Question

I have two questions:

1) TRUE or FALSE? Explain your answer

The isoelectric point (pI) of the tripeptide Lys-Lys-Lys (structure with no indication of ionization state shown below) will correspond to the pH at which each of the epsilon-amino groups is appoximately one-third protonated

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/structure7/142/mfcd00038220.png/_jcr_content/renditions/mfcd00038220-medium.png

2) What would you predict about the likely ratio of hydrophilic to hydrophobic residues in a series of globular monomeric proteins that range in size from 10,000 to 100,000 Da? How might the ratio of hydrophilic to hydrophobic residues change with protein size? Explain any assumptions you make in drawing your conclusions.

Answer 1

1) The pK of epsilon-amino group of Lys = 9.5

The pK of N-terminus amino group = 9.0

The pK of C-terminus carboxyl group = 3.5

The isoelectric point of the given tripeptide = (9.0 + 9.5)/2 = 9.25

At pH = 9.25

i) The charge on the C-terminus carboxyl group = -1 (pH > pK)

ii) The charge on the N-terminus amino group = 0 (pH > pK)

iii) The total charge on all the three epsilon-amino groups = +1 (pH < pK)

i.e. The charge on each epsilon-amino group = +1/3

Hence, the given statement is TRUE.