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22.2
Amino acid: a compound that contains both an amino group
and a carboxyl group. Describe Amino Acids (20 common
acids) – the units that make up proteins
Structure recognize…
 Alpha Carbon chiral center
 Carboxylic acid group bonded to an Alpha Carbon
 R Group
 Amine
R group
Alpha Carbon
Amine
Carboxylic Acid
-
-
COO
H
NH3 +
COO
+
H3 N
CH3
D-A lanine
H
OH
CH2 OH
D -Glycerald ehyde
H
CH3
L-Alan ine
CHO
the n aturally
occu rring form
the n aturally
occu rring form
CHO
HO
H
CH2 OH
L-Glycerald ehyde
• Nonpolar side chains
COO-
NH3
+
COONH3
+
Glycine
(Gly, G)
-
+
COO
NH3 +
S
COONH3 +
Leucin e
(Leu, L)
Meth ion in e
(Met, M)
H
-
COO
Isoleucin e
(Ile, I)
-
-
COO
N
H
COO
NH3
COO- Phen ylalan ine
(Phe, F)
+
NH3
A lanine
(A la, A)
N
H
NH3
+
Prolin e
(Pro, P)
Tryptoph an
(Trp , W)
COO- Valine
(Val, V)
+
NH3
• Polar side chains
COO-
H2 N
O
NH3 +
-
As paragine
(As n, N )
COO
HS
NH3
O
-
H2 N
COO
NH3
Glutamine
(Gln, Q)
+
HO
NH3
+
COO-
HO
NH3
+
Serine
(Ser, S)
OH
-
COO
+
Cysteine
(Cys, C)
Tyrosine
(Tyr, Y)
COO- Threon in e
(Thr, T)
NH3 +
• Basic and Acidic side chains
-
COO-
O
O
NH3
As partic acid
(As p, D )
+
NH2 +
H2 N
O
-
COO
N
H
NH3
+
Arginin e
(Arg, R)
-
-
O
COO Glutamic acid
+
(Glu, E)
NH
N
3
N
H
+
H3 N
COONH3
Histidine
(His , H)
+
-
COO
NH3
+
Lysine
(Lys, K)
22.3
Know zwitterions: compounds with + and – charges
Proteins will always have ionic character
O
R-CH-COH
O
R-CH-CO-
NH2
Un-ionized
form
NH3 +
Zwitterion
Protein in acidic and basic environments
O
+
+
H3 N-CH-C-O + H3 O
R
O
+
H3 N-CH-C-OH + H2 O
R
Same
O
+
H3 N-CH-C-O + OH
R
O
H2 N-CH-C-O + H2 O
R
• Isoelectric point, pI:
The pH at which an amino acid has no net charge:
both equal + & - charges
O
+
H3 N-CH-C-OH
R
pH 2.0
Net charge +1
OH
+
H3 O
O
+
H3 N-CH-C-O
R
pH 5.0 - 6.0
Net charge 0
OHH3 O+
Isoelectric environment
O
H2 N-CH-C-OR
pH 10.0
N et ch arge -1
 Using pH and pI info, know how to recognize the
structures of amino acids in acidic and in basic
solutions
Nonpolar &
polar side chains
alanine
asparagine
cys teine
glutamine
glycine
isoleucine
leucine
methionine
phenylalanine
proline
serine
threonine
tyros ine
tryptophan
valine
pI
6.01
5.41
5.07
5.65
5.97
6.02
6.02
5.74
5.48
6.48
5.68
5.87
5.66
5.89
5.97
Acidic
Side Chains
aspartic acid
glutamic acid
Bas ic
Side Chains
arginine
histidine
lysine
pI
2.77
3.22
pI
10.76
7.59
9.74
22.4
Be able to recognize cystine and the disulphide bond
+
2 H3 N-CH-COOCH2
SH
Cysteine
oxidation
reduction
+
H3 N-CH-COO
CH2
a disulfide
bon d
S
S
CH2
+
H3 N-CH-COO
Cystine
22.2
Understand the basic protein structure: numerous amino
acids linked by a peptide (amide) bonds (see pg475 for
amides)
Amide>>
peptide
bond
CH3
+
H 3N
O-
O
Alanine (Ala)
+
+
H 3N
O
OCH2 OH
Serine (Ser)
CH3 H
O
+
N
H 3N
O - + H2 O
O
CH2 OH
Alanyls erine
(Ala-Ser)
22.6
Combining Amino acids to form proteins
 Know how to recognize a peptide bond or linkage
 Know that when two amino acids react, the products
are a dipeptide and water
peptide
bond
CH3
+
H 3N
O-
O
Alanine (Ala)
+
+
H 3N
O
OCH2 OH
Serine (Ser)
CH3 H
O
+
N
H 3N
O - + H2 O
O
CH2 OH
Alanyls erine
(Ala-Ser)
22.6
Know the lingo
 Peptides – shortest chain length of amino acids
 Polypeptides – middle chain length of amino acids
 Proteins – longest chain length of amino acids
22.6
Know how to match the structure of a polypeptide to the
three letter abbreviation
By convention, peptides are written from the left, beginning with the
free -NH3+ group and ending with the free -COO- group on the right.
C-terminal amino acid: the amino acid at the end of the chain
having the free -COO- group.
N-terminal amino acid: the amino acid at the end of the chain
having the free -NH3+ group.
+
H 3N
N-terminal
amino acid
O
C6 H5
O
H
N
N
OH
O
OH
COOSer-Phe-Asp
C-terminal
amino acid
22.7
In “cartoons”, Recognize the
 peptide backbone
 side chains