The melanocyte-stimulating peptide hormone -melanotropin has the following seque

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, Tyr

10.1

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

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