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Lecture 3 Kumar
Amino acids and AA Transport
Alpha carbon
1
Mirror Images of Amino Acid
a
Mirror
image
a
Same chemical properties
Stereo isomers
2
3
4
5
6
7
8
9
pKa of Amino Acid Residues
Residues on amino acids can release or accept protons
-COOH
R -COOH
a
-Imidazole·H+
Cys -SH
Tyr -OH
His
-NH3+
R -NH3+
a
-COOR -COOa
+ H+ pKa = 1.8~2.4
+ H+ pKa = 3.9~4.3
-Imidazole + H+ pKa = 6.0
Cys -S+ H+ pKa = 8.3
Tyr -O+ H+ pKa = 10
His
-NH2
R -NH2
a
+ H+ pKa = 8.8~11
+ H+ pKa = 10~12.5
Smaller pKa releases proton easier
Only His has the residue with a neutral pKa (imidazole)
pKa of a carboxylic or amino groups is lower than pKa of the R residues
10
Acidic environment
NH2 H+
R-C-H
COOH
Neutral environment
Alkaline environment
NH2 H+
R-C-H
COO-
NH2
R-C-H
COO-
pK2 ~ 9
pK1 ~ 2
5.5
+1
0
Isoelectric point
-1
11
pKa of Amino Acids
Amino acids
Gly
G
Ala
A
Val
V
Leu
L
Ile
I
Ser
S
Thr
T
Met
M
Phe
F
Trp
W
Asn
N
Gln
Q
Pro
P
Asp
D
Glu
E
His
H
Cys
C
Tyr
Y
Lys
K
Arg
R
-COOH -NH2
2.34
2.34
2.32
2.36
2.36
2.21
2.63
2.28
1.83
2.38
2.02
2.17
1.99
2.09
2.19
1.82
1.71
2.20
2.18
2.17
9.60
9.69
9.62
9.68
9.68
9.15
10.4
9.21
9.13
9.39
8.80
9.13
10.6
9.82
9.67
9.17
10.8
9.11
8.95
9.04
-R
pH
two pKa
pK2
pI
pK1
pK1 + pK2
2
three pKa
3.86
4.25
6.0
8.33
10.07
10.53
12.48
pK3
pK2
pK1
?
?
pI ?
[OH-]
12
H
first
HOOC-CH2-C-COOH
NH3+
+1
pK1 = 2.1
H
second
HOOC-CH2-C-COO-
Aspartic acid
Isoelectric point is the average
of the two pKa flanking the
zero net-charged form
0
2.1 + 3.9
= 3.0
2
Isoelectric point
NH3+
pK2 = 3.9
H
-OOC-CH -C-COO2
-1
NH3+ third
pK3 = 9.8
H
-OOC-CH -C-COO2
NH2
-2
-2
pK3
-1
pK2
pK1
0
+1
[OH]
13
14
15
16
Learning objectives for lecture 3
• Know the structures of simple sugars
• Understand the difference between D and L forms
• Know the structural differences between isomers and
enantiomers
• Know the basis for denoting amino acids as neutral, acid or
basic
• Understand how pKa of amino acids are defined and how
are they used to calculate isoelectric points
• Know what makes amino acids good buffers at many pH
values
• Understand how amino acids are generally transported from
intestinal lumen.
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