Chapter Three

Biochemistry

Biochemistry is the study of chemical substances found in living systems and the chemical interactions of these substances with each other.

A biochemical substance is a chemical substance found within a living organism.

Copyright © Houghton Mifflin Company. All rights reserved.

3 –1

Biochemistry

Biochemistry

Biochemistry


3 –2

14.2 Intro to Carbohydrates

Carbohydrates are biomolecules that decompose to produce carbon and water. Their empirical formulas are approximately CH

2

O.

Carbohydrates are produced in plants by photosynthesis.

CO

2

+ H

2

O sunlight chlorophyll enzymes

Carbohydrates + O

2


3 –3

14.2 Intro to 14 Carbohydrates

Carbohydrates are scarce in animals, but account for ~75% of dry mass in plants.

Uses of carbohydrates:

Oxidized to provide energy

Serve as stored form of chemical energy

Supply carbon for biosynthesis in cells

Form structures of some cells and tissues

Are markers on cell surfaces


3 –4

14.3 Types of Carbohydrates

Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or substances that produce such compounds by hydrolysis.

H

HO

H

C

O

C OH

C H

H

H

C OH

C OH

CH

2

OH

Glucose, a polyhydroxy aldehyde


HO

H

H

C

C

CH

2

OH

C O

H

OH

C OH

CH

2

OH

Fructose, a polyhydroxy ketone

3 –5

14.3 Types of Carbohydrates

Carbohydrates are classified by size. Monosaccharides contain one polyhydroxycarbonyl unit. Larger carbohydrates contain chains of these units, linked by covalent bonds.

monosaccharide disaccharide oligosaccharide (3 - 10 monosaccharide units) polysaccharides contain many

(sometimes >10,000) monosaccharide units


3 –6

14.4 Types of Monosaccharides

Monosaccharides are classified by

1. The type of carbonyl group:

Aldose or Ketose

2. The number of carbons:

Triose (3 carbons) Tetrose (4 carbons)

Pentose (5 carbons) Hexose (6 carbons)

Competency XIII-1


3 –7

14.4 Types of Monosaccharides

H O

H

HO

H

H

C OH

C H

C OH

C OH

H C OH

H

Glucose, an aldohexose


H

H

H

H

H

C OH

C O

C OH

C OH

C OH

H

Ribulose, a ketopentose

3 –8

14.5 Handedness

Shapes of molecules are incredibly important in biochemistry. Molecules that have the same formula but different shape are called isomers.

There are several types of isomers:

Constitutional Isomers

Stereoisomers

Geometric

Optical


3 –9

14.5 Handedness

Types of isomers:

Constitutional isomers, or structural isomers, are isomers that differ in their bonding sequence or connectivity.

H

3

C H

C C

H CH

3

trans-2-butene

H CH

3

H

2

C CH

2

C C

H

2

C CH

2

H CH

3 cyclobutane 2-methylpropene


3 –10

14.5 Handedness

Types of isomers:

Stereoisomers are isomers that differ only in how their atoms are oriented in space. The connectivity is the same in all the isomers.

There are two types of stereoisomers.

Geometric isomers, or cis-trans isomers:

H H

C C

H

3

C CH

3

cis-2-butene


H

3

C H

C C

H CH

3

trans-2-butene

3 –11

14.5 Handedness

Types of isomers:

Optical isomers are molecules that interact with polarized light.

The simplest of these are nonsuperimposable mirror images of each other. They are called enantiomers.


3 –12

14.5 Handedness

Molecules that can have enantiomers must have chiral centers.

Chiral centers are tetrahedral carbons with four different substituents. The substituents can be individual atoms or functional groups.

J

M

L

C

1

K


K

J

C

L

M

2

3 –13

14.5 Handedness

Pairs of molecules with nonsuperimposabl e mirror images are called enantiomers.


3 –14

14.5 Handedness

Carbohydrates have many stereoisomers.

Glyceraldehyde, the simplest carbohydrate, has enantiomers. It is an aldotriose.


3 –15

14.5 Handedness

Each enantiomer in a pair has the same properties unless it interacts with another chiral substance. Biological molecules are usually chiral.

O O

H

3

C

CH

3

C

C

H

(S)-(+)-Carvone

H odor of caraway

H H

CH

3

C

C

H

H

(R)-(



)-Carvone odor of spearmint

CH

3


3 –16

14.5 Handedness

Compounds can have more than one chiral center. The number of stereoisomers is 2 n , where n is the number of chiral centers.

H

H

HO

C

O

C OH

C H

HO

HO

H

C

O

C H

C H

H

H

C

C

OH

OH

CH

2

OH glucose


H

H

C OH

C OH

CH

2 mannose

OH

H

HO

H

C

O

C OH

C H

HO

H

C H

C OH

CH

2

OH galactose

3 –17

14.5 Handedness

Stereoisomers of compounds with more than one chiral center that are not enantiomers

(mirror images) are called diastereomers.

A

G

H

C

F

C

D

E

A

E

D

C

C

H

G

F

E

G

H

C

C

D

A

F

E

A

D

C

C

H

G

F

A B C D

A & B, C & D, pairs of enantiomers

A is a diastereomer of C & D.


