


Naming of glycosides specifies the sugar component

Acetals of glucose and fructose are glucoside and fructoside

If an acetal linkage is formed between the hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another, this forms a disaccharide

In polysaccharides, large numbers of monosaccharides are linked together through acetal linkages

Figure 7.18 Methyl Glucoside Formation

Polymerization of Monosaccharide Subunits

• Through glycosidic linkages at the hemiacetal carbons, many monosaccharide subunits can be put together to form long, branching chains via 1,4 or 1,6 linkages.

• These can be between 2 α, 2 β, or between an α and a β.


Glycosylation Reactions attach sugars or glycans (sugar polymers) to proteins or lipids

Catalyzed by glycosyl transferases, glycosidic bonds are formed between anomeric carbons in certain glycans and oxygen or nitrogen of other types of molecules, resulting in N- or O-glycosidic bonds

Chemical Reactions of Sugars


Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily oxidized. Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy group

on adjacent carbon. At the same time, the cyclic forms are converted to open-chain forms and also react. Reducing sugars: Monosaccharides that can be oxidized Oxidation of carbohydrates to CO2 and H2O very important at cellular level, serves as

source of heat and energy.

Reducing sugar + Cu(II) oxidized cmpd + Cu2ODeep blue solution

Red-orange precipitate


Glycation: Reaction of reducing sugars with nucleophilic nitrogen atoms in a nonenzymatic reaction

Most researched example of the glycation reaction is the nonenzymatic glycation of protein (Maillard reaction)


Figure 7.20 The Maillard Reaction

The Schiff base that forms rearranges to a stable ketoamine, called the Amadori product

Can further react to form advanced glycation end products (AGEs)

Promote inflammatory processes and involved in age-related diseases


Important Monosaccharides Glucose (D-Glucose) —originally called dextrose

The primary fuel for living cells Precursor for Vitamin C synthesis Preferred energy source for brain cells and cells

without mitochondria (erythrocytes) Modified subunits can form long polymer chains starch, cellulose, glycogen

Figure 7.21 a-D-glucopyranose


Fructose (D-Fructose) is often referred to as fruit sugar, because of its high content in fruit

On a per-gram basis, it is twice as sweet as sucrose; therefore, it is often used as a sweetening agent in processed food

Figure 7.22 b-D-fructofuranose

Fructose (D-fructose)

http://upload.wikimedia.org/wikipedia/commons/1/1e/Fructose-isomers.jpg

Fruit sugar Absorbed directly into bloodstream during

digestion (like glucose and galactose) Anomeric carbon is C2

70%

22%

Hemiketal (on C2)

1

23

456

Other Important Monosaccharides


Galactose Synthesized in mammary gland, incorporated into milk lactose Component of the antigens present on blood cells that determine

blood type Used to synthesize other biomolecules Galactosemia is a genetic disorder resulting from a missing enzyme

in galactose metabolism

Section 7.2: Disaccharides

DisaccharidesTwo monosaccharides linked by a glycosidic bond Linkages are named by a- or b-conformation and

by which carbons are connected (e.g., a(1,4) or b(1,4))

Figure 7.27 Glycosidic Bonds

DisaccharidesMaltose

Lactose Sucrose



Disaccharides Continued Lactose (milk sugar) is the

disaccharide found in milk One molecule of galactose linked

to one molecule of glucose (b(1,4) linkage)

It is common to have a deficiency in the enzyme that breaks down lactose (lactase)

Lactose is a reducing sugar

Figure 7.28 a- and b-lactose


Disaccharides Continued Sucrose is common table sugar

(cane or beet sugar) produced in the leaves and stems of plants

One molecule of glucose linked to one molecule of fructose, linked by an a,b(1,2) glycosidic bond

Glycosidic bond occurs between both anomeric carbons

Sucrose is a nonreducing sugar

Figure 7.31 Sucrose

Other Important Monosaccharides

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011: http://www.istpace.org/Web_Final_Report/WP_4_chem_subsystems/descr_chem_subsystems/pna_intro/pna_intro.html

Ribose and Deoxyribose Used to synthesize RNA and DNA Used in protein synthesis

Phosphodiester bond

Section 7.3: Polysaccharides

Polysaccharides (glycans) are composed of large numbers of monosaccharides connected by glycosidic linkages Smaller glycans made of 10 to 15 monomers

called oligosaccharides, most often attached to polypeptides as glycoproteins

Two broad classes: N- and O-linked oligosaccharides


O-Glycosidic linkages attach glycans to the side chain hydroxyl of serine or threonine residues or the hydroxyl oxygens of membrane lipids

Figure 7.32 Oligosaccharides Linked to Polypeptides

N-linked oligosaccharides attached to polypeptides by an N-glycosidic bond with side chain amide nitrogen from the asparagine

Three major types of asparagine-linked oligosaccharides: high mannose, hybrid, and complex


Homoglycans Have one type of monosaccharide and are found

in starch, glycogen, cellulose, and chitin (glucose monomer)

