Download Chapter 14: Carbohydrates

Chapter 16: Amino
Acids, Proteins and
Enzymes
Chem 20
El Camino College
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Proteins
Proteins are polymers made of 20 different αamino carboxylic acids.
 A single protein molecule contains hundreds or
even thousands of amino acid units.
 An animal body has tens thousands of different
proteins

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Proteins
Proteins form components of the body such as
muscles, hair, and nails
 Enzymes are proteins that act as tiny “machines”
in cellular processes
 Hemoglobin is a protein that carries oxygen in
the blood
 Proteins can also act as storage molecules.
Some hormones are proteins.

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α Amino Acids
An amino acid contains two functional groups:
 A carboxylic group, -COOH
 An amine group –NH2 bonded to the carbon α
 There are 20 different amino acids in humans

R
H
H
C
N
OH
C
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H
O
α Amino Acids
Under physiological conditions, the –COOH
group loses H+ to give –COO- and the –NH2
group gains H+ to give –N H3 ⁺.
 Thus the amino acid is a dipolar ion, called
Zwitterion.

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Types of Amino Acids
AAs are classified as
Nonpolar
• nonpolar with
hydrocarbon side chains
• polar with polar or ionic
side chains.
• acidic with acidic side
chains.
Acidic
• basic with –NH2 side
chains.
Polar
Basic
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Nonpolar Amino Acids
An amino acid is nonpolar when the R group is
hydrophobic.
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Polar Amino Acids
An amino acid is polar when the R group is
polar ( has OH, SH, -CONH- group) .
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Acidic and Basic Amino Acids
An amino acid is
• acidic when the R group is a carboxylic
acid.
• basic when the R group is an amine.
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Amino Acids

Every amino acid, except glycine, contains at
least one chiral center. For example alamine
can have two isomers
In proteins, only L amino acids are found10
Amino Acids

Draw the spatial formulas for serine (R=CH2OH)
and lysine (R=CH2CH2CH2CH2NH3+) as they
exist in the body
HOH2C
H
H
H3NH2CH2CH2CH2C
C
N
O
H
C
H
H
C
O
N
C
H
O
H
H
O
serine
lysine
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Amino Acids
Amino acids can act as buffers by using up H+
ions or OH- ions that are added
 isoelectric point is the pH at which the positive
and negative charges of an amino acid are
equal.

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Peptide Formation
Peptide is amide formed between carboxyl group
of an amino acid and amino group of the next
amino acid. The bond is called peptide bond.
 By convention the N-terminal amino acid residue
is written at the left, and the C-terminal amino acid
residue at the right.

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Peptide Formation
The peptide bond is planar (C=O and NH in a
single plane), and the bond between C and N
does not rotate
.

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The Peptide Bond

The peptide bond is planar (flat), and the bond
between C and N does not rotate
H3C
H
H
C
120o
120o
N
120o C 120o
H
O
H
O
N 120o
C
120o
C
O
H
H
H
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The Peptide Bond
The peptide bond exists in another form,
obtained by moving the electrons around
 Different forms are called resonance structures

H
H
N
N
C
: O:
:O:
:
C
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Draw the tripeptide that forms when
serine, glycine, and methionine react.
HOH2C
H
H
C
N
N
C
H
H
C
O
H
HOH2C
H
O
H3CSH2CH2C
H
O
C
H
H
H
O
H3CSH2CH2C
O
N
N
C
H
O
O
N
C
H
O
H
C
C
C
C
H
H
H
C
H
H
O
N
C
H
O
H
H
H
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Protein Structure

Protein structure has 4 components
Primary structure (1o)
 Secondary structure (2o)
 Tertiary structure (3o)
 Quaternary structure (4o)

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Primary Structure of Proteins
The primary structure of a protein is
• the particular sequence of amino acids.
• the backbone of a peptide chain or protein.
CH3
CH3
CH
CH3 O
H3N
CH
C
S
CH3
CH O
N
H
CH
C
N
SH
CH2
CH2 O
CH2 O
CH
H
C
N
CH
C
O-
H
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
Ala-Leu-Cys-Met
1o Structures
• The nonapeptides oxytocin and vasopressin
have similar 1o structures.
• Only the amino acids at positions 3 and 8
differ.
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1o Structure of Insulin
Insulin
• was the first protein to have its
1o structure determined.
• has a primary structure of 2
linked polypeptide chains
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2o Structures
The three most common forms of 2o
structure are
• the alpha helix
• the beta pleated sheet
• the triple helix
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2o Structure – Alpha Helix
The 2o structure of an alpha
helix is
• a 3-D arrangement of amino
acids in a polypeptide chain
• held by interactions between
the –N-H gps and C=O gps
• a corkscrew shape that looks
like a coiled “telephone cord”.
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2o Structure–Beta Pleated Sheet
The 2o structure of a beta pleated sheet
• consists of polypeptide chains arranged
side by side.
• has R groups above and below the sheet.
• is typical of fibrous proteins such as silk.
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2o Structure – Triple Helix
The structure of a triple helix is
• 3 polypeptide chains woven together.
• found in collagen, connective tissue, skin,
tendons, and cartilage.
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3o Structure
The 3o structure
• the globular structure of a protein.
• alpha-helices and beta-sheets wrap
around each other to form a 3-D shape.
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4o Structure
The 4o structure
• combination of 2 or
more protein units.
• of hemoglobin
consists of 4
polypeptide chains as
subunits.
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Summary of Protein
Structure
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Denaturation Involves:
• disruption of 2o, 3o and 4o protein structures.
• heat and organic cmpds that break apart
interactions between chains.
• acids and bases that break interactions
between polar R groups and disrupt ionic bonds.
• heavy metal ions that disrupt bonds between
chains
• agitation that stretches peptide chains
interactions break.
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Applications of Denaturation
Denaturation of protein
occurs when
an egg is cooked
the skin is wiped with alcohol
heat is used to cauterize blood
vessels
instruments are sterilized in
autoclaves.
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