Download Amino Acids and Proteins

Amino Acids and Proteins
Larry Scheffler
Lincoln High School
Portland OR
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Amino Acids
Amino acids have both
 a carboxyl group
-COOH
 an amino group
-NH2
in the same molecule..
2
Amino Acid Structure
The general formula of an amino acid is
shown here
The group designated by R is usually a
carbon chain but other
structures are also
possible
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Amino Acid Structure
Amino acids may be
characterized as a, b , or
g amino acids depending
on the location of the
amino group in the
carbon chain.
a are on the carbon
adjacent to the carboxyl
group.
b are on the 2nd carbon
g on the 3rd carbon from
the carboxyl group
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Amino Acids - Proteins
Amino acids are the building blocks of
proteins. Proteins are natural
polymers of successive amino acids
There are 20 different amino acids
that make up human proteins
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a- amino acids
Amino acids found in
proteins are aamino acids. The
amino group is
always found on the
carbon adjacent to
the carboxyl group
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Amino Acid Functions
1.
2.
Amino acids are the building blocks of
proteins
Some amino acids and their derivatives
function as neurotransmitters and other
regulators
Examples Include
L-dopamine
Epinephrine
Thyroxine
Histidine
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Amino Acids and Proteins
Amino acids
forming proteins
may be
characterized as
Acidic, Basic, or
neutral
depending on
the character of
the side chain
attached.
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Acidic Amino Acids
There are
two acidic
amino acids.
There are
two carboxyl
groups and
only one
amino group
per molecule
(asp)
(glu)
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Basic Amino Acids I
These amino
acids are
basic. They
have more
amino groups
than carboxyl
groups
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Basic Amino Acids II
These amino
acids are also
basic. They
have more
amino groups
than carboxyl
groups
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Neutral Amino Acids I
These amino
Acids are
considered
neutral. There
is one carboxyl
group per amino
group
(ala)
(gly)
((leu)
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Neutral Amino Acids II
(Ser)
(Tyr)
(Val)
(Trp)
(Cys)
(Met)
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Neutral Amino Acids III
(Ile)
(Thr)
(Asp)
(Phe)
(Gln)
(Pro)
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Amino Acids and Optical
Isomers


Except for glycine, all amino acids have a
chiral carbon atom. Therefore they can
have optical isomers
The amino acids found in proteins are all
levarotatory or L forms.
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Amino Acids are Amphoteric

Amino acids are amphoteric. They are capable of
behaving as both an acid and a base, since they have
both a proton donor group and a proton acceptor
group.

In neutral aqueous solutions the proton typically
migrates from the carboxyl group to the amino group,
leaving an ion with both a (+) and a (-) charge.
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The Zwitterion
This dipolar ion form is known as a Zwitterion.
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Essential Amino Acids


Of the 20 amino acids that make up
proteins 10 of them can be
synthesized by the human body
The other 10 amino acids must be
acquired from food sources. These
amino acids are known as essential
amino acids
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Essential Amino Acids
Essential amino acids Non-Essential amino acids
 Arginine
 Alanine (from pyruvic acid)
 Histidine
 Asparagine (from aspartic acid)
 Isoleucine
 Aspartic Acid (from oxaloacetic acid)
 Leucine
 Cysteine
 Lysine
 Glutamic Acid (from oxoglutaric acid)
 Methionine
 Glutamine (from glutamic acid)
 Phenylalanine
 Glycine (from serine and threonine)
 Threonine
 Proline (from glutamic acid)
 Tryptophan
 Serine (from glucose)
 Valine
 Tyrosine (from phenylalanine)
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Essential Amino Acids
Complete protein



Contains all 10
essential amino acids
Proteins derived from
animal sources are
complete proteins
Beans contain some
complete protein as
well
Incomplete protein



Lack one of more of the
essential amino acids
Most vegetable proteins
are incomplete proteins
Beans are an exception
to this generalizations
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Peptide Bond
When two amino acids combine, there is
a formation of an amide and a loss of a
water molecule
+ H2O
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Proteins- Levels of Structure
Amino acids can undergo condensation
reactions in any order, thus making it possible
to form large numbers of proteins.
Structurally, proteins can be described in four
ways.
1. Primary
2. Secondary
3. Tertiary
4. Quaternary structure.
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Primary Structure
The primary structure of a protein is defined by
the sequence of amino acids, which form the
protein. This sequence is determined by the
base pair sequence in the DNA used to create it.
The sequence for bovine insulin is shown below
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Secondary Structure

The secondary structure describes the way that the
chain of amino acids folds itself due to intramolecular
hydrogen bonding
Two common secondary structures are
the a-Helix 
and the b- sheet 
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Tertiary Structure

The tertiary structure
maintains the three
dimensional shape of
the protein.

The amino acid chain
(in the helical, pleated
or random coil form)
links itself in places to
form the unique twisted
or folded shape of the
protein.
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Tertiary Structure

There are four ways in which parts of the amino acid
chains interact to stabilize its tertiary shape.. They include:
I.--
Covalent bonding, for
example disulfide bridges
formed when two cysteine
molecules combine in which
the –SH groups are oxidized:
II.-- Hydrogen bonding between
polar groups on the side chain.
III.-- Salt bridges (ionic bonds)
formed between –NH2 and –
COOH groups
IV.-- Hydrophobic interactions.
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Quaternary Structure
Many proteins are not single strands
The diagram below shows the quaternary structure of
an enzyme having four interwoven amino acid strands
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Denaturing Proteins


The natural or native structures of
proteins may be altered, and their
biological activity changed or destroyed
by treatment that does not disrupt the
primary structure.
Following denaturation, some proteins
will return to their native structures under
proper conditions; but extreme
conditions, such as strong heating,
usually cause irreversible change.
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Denaturing Proteins

Heat
hydrogen bonds are broken by increased translational
and vibrational energy.(coagulation of egg white
albumin on frying.)

Ultraviolet
Radiation
Similar to heat
(sunburn)

Strong Acids or
Bases
salt formation; disruption of hydrogen bonds.
(skin blisters and burns, protein precipitation.)

Urea
competition for hydrogen bonds.
(precipitation of soluble proteins.)

Some Organic
Solvents
(e.g. ethanol & acetone) change in dielectric constant
and hydration of ionic groups.
(disinfectant action and precipitation of protein.)

Agitation
shearing of hydrogen bonds.
(beating egg white albumin into a meringue.)
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Sickle Cell Anemia


A small change in
the sequence of
the primary
structure can have
a significant
impact on protein
structure
In sickle cell
anemia a glutamic
acid is replaced by
a valine in the
amino acid
sequence
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Ninhydrin Reaction

Triketohydrindene hydrate, commonly known as
ninhydrin, reacts with amino acids to form a
purple colored imino derivative, This derivative
forms a useful test for amino acids, most of which
are colorless.
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Protein Tests: Biuret

Biuret reagent is a light blue
solution containing Cu2+ ion
in an alkaline solution.
Biuret turns purple when
mixed with a solution
containing protein. The
purple color is formed when
copper ions in the biuret
reagent react with the
peptide bonds of the
polypeptide chains to form a
complex.
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Xanthroprotic Test

Concentrated Nitric acid will form a yellow
complex with tryptophan and Tyrosine side
chains in proteins
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Disulfide Bridge Test

Disulfide bridges will react with Pb2+
ion from lead acetate in an acidfied
solution. A black precipitate indicates
the presence of disulfide-bonded
cysteine in proteins.
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