24.3 amino acids and their polymers 1 > copyright © pearson education, inc., or its affiliates....

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved..

24.3 Amino Acids and 24.3 Amino Acids and Their PolymersTheir Polymers

1

>>

Chapter 24The Chemistry of Life

24.1 A Basis for Life24.2 Carbohydrates

24.3 Amino Acids and Their Polymers

24.4 Lipids24.5 Nucleic Acids24.6 Metabolism



2

>>

Strength-building exercises can cause your muscles to become larger and stronger. This could not happen withoutamino acids.

CHEMISTRY & YOUCHEMISTRY & YOU

Why do your muscles need amino acids?



3

>>

Amino Acids

What is the general structure of an amino acid?

Amino AcidsAmino Acids



4

>>

Many biological compounds contain nitrogen in addition to carbon, oxygen, and hydrogen.


• Some of the most important nitrogen-containing molecules in organisms are amino acids.

• In fact, the polymers of amino acids make up more than one-half of the dry weight of your body.



5

>>

An amino acid is any compound that contains an amino group (—NH2) and a carboxyl group (—COOH) in the same molecule.

• For chemists and biochemists, however, the term is usually reserved for the 20 common amino acids that are formed and used by living organisms.




6

>>

Amino acids consist of a carboxyl group, an amino group, a hydrogen, and an R-group side chain that are all covalently bonded to a central carbon atom.




7

>>

The chemical nature of the side-chain group accounts for the differences in properties of the 20 amino acids.

• In some amino acids, the side chains are nonpolar aliphatic or aromatic hydrocarbons.

• In other amino acids, the side chains are neutral but polar.

• In still others, the side chains are acidic or basic.




8

>>

Because the central carbon of an amino acid is asymmetric, these compounds can exist as enantiomers.

• Enantiomers may be right- or left-handed.

• Nearly all the amino acids found in nature are of the left-handed, or L, form.




9

>>

Common Amino Acids

Name Symbol Name Symbol

Alanine Ala Leucine Leu

Arginine Arg Lysine Lys

Asparagine Asn Methionine Met

Aspartic acid Asp Phenylalanine Phe

Cysteine Cys Proline Pro

Glutamine Gln Serine Ser

Glutamic acid Glu Threonine Thr

Glycine Gly Tryptophan Trp

Histadine His Tyrosine Tyr

Isoleucine Ile Valine Val

Interpret DataInterpret Data

The table below gives the names of amino acids with their three-letter abbreviations.



10

>>

Does the table in this lesson show all existing amino acids?



11

>>

Does the table in this lesson show all existing amino acids?

No, it shows the 20 common amino acids that are formed and used by living organisms. These are the amino acids that are important to chemists and biochemists.



12

>>

Peptides and Proteins

What determines the differences in the chemical and physiological properties of peptides and proteins?

Peptides and ProteinsPeptides and Proteins



13

>>

A peptide is any combination of amino acids in which the amino group of one amino acid is united with the carboxyl group of another amino acid.


• The amide bond between the carboxyl group of one amino acid and the nitrogen in the amino group of the next amino acid in the peptide chain is called a peptide bond.



14

>>

Peptide bonds always involve the central amino acid and central carboxyl groups.

• The side chains are not involved in the bonding.


Amino acid Amino acid Peptide



15

>>

A free amino group is at one end of the peptide.

• The convention is to write the peptide formula so that the free amino group is at the left end.

• There is also a free carboxyl group, which appears at the right end of the molecule.

Amino acid Amino acid Peptide




16

>>

More amino acids may be added to the peptide in the same fashion to form long chains by condensation polymerization.

• The order in which the amino acids of a peptide are linked is called the amino acid sequence of that molecule.

• The amino acid sequence of a peptide is conveniently expressed using the three-letter abbreviations for the amino acids.




17

>>

For example, Asp—Glu—Gly represents a peptide containing three amino acids.

• This tri-peptide contains aspartic acid, glutamic acid, and glycine, in that order, with the free amino group assumed to be on the left end (on the Asp) and the free carboxyl group on the right end (on the Gly).


– Note that Asp—Glu—Gly is a different peptide from Gly—Glu—Asp because the order of amino acids is reversed, and thus the free amino group and free carboxyl group are on different amino acids.



