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 appr

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.

Solutions

Expert Solution

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

gladiator answered 1 year ago

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