Question

Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.

Short answer

The pI value of the an amino acid is the average of the pKa values of its similarly ionizing groups. $\mathrm{\alpha }$amino and $\epsilon$amino groups are the similar ionizing groups present in lysine. The average of their pKa value gives pI value of lysine.

Step-by-step solution

Step-by-Step SolutionStep 1: The structure of lysine

The structure of Lysine is as follows:

Structure of Lysine

Isoelectric point of lysine

Lysine is an amino acid with 3 ionisable side groups which are$\mathbf{\alpha }$-COOH, $\mathbf{\alpha }$-amino and $\mathbf{\epsilon }$-amino.Their pKa values are 2.2, 9.2, 10.8, respectively. The pI value of an amino acid is the average of the pKa values of its positively charged amino groups.

Thus, isoelectric point can be calculated as,

$$\begin{gathered} \text{pI=}\frac{\text{9.2+10.8}}{\text{2}} \\ \text{=10} \end{gathered}$$