AMINO ACID AND PROTEINS

1
Chapter 20

• AMINO ACID AND PROTEINS

Video 1
2
20 .1 INTRODUCTION
amino acid
amino group NH2
carboxyl group -COOH
Video 2
3

• Amino acids can join together into long chains by
  forming amide bonds between the NH2 of one amino
  acid and the COOH of another
• peptides - chains with fewer than 50
• amino acids
• protein is reserved for longer chains.

4

• Amino acids commonly found in proteins are
  ?-amino acid, meaning that the amino group in
  each is a substituent on the ? carbon atom
• ? carbon atom the one next to
• the carbonyl group.

5
General structure of ?-amino acid

6
E
X
A
M
P
L
E
7

• The hydrogen atom and R group are attached to ?
  carbon
• The differences of R group in each amino acid
  give the unique characteristic of the amino
  acids.
• There are 20 different amino acids commonly found
  incorporated into proteins.

Video 3
8
Classification of 20 Amino Acids in Protein.
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
Naming Amino Acids

• The carboxyl group as the parent name whereas
  amino group as substituent group
• Alanine
• CH3 CH COOH
• NH2

Common name
2
1
3
2-amino
propanoic acid
Video 4
16
Example
Serine
2-amino-3-hydroxypropanoic acid
17
Example
Aspartic acid
HOOC CH2 CH COOH
NH2
2-aminobutanedioic acid
18
Example
Lysine
H2N CH2CH2CH2CH2 CH COOH
NH2
2,6-diaminohexanoic acid
19
Physical Properties

• Amino acids can undergo an internal acid-base
  reaction, in which a proton is transferred from
  the carboxyl to the amino group to form dipolar
  ion called zwitterion.

Video 5
20
Zwitterion

• a form in which the carboxyl group is present as
  a carboxylate ion, -CO2- , and the amino group is
  present as an aminium ion, -NH3

Video 6
21
Uncharged structure
dipolar ion / zwitterion
22

• The dipolar nature of amino acids give to
• their unique properties-
• Amino acids have high melting points, generally
  over 2000C.
• Amino acids are more soluble in water.
• Amino acids are amphoteric or ampholite (
  amphoteric electrolyte ) which
• can react as acid or base
• depending on the pH of
• the solution

23
OH-
OH-
H
H
Acid
pHpI
base
cation
zwitterion
anion
Amino acids as acid
Amino acids as amphoteric
Amino acids as base
24

• In acidic solutions all amino acids are present
  primarily as cations (amino acids are base).
• In basic solutions they are
• present as anions (amino acids
• are acid).

25
Isoelectric point (pI)

• zwitterion is electrically neutral and exists at
  specific pH. This particular pH is called
  isoelectric points.
• Each amino acid has its specific isoelectric
  point.
• For example , isoelectric point,
• pI - alanine is at pH 6.02.
• At this pH, alanine exists as
• zwitterions.

Video 7
26
Exercise

• The isoelectric point for Lysine (
  2,6-diaminohexanoic acid ) is 9.5.
• Draw the structural formulae for Lysine at
• (i) pH 1 (ii) pH 9.5 (iii) pH 12