Question

Consider a protein with the acidic side chains,

Amino Acid side-chain Arginine	pKa = 12.48
Aspartic Acid	PKa = 3.90
Cysteine	pKa = 8.33
Glutamic acid	pKa = 4.07
Histidine	pKa = 6.04
Lysine	pKa =10.79
Tyrosine	pKa =10.13

Given that the pH of blood is about 7.3, how many of the above side chains would be mostly in their ionic form (A-) in blood?

1. 2
2. 3
3. 4
4. 5

Answer 1

All the amino acids COOH having pKa higher than 7.3 will ionize to give the anion.

So aspartic acid, glutamic acid, histidine have pKa lower than 7.3. Hence will ionize to give respective anion, A^{​​​​​​-.}

So answer is 3 amino acids will be present in ionic form.