18

>>

In theory, the process of adding amino acids to a peptide chain can continue indefinitely.


• A peptide with more than ten amino acids is a polypeptide.

• A peptide with more than 100 amino acids is a protein.



19

>>

Proteins are an important class of biomolecules.

• For example, your skin, hair, nails, and muscles are all made of proteins.

• Proteins are needed for almost all chemical reactions that occur in the body.




20

>>

We can make some of the amino acids that our cells use to make proteins.• Other amino acids

must be obtained by


eating foods rich in proteins.

• Beans and brown rice are good sources of amino acids.



21

>>





22

>>


Your muscles need amino acids because muscles are made of proteins, which are made from amino acids. Your body produces some of these amino acids; others must be obtained from protein-rich food.




23

>>

Differences in the chemical and physiological properties of peptides and proteins result from differences in the amino acid sequence.


• As many as 20100 different amino acid sequences are possible for a protein of 100 amino acids containing a combination of the 20 different amino acids.



24

>>

• The figure below represents a long peptide chain of a protein.


Protein molecules are folded into relatively stable three-dimensional shapes.



25

>>

• This figure shows how sections of a peptide chain may coil into a regular spiral, known as a helix.





26

>>

• Peptide chains may also be arranged side by side to form a pleated sheet, as shown below.





27

>>

• Irregular folding of the chains can also occur.





28

>>

The three-dimensional shape of a protein is determined by interactions among the amino acids in its peptide chains.

• Protein shape is maintained partly by hydrogen bonds between adjacent folded chains.




29

>>

Covalent bonds also form between sulfur atoms of cysteine side chains that are folded near each other.

• In that way, separate polypeptide chains may be joined into a single protein.




30

>>

The three-dimensional structure of myoglobin, the oxygen storage protein of muscle tissue, is shown here.


• The peptide chains of most of the myoglobin molecule are twisted into helixes.



31

>>

Myoglobin also contains a nonprotein structure called heme.


• Heme contains four linked rings with an iron(II) ion (Fe2+) at the center.

• Molecular oxygen binds to the heme iron.



32

>>

How many amino acids are in a peptide? How many are in a polypeptide? How many are in a protein?



33

>>

How many amino acids are in a peptide? How many are in a polypeptide? How many are in a protein?

A peptide contains 2 or more amino acids joined together by a peptide bond. A polypeptide contains 10 or more amino acids. A protein contains about 100 or more peptides.



34

>>

Enzymes

How do enzymes affect the rates of reactions in living things?

EnzymesEnzymes



35

>>

Enzymes

How do enzymes affect the rates of reactions in living things?

• Enzymes are proteins that act as biological catalysts.

EnzymesEnzymes



36

>>

Enzymes increase the rates of chemical reactions in living things.

EnzymesEnzymes



37

>>

In 1926, the American chemist James B. Sumner reported the first isolation and crystallization of an enzyme.

• The enzyme he isolated was urease.

• Urease hydrolyzes urea, a constituent of urine, into ammonia and carbon dioxide.

ureaseH2N–C–NH2(aq) + H2O(l) 2NH3(g) + CO2(g)

O

Urea Water Ammonia Carbon dioxide

EnzymesEnzymes



38

>>

Since the discovery of urease, thousands of enzymes have been isolated and structurally characterized as proteins.

EnzymesEnzymes



39

>>

In addition to being able to promote reactions, enzymes have two other properties of true catalysts.

EnzymesEnzymes



40

>>


• First, they are unchanged by the reaction they catalyze.

EnzymesEnzymes



41

>>


• First, they are unchanged by the reaction they catalyze.

• Second, they do not change the normal equilibrium position of a chemical system.

EnzymesEnzymes

– The same amount of product is eventually formed whether or not an enzyme is present.



42

>>

Few reactions in cells ever reach equilibrium, however.

• The products tend to convert rapidly to another substance in a subsequent enzyme-catalyzed reaction.

– According to Le Châtelier’s principle, such removal of a product pulls the reaction toward completion.

EnzymesEnzymes



43

>>

Enzymes catalyze most of the chemical changes that occur in the cell.

How Enzymes Work

EnzymesEnzymes

• Substrates are the molecules on which an enzyme acts.



44

>>

In a typical enzymatic reaction, the substrate interacts with side chains of the amino acids on the enzyme.

