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Introduction to Biochemistry

Carbohydrates

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Carbohydrates

Carbohydrates are

a major source of energyfrom our diet.

composed of the elementsC, H and O.

also called saccharides,

which means sugars.

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Carbohydrates

Carbohydrates

are produced byphotosynthesis in plants.

such as glucose aresynthesized in plantsfrom CO2, H2O, andenergy from the sun.

are oxidized in living cellsto produce CO2, H2O,and energy.

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Types of Carbohydrates

The types of carbohydrates are

monosaccharides, the simplest carbohydrates. disaccharides, which consist of two monosaccharides. polysaccharides, which contain many monosaccharides.

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Monosaccharides

Monosaccharides consist of

3-6 carbon atoms typically.

a carbonyl group (aldehyde or ketone). several hydroxyl groups. 2 types of monosaccharide structures:

Aldoses and ketoses

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Aldoses

Aldoses are monosaccharides

with an aldehyde group with many hydroxyl (-OH)

groups.triose (3C atoms)

tetrose (4C atoms)

pentose (5 C atoms)

hexose (6 C atoms)

O

CH aldose

HCOH

HCOH

CH2OH

Erythose, an aldotetrose

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Ketoses

Ketoses are monosaccharides

with a ketone group with many hydroxyl (-OH)

groups.

CH2OH

C=O ketose

HCOH

HCOH

HCOH

CH2OH

Fructose, a ketohexose

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Learning Check

Identify each as aldo- or keto- and as tetrose,pentose, or hexose:

H

CH2OH

OHC

H

H

H

OH

OH

OH

C

C

C

HC

O

CH2OH

HHO

CH2OH

O

H OHC

C

C

aldohexoseketopentose

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Structures ofMonosaccharides

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Fischer Projections

A Fischer projection

is used to represent carbohydrates. places the most oxidized group at the top. shows chiral carbons as the intersection of vertical and

horizontal lines.

Copyright 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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D and L Notations

In a Fischer projection, the OH group on the chiral carbon farthest from the carbonyl group

determines an L or D isomer.

left is assigned the letter Lfor the L-form.

right is assigned the letter Dfor the D-form.

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Examples of D and L Isomers ofMonosaccharides

D-glucose D-ribose L-galactose

O

CH2OH

H OH

H OH

HO HOHH

C H

CH2OH

H OH

H OH

OHHHC

OH

O

CH2OH

HO H

H OH

H OH

HHO

C

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D-Glucose

D-glucose is

found in fruits, cornsyrup, and honey.

an aldohexose with

the formula C6H12O6. known as blood sugar

in the body.

the monosaccharide in

polymers of starch,cellulose, andglycogen.

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D-Fructose

D-fructose

is a ketohexoseC6H12O6.

is the sweetestcarbohydrate.

is found in fruit juicesand honey.

converts to glucose inthe body.

H OH

CH2

OH

C

HOH OH

HC

O

C

C

CH2OH

D-Fructose

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Cyclic Structures

Cyclic structures

are the prevalent form of monosaccharides with 5 or 6carbon atoms.

form when the hydroxyl group on C-5 reacts with thealdehyde group or ketone group.

O

O

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H

OHH

OH

C

H H

OH OH

C C CH

O

CHOCH2

Drawing the Cyclic Structure forGlucose

STEP 1 Number the carbon chain and turn clockwise toform a linear open chain.

HHO

H

CH2OH

OHC

H

H

OH

OH

C

C

C

OH

C1

2

3

4

5

6

6 5 4 3 2 1

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Cyclic Structure for Glucose(cont)

OH

OH

OHOH

CH2OH

O

-D-Glucose -D-Glucose

OH

OH

OH

OH

CH2OH

O

STEP 3 Write the newOH on C1 down for the form. up for the form.

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Summary of the Formation ofCyclic Glucose

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-D-Glucose and -D-Glucose inSolution

When placed in solution, cyclic structures open and close. -D-glucose converts to -D-glucose and vice versa. at any time, only a small amount of open chain forms.

-D-glucose D-glucose (open) -D-glucose

(36%) (trace) (64%)

OH

CH2OH

OH

OC

H

OH

OHOH

OH

OHOH

CH2OH

OOH

OH

OHOH

CH2OH

O

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Cyclic Structure of Fructose

Fructose is a ketohexose. forms a cyclic structure. reacts the OH on C-5 with the C=O on C-2.

D-fructose -D-fructose-D-fructose

O CH2OH

OH

OH

OH

CH2OH O OH

CH2OH

OH

OH

CH2OH

H OH

H OH

HHO

O

CH2OH

C

C

C

C

CH2OH

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Disaccharides

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Important Disaccharides

A disaccharideconsists of two monosaccharides.

Monosaccharides Disaccharide

glucose + glucose maltose + H2Oglucose + galactose lactose + H2O

glucose + fructose sucrose + H2O

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Maltose

Maltose is

a disaccharide also known as malt sugar. composed of two D-glucose molecules.

obtained from the hydrolysis of starch. used in cereals, candies, and brewing. found in both the - and - forms.

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Formation of Maltose

Free -OH

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Lactose

Lactose

is a disaccharide of -D-galactose and -or-D-glucose.

contains a -1,4-glycosidic bond.

is found in milk andmilk products.

-form-form

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Sucrose

Sucrose or table sugar is obtained from sugar cane and sugar beets. consists of -D-glucose and -D-fructose.. has an ,-1,2-glycosidic bond.

-D-glucose

-D-fructose

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Sweetness of Sweeteners

Sugars and artificialsweeteners

differ insweetness.

are compared tosucrose (tablesugar), which is

assigned a valueof 100.

60 000

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Learning Check

Identify the monosaccharides in each of the following:A. lactose

(1) -D-glucose (2) -D-fructose (3) -D-galactose

B. maltose

(1) -D-glucose (2) -D-fructose (3) -D-galactose

C. sucrose(1) -D-glucose (2) -D-fructose (3) -D-galactose

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Polysaccharides

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Polysaccharides

Polysaccharides

are polymers of D-glucose. include amylose and amylopectin,

starches made of

-D-glucose.

include glycogen (animal starch inmuscle), which is made of -D-glucose.

include cellulose (plants and wood),which is made of -D-glucose. -D-Glucose

O

CH2OH

OH OH

OH

OH

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Structures of Amylose andAmylopectin

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Amylose

Amyloseis

a polymer of -D-glucose molecules.

linked by -1,4glycosidic bonds.

a continuous(unbranched) chain.

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Amylopectin

Amylopectin

is a polymer of -D-glucose molecules.

is a branched-chainpolysaccharide.

has -1,4-glycosidicbonds between theglucose units.

has -1,6 bonds tobranches.

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Dextrins

Starches like amylose and amylopectinhydrolyze to dextrins (smallerpolysaccharides)

Contain 3-8 glucose units

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Glycogen

Glycogen

is the polysaccharidethat stores -D-glucose

in muscle. is similar to amylopectin,

but is more highlybranched.

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Cellulose

Cellulose

is a polysaccharideof glucose units inunbranched chains.

has -1,4-glycosidicbonds.

cannot be digestedby humans because

humans cannotbreak down -1,4-glycosidic bonds.