Question

a. What percentage of the $\mathbf{\alpha }$-amino group of lysine will be protonated at its pI?<25%50% >75%

b. Answer the same question for the$\mathbf{\epsilon }$-amino group of lysine.

Short answer

1. >75%
2. >75%

Step-by-step solution

Step-by-Step SolutionStep 1:Structure of Lysine

The structure of Lysine is shown below:

Structure of Lysine

Step 2:α-amino and ε-amino group of lysine

Lysine is an amino acid with 3 ionisable side groups which are $\mathbf{\alpha }$-COOH, $\mathbf{\alpha }$-amino and $\mathbf{\epsilon }$-amino.There pKa values are 2.2, 9.2, 10.8, respectively. Thus, isoelectric point can be calculated as:

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

Thus, percentage of the$\mathrm{\alpha }$amino group of lysine will be protonated and its pI will be,

$\frac{9.2}{10}\times 100$= 92%

And, percentage of$\epsilon$amino group of lysine will be more than 75%.

Hence,more than 75% of $\alpha$-amino group and$\epsilon$-amino group will be protonated at its pI.