How Enzymes Work

EnzymesEnzymes



45

>>

A substrate molecule must make contact with, and bind to, an enzyme molecule before the substrate can be transformed into the product.

How Enzymes Work

EnzymesEnzymes



46

>>

The place on an enzyme where a substrate binds is called the active site.

How Enzymes Work

EnzymesEnzymes

• An active site is usually a pocket or crevice formed by folds in the peptide chains of the enzyme protein.



47

>>

The peptide chain of an enzyme is folded in a unique way to accommodate the substrate at the active site.

How Enzymes Work

EnzymesEnzymes



48

>>

Since the active site of each enzyme has a distinctive shape, only a specific substrate molecule can fit into the enzyme, similar to how only one key shape will fit into a certain lock.

How Enzymes Work

EnzymesEnzymes

• Thus, each enzyme can catalyze only one chemical reaction at a time.



49

>>

An enzyme-substrate complex is formed when an enzyme molecule and a substrate molecule are joined.

How Enzymes Work

EnzymesEnzymes



50

>>

Carbonic anhydrase catalyzes the reversible breakdown of carbonic acid to carbon dioxide and water.

How Enzymes Work

EnzymesEnzymes

H2CO3(aq) CO2(g) + H2O(l)

carbonic anhydrase

Carbonic acid Carbon dioxide

Water

• One molecule of carbonic anhydrase can catalyze the breakdown of 36 million molecules of carbonic acid in one minute!



51

>>

The figure below shows a model of the enzyme-substrate complex formed between carbonic anhydrase and its substrate, carbonic acid.

How Enzymes Work

EnzymesEnzymes



52

>>

• Some enzymes can directly catalyze the transformation of biological substrates without assistance from other substances.

• Other enzymes need nonprotein coenzymes, also called cofactors, to assist the transformation.

Coenzymes

EnzymesEnzymes



53

>>

Coenzymes are metal ions or small organic molecules that must be present for an enzyme-catalyzed reaction to occur.

Coenzymes

EnzymesEnzymes

• Many water-soluble vitamins, such as B vitamins, are coenzymes.

• Metal ions that act as coenzymes include the cations of magnesium, potassium, iron, and zinc.



54

>>

Liver contains high levels of catalase.

EnzymesEnzymes

• When a small amount of crushed liver cells is added to a solution of hydrogen peroxide, oxygen gas is rapidly evolved.

Coenzymes



55

>>

The enzyme catalase includes an iron(III) ion in its structure.

EnzymesEnzymes

• Catalase catalyzes the breakdown of hydrogen peroxide to water and oxygen.

Coenzymes

2H2O2(aq) 2H2O(l) + O2(g)catalase



56

>>

What kind of biological molecule is an enzyme? How does an enzyme act?



57

>>

What kind of biological molecule is an enzyme? How does an enzyme act?

An enzyme is a protein. It acts as a catalyst—it speeds reaction time, does not affect a reaction’s equilibrium point, and is not used up by a reaction.



58

>>

An amino acid has a carboxyl group, an amino group, a hydrogen atom, and an R group bonded to a central carbon atom.

Differences in the amino acid sequence result in differences in the properties of peptides.

Enzymes increase reaction rates.

Key ConceptsKey Concepts



59

>>

• amino acid: an organic compound having amino (—NH2) and carboxyl (—COOH) groups in the same molecule; proteins are made from the 20 naturally occurring amino acids

• peptide: an organic compound formed by a combination of amino acids in which the amino group of one acid is united with the carboxyl group of another through an amide bond

Glossary TermsGlossary Terms



60

>>

• peptide bond: the bond between the carbonyl group of one amino acid and the nitrogen of the next amino acid in the peptide chain; the structure is

• protein: any peptide with more than 100 amino acids




61

>>

• enzyme: a protein that acts as a biological catalyst

• substrate: a molecule on which an enzyme acts

• active site: a groove or pocket in an enzyme molecule into which the substrate (reactant molecule) fits; where the substrate is converted to products




62

>>

Proteins are polymers of amino acids and are needed for most chemical reactions in cells.

BIG IDEABIG IDEA

Chemistry as the Central Science



63

>>

END OF 24.3END OF 24.3

24.3 amino acids and their polymers 1 > copyright © pearson education, inc., or its affiliates....

Documents