Amino Acids and Proteins

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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.. 

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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 a- amino acids.
The amino group is always found on the carbon
adjacent to the carboxyl group

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Amino Acid Functions

• 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. 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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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.
• Primary
• Secondary
• Tertiary
• 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

• -- Covalent bonding, for example disulfide
  bridges formed when two cysteine molecules
  combine in which the SH groups are oxidized
• -- Hydrogen bonding between polar groups on the
  side chain.
• -- Salt bridges (ionic bonds) formed between NH2
  and COOH groups
• -- 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
hydrogen bonds are broken by increased
translational and vibrational energy.(coagulation
of egg white albumin on frying.) Similar to
heat(sunburn) salt formation disruption of
hydrogen bonds.(skin blisters and burns, protein
precipitation.) competition for hydrogen
bonds.(precipitation of soluble
proteins.) (e.g. ethanol acetone) change in
dielectric constant and hydration of ionic
groups.(disinfectant action and precipitation of
protein.) shearing of hydrogen bonds.(beating
egg white albumin into a meringue.)

• Heat
• Ultraviolet Radiation
• Strong Acids or Bases
• Urea
• Some Organic Solvents
• Agitation

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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

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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.