In: Biology
The amino acid sequence of human adrenocorticotropin, a polypeptide hormone, is: Ser-Tyr-Ser-Met-Glu-His-Glu-Arg-Trp-Gly-Lys-Pro-Val-Gly-Lys-Lys-Arg-Arg-Pro-Val-Lys-Val-Tyr-Pro-Asp-Ala-Gly-Glu-Asp-Gln-Ser-Ala-Glu-Ala-Glu-Pro-Leu-Glu-Phe a)What is the approximate net charge of this molecule at pH 3? ph 7? ph 9? Assume that the pKs of the terminal -NH3+ and -COOH groups are 7.8 and 3.6, respectively.
pH < pKa : protonated form
pH > pKa :deprotonated form
Positively charged amino acids : Arginine , Histidine and Lysine have one extra NH3+ group
Side chain pKa value : Lys - 10.5, His - 6.0, Arg - 12.5
Negatively charged amino acids : Glutamic acid and Aspartic acid have one extra COO- group
Side chain pKa value : Glu -4.3, Asp - 3.9
At pH 3 :
pKs of the terminal -NH3+ and -COOH groups are 7.8 and 3.6, respectively.
Amino terminal pH (3) < pKa (7.8) - Protonated form +1 (NH3+)
Carboxy terminal pH (3) < pKa (3.6) - Protonated form 0 (COOH)
Lys side chain pH < pKa Protonated form +1 (NH3+)
Arg side chain pH < pKa Protonated form +1 (NH3+)
His side chain pH < pKa Protonated form +1 (NH3+)
Glu and Asp side chain pH < pKa Protonated form : 0 (COOH)
Hence, there are 5 Glutamic acid (0), 4 lysine (+4), 1 histidine (+1), 3 arginine (+3) and 1 aspartic acid (0)
Amino terminal (+1) + Lysine (+4) + Histidine (+1) + Arginine (+3) = 9
The net charge at pH 3 is +9
At pH 7 :
Amino terminal (+1) + Lysine (+ 4) + Histidine (0) + Arginine (+3) + glutamic acid (0) + aspartic acid (0) + Carboxy terminal (-1)
Net charge at pH 7 = 8 - 1 = +7
At pH 9 :
Amino terminal (0) + Lysine (+ 4) + Histidine (0) + Arginine (+3) + glutamic acid (0) + aspartic acid (0) + Carboxy terminal (-1)
Net charge at pH 9 = 7 - 1 = +6