The melanocyte-stimulating peptide hormone -melanotropin has the following seque
ID: 58032 • Letter: T
Question
The melanocyte-stimulating peptide hormone -melanotropin has the following sequence:
Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val
Assume that the pKa values for the N-terminal as 7.9 and for the C-terminal as 3.8. For its side chains, you may look up their pKa values from the table below.
10.1
Calculate the net charge of -melanotropin at pH= 11, pH = 5, pH = 1, and the isoelectric point.
Name Abbreviations 1- and 3-letter pKa of Ionizing Side Chain Arginine R, Arg 12.5 Aspartic acid D, Asp 3.9 Cysteine C, Cys 8.3 Glutamic acid E, Glu 4.2 Histidine H, His 6.0 Lysine K, Lys 10.0 Tyrosine Y, Tyr10.1
Explanation / Answer
Given sequence: Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val
Name
Abbreviations 1- and 3-letter
pKa of Ionizing Side Chain
Arginine
R, Arg
12.5
Aspartic acid
D, Asp
3.9
Cysteine
C, Cys
8.3
Glutamic acid
E, Glu
4.2
Histidine
H, His
6.0
Lysine
K, Lys
10.0
Tyrosine
Y, Tyr
10.1
The peptide NH3+ has pKa of 7.9 and COOH has a pKa of 3.8. The Ka is the acid dissociation constant which is a quantitative measure of the strength of an acid in the solution. The larger the value of pKa, the smaller is the extent of dissociation and therefore, the increasing order of the acid will reflect the decreasing order of pKa. SO, the pKa values are arranged from lowest to highest in the following order:
Name
Abbreviations 1- and 3-letter
pKa of Ionizing Side Chain
Carboxy terminal COOH
3.8
Glutamic acid
E, Glu
4.2
Histidine
H, His
6.0
Amino terminus (NH3+)
7.9
Lysine
K, Lys
10.0
Tyrosine
Y, Tyr
10.1
Arginine
R, Arg
12.5
If, pH<pKa the H+ is on (the COO- will be neutral COOH but NH3+ will remain as NH3+ only) and at pH>pKa the H+ is off.
If the pH is lower than the pKa, then the compound will be protonated. If the pH is higher than the pKa, then the compound will be deprotonated.
A further consideration is the charge on the compound. Acids are neutral when protonated and negatively charged (ionized) when deprotonated. Bases are neutral when deprotonated and positively charged (ionized) when protonated.
So, at pH = 1, which is low and highly acidic, the amino acid will act as a base and accept proton from the acidic medium and thus, they will have positive charge. Both COOH and NH2 will be protonated (i.e. NH2 will become NH3+ and COO- will become COOH). The, charge of the peptide at pH 1is as follows:
Name
Charge at pH 1
Carboxy terminal COOH
0
Glutamic acid
0
Histidine
0
Amino terminus (NH3+)
+1
Lysine
+1
Tyrosine
+1
Arginine
+1
Thus, the net charge is + 4
At pH = 5,
Name
Charge at pH 5
Carboxy terminal COOH
-1
Glutamic acid
-1
Histidine
+ 1
Amino terminus (NH3+)
+1
Lysine
+1
Tyrosine
0
Arginine
+1
Thus, the net charge is +2
At pH = 11. The amino acid will acts as an acid and lose it H of COOH and will carry negative charge. Both COOH and NH2 are deprotonated.
Name
Charge at pH 11
Carboxy terminal COOH
-1
Glutamic acid
-1
Histidine
0
Amino terminus (NH3+)
0
Lysine
0
Tyrosine
-1
Arginine
+1
Thus, the net charge is -2.
Name
Abbreviations 1- and 3-letter
pKa of Ionizing Side Chain
Arginine
R, Arg
12.5
Aspartic acid
D, Asp
3.9
Cysteine
C, Cys
8.3
Glutamic acid
E, Glu
4.2
Histidine
H, His
6.0
Lysine
K, Lys
10.0
Tyrosine
Y, Tyr
10.1