3 –18

14.5 Handedness

In carbohydrates, handedness is shown in

Fischer projections. The right-handed isomer is the D-(dextro) isomer; the lefthanded isomer is the L-(levo) isomer.

H

C

O

CHO

HO C H HO H

H

HO

C

H

CH

L-glyceraldehyde

2

OH


H

CHO

O H

C

OH H C OH

CH

2

OH

H

C

H

OH

D-glyceraldehyde

3 –19

14.5 Handedness

In carbohydrates with many chiral centers, the carbon furthest from the carbonyl group is used for this designation.

H

HO

H

H

C

O

H

OH

H

OH

OH

Naturally occuring carbohydrates are all Disomers.

CH

2

OH

D-glucose

HO

H

O

C

H

H

OH

HO

HO

HO CH

2

L-glucose

H

H


3 –20

14.7 Cyclization

Monosaccharides contain carbonyl and hydroxyl groups. These react to form hemiacetals. The reactions are spontaneous, intramolecular, and form cyclic products.

O

Ring size:

6 atoms pyranose

5 atoms furanose

Pyran

O

Furan


3 –21


Ring size:

6 atoms pyranose

5 atoms furanose


3 –22


Hemiacetals can form in two orientations:

CH

2

OH

C

H

C

H

OH

OH

C

O

H

C

H OH



-D-glucose

H

C

OH

CH

2

OH

C

H

C

H

OH

OH

C

O

H

C

H OH



-D-glucose

C

OH

H

36%


64%

3 –23

14.8 RXN’s of Monosaccharides

Aldoses are easily oxidized. Benedict’s test with Cu 2+ can be used to detect them.

O

C

H

H C

R

OH + Cu

2+ blue sol'n

O

C

OH

H C

R

OH Cu

2

O reddish solid

Tollens’ test, which produces metallic silver, is also useful.


3 –24


Positive Benedict’s test.


3 –25


Carbonyl groups in monosaccharides can be reduced to hydroxyl groups. The products are sugar alcohols.

HO

H

H

H

C

O

H OH

H

OH

OH

CH

2

OH

D-glucose

H

2 catalyst

H

HO

H

CH

2

OH

OH

H

OH

H OH

CH

2

OH

D-glucitol, a.k.a. sorbitol


3 –26


In the presence of alcohols, monosaccharides form cyclic acetals called glycosides.

Glucose + CH

3

OH

H

3

O

1+

catalyst

CH

2

OH

H

C

C

H

OH

OH

C

O

H

C

C

H

O

H OH methyl-



-D-glucoside


CH

3

CH

2

OH

H

C

C

H

OH

OH

C

O

H

C

C

O

H

H OH methyl-



-D-glucoside

CH

3

3 –27

14.9 Disaccharides

Disaccharides contain two monosaccharide units. They are bonded together through acetal/glycoside linkages. One monosaccharide supplies the carbonyl in hemiacetal form; the other provides the alcohol.

R O R

H H

C

OH

+

HO

C

R hemiacetal alcohol

R

- H

2

O


R O R

H H

C C

O

R R acetal, a.k.a. glycoside

3 –28


Maltose is composed of two glucose units joined by an



(1



4) glycoside linkage.


3 –29


Lactose is composed of galactose and glucose units united in a



(1



4) linkage.

CH

2

OH

OH

C

C

H

OH

H

C

O

H

C

H OH



-D-galactose unit


H

C

H

O

C

CH

2

OH

C O

H

OH

C

H

C

H OH

D-glucose



or



C

OH

H

3 –30


Lactose is the sugar found in milk.

Lactose intolerance occurs when a person does not produce the enzyme to hydrolyze the



(1



4) glycoside linkage. Infants and children have the enzyme, but many adults do not.


3 –31


Sucrose contains glucose and fructose units joined in an



,



(1



2) linkage.


CH

2

OH

C

H

C

H

OH

OH

C

O

H

C

H OH

O

C

H

HO CH

2

C

H

H

C

OH

O

HO

C

CH

2

C

OH

H

3 –32

14.10 Polysaccharides

The major polysaccharides are polymers of glucose. In cellulose, the glucose units are joined by



(1



4) glycoside linkages.


3 –33


Cellulose is the major structural carbohydrate in plants.

Animals lack the enzyme cellulase, which catalyzes hydrolysis of the



(1



4) glycoside linkages. They (we!) cannot metabolize cellulose for nutrition.

Grazing animals and termites have bacteria in their guts that produce the enzyme; these animals can feed on grass and wood.


3 –34


Starches are also polymers of glucose. They are used for energy storage in plants. There are two forms, amylose and amylopectin.

Amylose is a straight-chain polymer in which glucose units are linked by



(1



4) glycoside linkage.

Amylopectin is a branched-chain polymer.

The branches are formed by



(1



6) linkages.


3 –35


Comparison of amylose and amylopectin structures.


3 –36


Amylopectin, showing glycoside linkages.


3 –37


Glycogen, sometimes called animal starch, is structurally similar to amylopectin.

Glycogen is formed when excess glucose is present in the blood; the process is called glycogenesis. Glycogen is stored in the liver and muscle tissue.

When blood glucose is low, glycogen is hydrolyzed to release glucose. The process is called glycogenolysis.


3 –38

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