Amino acids and proteins

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LESSON 20.2

• Amino acids and proteins
• Chemical properties of amino acids

AIM
To study the reactions of amino acids,
including some reactions involving the side
chain
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20.2 Chemical Properties

• Reaction of Amino Group (-NH2) with Halide Acid
  (HX)
• The amino group is basic, so it reacts with
  halide acids to form salts.
• General equation is-

R NH2 HX R NH3X
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Example
HCl
Alanine
Alininium Chloride
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• With Nitrous Acid ( HNO2 )
• The reaction between amino group and nitrous
  acid

General equation
Observation Bubbling of gas (N2)
Video 2
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Example
H
H
CH3 C COOH HNO2 CH3 C COOH H2O
NH2
OH
Observation Bubbling of gas (N2)
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• Reaction of Carboxyl Group ( -COOH )
• with sodium hydroxide
• carboxyl group reacts with sodium hydroxide to
  form salt and water
• General equation

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Example
NaOH
2-aminopropanoic acid ( alanine )
H2O
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• with alcohol ( ROH ) in the presence of an acid
  catalyst
• Reaction between carboxyl group with alcohol
  group catalyzed by acid to form ester

Video 3
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Example
H
O
H
CH3 C C
CH3OH
?
O-
NH3
H
O
CH3 C C
H2O
OCH3
NH3
Methyl-2-aminopropanoate
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PEPTIDE BOND

• Peptides are amino acid polymers in which the
  individual amino acid units, are linked together
  by amide bonds, or peptide bonds.
• 2 amino acids dipeptide
• 3 amino acids tripeptide
• 15 30 amino acids oligopeptide
• gt 30 amino acids polipeptide

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PEPTIDE BOND

• Amino group of a substance reacts with carboxyl
  group of another amino acid to form a peptide
  bond by elimination of water.

• This is condensation reaction.

Video 4
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Notes N-terminal (with free NH2) is always
written on the left and C-terminal (with free
COOH) at the right side Naming Starting from
N-terminal, using 3-letter abbreviations for all
the ?-amino acids residue.
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Video 6
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• N ?-amino acids form N! polypeptides.
• Example
• 5 ?-amino acids form 120 pentapeptides ( 5!
  5 x 4 x 3 x 2 x 1 )
• The ?-amino acids in the
• peptides are known as
• ?-amino acid residues.

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Exercise

• Write chemical equations for the formation of 2
  possible dipeptides from glysine (
  2-aminoethanoic acid ) and alanine (
  2-aminopropanoic acid )

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Exercise

• The structural formulae of 3 amino acids are
  given below

phenylalanine
valine
Tyrosine
Write an equation to show a tripeptide
consisting of 3 amino acids in the order
phenylalanine-valine-tyrosine can be formed.
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Importance of amino acids

• Human beings can synthesize about half of the
  amino acids needed to make proteins.

• Other amino acids, called the essential amino
  acids (amino acid that cannot synthesized by the
  human), must be provided in the diet.

• The 10 essential amino acids are

arginine (Arg) methionine
(Met) threonine (Thr) leucine
(Leu) histidine (His) isoleucine
(Ile) trypthophan (Trp) phenylalanine
(Phe) valine (Val) lysine (Lys)
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Importance of proteins

• Proteins are the most abundant organic molecules
  in animals, playing important roles in all
  aspects of cell structure and function.

• Examples of protein functions

Class of protein Examples
Functions
structural protein collagen, keratin
tendons, skin hair, nails
enzymes DNA polymerase repair DNA
transport protein hemoglobin
transport O2