Question

biochemistry question

use pK_a = 3.2 the a-COOH group, pK_BH = 8.0 for the a-NH₂ group, and the following values for dissociation constants for side-chain groups.

Amino Acid                          Dissociation Constant of Side Chain Group

Aspartic Acid                                       3.9 (pK_a of COOH)

Glutamic Acid                                     4.2 (pK_a of COOH)

Histidine                                              6.0 (pK_BH of =N)

Lysine                                                  10.5 (pK_BH of NH₂)

using the information above. (a) Draw structures of the predominant ionic form of the tripeptide His-Gly-Glu at each of the following pH values: pH 4, 7, and 10. (b) Calculate the isoelectric pH (pI) of this tripeptide.

Answer 1

Amino Acid                          Dissociation Constant of Side Chain Group

Aspartic Acid                                         3.9 (pKa of COOH)

Glutamic Acid                                       4.2 (pKa of COOH)

Histidine                                                6.0 (pKBH of =N)

Lysine                                                  10.5 (pKBH of NH2)

using the information above. (a) Draw structures of the predominant ionic form of the tripeptide His-Gly-Glu at each of the following pH values: pH 4, 7, and 10. (b) Calculate the isoelectric pH (pI) of this tripeptide.

The K_a value is a value used to describe the tendency of compounds or ions to dissociate. The K_a value is also called the dissociation constant, the ionisation constant, and the acid constant.

HB+ <=> H+ + B

The definition of K_a is: [H⁺]^.[B] / [HB], where B is the conjugate base of the acid HB.

The pK_a value is defined from K_a, and can be calculated from the K_a value from the equation

pKa = -Log₁₀(K_a) we can also write this formula as pKa = pH + log(protonated/unprotonated)

Predominant Ionic form of below peptide at pH 4, 7 and 8 (please find images separately attached)

Calculating PI of peptide,

PI is the average of all PKa values representing protonation and de-protonation of neutral form of the peptide.

Here ionisable groups are                                      Pka for them

Carboxyl terminus (COOH-)                                   pKa = 3.2

Amino terminus (NH2-)                                          pKBH = 8.0

Imidazole ring of HIS                                                6.0 (pKBH of =N)

Carboxyl side chain of Glu                                       4.2 (pKa of COOH)

For that we have to calculate at which pH net charge on the peptide will be zero,

At PH 4 charge on the peptide is,

peptide N-terminus	Glutamic Acid carboxyl group	peptide C-terminus	Imidazole ring of HIS	Net charge
+1	-1	-1	+1	+1

At PH 7 charge on the peptide is,

peptide N-terminus	Glutamic Acid carboxyl group	peptide C-terminus	Imidazole ring of HIS	Net charge
+1	-1	-1	+1	0

At PH 10 charge on the peptide is

peptide N-terminus	Glutamic Acid carboxyl group	peptide C-terminus	Imidazole ring of HIS	Net charge
0	-1	-1	0	-2

If we increase the pH above 8 peptide N terminus is losing its proton and net charge on peptide is becoming -2 hence PI fir this tripeptide should lie between 7 to 8 taking average 7+8/2 we get 7.5 so PI here would be 7.5