Starch and glycogen are energy storage molecules while chitin and cellulose perform structural roles

Chitin is part of the cell wall of fungi and arthropod exoskeleton

Cellulose is the primary component of plant cell walls

No fixed molecular weight, because the size is a reflection of the metabolic state of the cell producing them


Starch—the energy reservoir of plant cells and a significant source of carbohydrate in the human diet

Two polysaccharides occur together in starch: amylose and amylopectin

Amylose is composed of long, unbranched chains of D-glucose with a(1,4) linkages between them

Figure 7.33 Amylose


Amylose typically contains thousands of glucose monomers and a molecular weight from 150,000 to 600,000 Da

The other form is amylopectin, which is a branched polymer containing both a(1,6) and a(1,4) linkages

Branch points occur every 20 to 25 residues

Figure 7.33 Amylose

• Components of plant starch built from glucose residues.Plant starches

Amylose (10-20%)No branching

Amylopectin (80-90%)Branching 24-30 residues


Glycogen is the carbohydrate storage molecule in vertebrates found in greatest abundance in the liver and muscle cells

Up to 8–10% of the wet weight of liver cells and 2–3% in muscle cells

Similar in structure to amylopectin, with more branch points

More compact and easily mobilized than other polysaccharides

Structure of glycogen

• Polymer built from glucose subunits.• Glucose in Glycogen are connected via α-1,4 or α-1,6

linkage.• α-1,4 linkage makes a linear chain, α-1,6 linkage makes a

branch (~every 10 residues).• The end glucose residues without open 1’-OH is called

nonreducing ends.• Branches provide more non-reducing ends for rapid

degradation.


Figure 7.34 (a) Amylopectin and (b) Glycogen

Glycogen breakdown Glycogen cleaved from nonreducing ends of chain by the enzyme glycogen

phosphorylase. Produces monomers of glucose-1-phosphate, which is then converted to

glucose 6-phosphate by phosphoglucomutase. G6P monomers produced have three possible fates:

G6P can continue on the glycolysis pathway and be used as fuel. G6P can enter the pentose phosphate pathway to produce NADPH and

5-carbon sugars. In liver and kidney, G6P can be dephosphorylated back to glucose by the

enzyme glucose 6-phosphatase. This is the final step in the gluconeogenesis (formation of glucose) pathway.

• Most important structural polysaccharide and single most abundant organic compound on earth.

• Provides strength and rigidity to plant cell walls.• Wood is ~50% cellulose.• Contains 300-3000 glucose subunits.• Form extended straight chains that hydrogen bond with

parallel chains, creating long, rigid fibers.• Undigestible by humans.

Cellulose


Pairs of unbranched cellulose molecules (12,000 glucose units each) are held together by hydrogen bonding to form sheetlike strips, or microfibrils

Each microfibril bundle is tough and inflexible with a tensile strength comparable to that of steel wire

Important for dietary fiber, wood, paper, and textiles

Figure 7.36 Cellulose Microfibrils


Heteroglycans High-molecular-weight carbohydrate polymers

that contain more than one type of monosaccharide

Major types: N- and O-linked glycosaminoglycans (glycans), glycosaminoglycans, glycan components of glycolipids, and GPI (glycosylphosphatidylinositol) anchors

GPI anchors and glycolipids will be discussed in Chapter 11

Section 7.3: PolysaccharidesHeteroglycans Continued

N- and O-Glycans—many proteins have N- and O-linked oligosacchaarides

N-linked (N-glycans) are linked via a b-glycosidic bond

O-linked (O-glycans) have a disaccharide core of galactosyl-b-(1,3)-N-acetylgalactosamine linked via an a-glycosidic bond to the hydroxyl of serine or threonine residues

Glycosaminoglycans (GAGs) are linear polymers with disaccharide repeating units

Five classes: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and heparin sulfate, and keratin sulfate

Varying uses based on repeating unit

Section 7.4: Glycoconjugates

Glycoconjugates result from carbohydrates being linked to proteins and lipids

Proteoglycans Heavily glycosylated

proteins; a core protein with one or more GAG chains; high carbohydrate content (about 95%)

Occur on cell surfaces or are secreted to the extracellular matrixFigure 7.38 Proteoglycan

Aggregate From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 7.4: Glycoconjugates Glycoproteins

Proteins covalently linked to carbohydrates through N- and O-linkages

Carbohydrate could be 1%–85% of total weight Glycoproteins can act as cell surface receptors

Section 7.4: Glycoconjugates

The glycocalyx is a glycoprotein-polysaccharide covering that surrounds the cell membranes of some bacteria, epithelia and other cells. Most animal epithelial cells have a fuzz-like coat on the external surface of their plasma membranes.

https://en.wikipedia.org/wiki/Glycocalyx

Download - Chapter 7 Carbohydrates. Carbohydrates are the most abundant biomolecule in nature Chapter 7: Overview Functions of carbohydrates 1.Provide energy through

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