General organic and biological chemistry structures off life CH 19 1 proteins amino acids GOB structures 5th ed

Lecture Presentation

Chapter 19
Amino Acids and
Proteins

Karen C. Timberlake
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Chapter 19 Amino Acids and Proteins
• Radiology technicians work
with radiologists to produce
images that allow for the
diagnosis and treatment of
certain medical conditions.
• Technicians operate
imaging equipment such
as X-ray, computed
tomography (CT), and
positron emission
tomography (PET).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Chapter 19 Readiness
Key Math Skill
• Calculating pH from [H3O+] (11.6)
Core Chemistry Skills
• Writing Equations for Reactions of Acids and Bases
(11.7)
• Naming Carboxylic Acids (16.1)
• Forming Amides (18.5)

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


19.1 Proteins and Amino Acids
Protein molecules,
compared with many of
the compounds we
have studied, can be
gigantic.
The horns of animals
are made of proteins.
Learning Goal Classify proteins by their functions. Give
the name and abbreviations for an amino acid, and draw
its zwitterion.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Functions of Proteins
Proteins
• in the body are polymers made from 20 different
amino acids.
• differ in characteristics and functions that depend on
the order of amino acids that make up the protein.
• form structural components such as cartilage,
muscles, hair, and nails.
• function as enzymes to regulate biological reactions
such as digestion and cellular metabolism.
• including hemoglobin and myoglobin transport oxygen
in the blood.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Structural Classification of Proteins and
Functions

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Amino Acids

Amino acids, the molecular building blocks of proteins, have
a central carbon atom called the α carbon, bonded to
• two functional groups: an ammonium group (— NH3+) and
a carboxylate group (— COO−)
• a hydrogen atom and an R group or side chain in addition
to the carboxylate and ammonium groups.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Zwitterions
At physiological pH, the
• ionized ammonium and
carboxylate groups give an
amino acid a balance of
positive and negative
charge, which gives an
overall zero charge.
• neutral amino acid, which is
called a zwitterion, occurs at
a specific pH value known
as its isoelectric point (pI).
Core Chemistry Skill Drawing the Zwitterion for an Amino Acid
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Classification of Amino Acids
Amino acids are classified using their specific R groups. The
• nonpolar (hydrophobic) amino acids have hydrogen,
alkyl, or aromatic R groups.
• polar amino acids have R groups that interact with
water, which makes them hydrophilic.
• polar neutral amino acids contain an hydroxyl
(— OH), a thiol (— SH), or an amide (— CONH2) R group
• polar acidic amino acids contain a carboxylate
(— COO−) R group.
• polar basic amino acids contain an ammonium
(— NH3+) R group.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Classification of Amino Acids
A summary of amino acid types and their interactions
with water

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Nonpolar Amino Acids
An amino acid is nonpolar when the R group is H,
alkyl, or aromatic.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Polar Amino Acids, Neutral R Groups
An amino acid is polar when the R group is an alcohol,
a thiol, or an amide.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Amino Acids, Charged R Groups
An amino acid is charged when the R group is a
carboxylate (— COO−) or an ammonium (— NH3+)
group.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Study Check
Draw the zwitterions for the amino acids serine
and aspartate.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Solution
Draw the zwitterions for the amino acids serine
and aspartate.
ANALYZE      Given          Need
THE           serine         draw the zwitterions
PROBLEM        aspartate        
 

Serine

Aspartate

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Amino Acid Abbreviations

Amino acids have
• three-letter abbreviations derived from their names, given in
Table 19.2.
• one-letter abbreviations to allow the faster transfer of data.
Of the 20 amino acids,
• 11 have one-letter abbreviations that are the same as the
first letter in their names and 9 use different letters.
• arginine R and tyrosine Y use the second letters in their
names; phenylalanine is F; and lysine is K, which is near L in
the alphabet.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Amino Acid Stereoisomers
Amino acids can exist as D and L stereoisomers called
enantiomers. We draw Fischer projections for α-amino acids
by placing
• the carboxylate group at the top.
• the R group at the bottom.
• the — NH3+ group of the L isomer on the left.
• the — NH3+ group of the D isomer on the right.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Study Check
Identify the following amino acids as D or L
enantiomers:
A.

COO –

+
B.
H3N––C––H
CH2

H

COO –
+
C ––NH3
CH2
OH

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Solution
Identify the following amino acids as D or L
enantiomers:

A.

COO –

+
B.
H3N––C––H
CH2

H

COO –
+
C ––NH3
CH2
OH

L-Phenylalanine
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

D-Serine
© 2016 Pearson Education, Inc.


Chemistry Link to Health:
Essential Amino Acids
Of the 20 amino acids used to build the proteins in the body,
• only 11 can be synthesized in the body;
• the other 9 are essential amino acids that must be

obtained from the proteins in the diet.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Chemistry Link to Health:
Essential Amino Acids
Complete proteins such as
eggs, milk, meat, and fish
contain all of the essential
amino acids.
Incomplete proteins from
plants such as grains,
beans, and nuts are
deficient in one or more
essential amino acids.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

19.2 Amino Acids as Acids and Bases
When an amino acid with
positive and negative
charges is overall neutral
in charge, it is said to be at
its isoelectric point (pI).
Ball-and-stick model of
glycine at its pI of 6.0.
Learning Goal Using the zwitterion for an amino acid at its
isoelectric point, draw its ionized structures at pH values
above or below its isoelectric point.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Isoelectric Point
The isoelectric point of an amino acid is the pH
at which
• the charged groups on an amino acid are
balanced.
• the amino acid is neutral.
An amino acid can exist as
• a positive ion if a solution is more acidic (lower pH)
than its pI.
• a negative ion if a solution is more basic (higher
pH) than its pI.
General, Organic, and Biological Chemistry: Structures of Life, 5/e

Karen C. Timberlake

© 2016 Pearson Education, Inc.


Ionized Forms of Amino Acids
The pI values for nonpolar and and polar neutral amino
acids are from pH 5.1 to 6.3.

Alanine has a zero overall charge at its pI of 6.0 with a
carboxylate anion (— COO−) and an ammonium cation
(—NH3+).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.