The `pK_(a)` values of ionisable groups in lysine are 2.18, 8.95 and 10.79 respectively. Find isoelectric point of lysine.

Answer

To find the isoelectric point (pI) of lysine, we will follow these steps:

### Step 1: Identify the pKa values
The given pKa values for lysine are:
- pKa1 = 2.18 (carboxyl group)
- pKa2 = 8.95 (amino group)
- pKa3 = 10.79 (side chain amino group)

### Step 2: Determine the relevant pKa values for calculating pI
The isoelectric point (pI) is calculated using the average of the pKa values that correspond to the protonation and deprotonation states around the zwitterionic form. For lysine, which is a basic amino acid, we consider the pKa values that are greater than 7.

### Step 3: Identify the pKa values greater than 7
From the provided pKa values:
- pKa2 = 8.95
- pKa3 = 10.79

These are the relevant pKa values for calculating the isoelectric point.

### Step 4: Calculate the average of the relevant pKa values
To find the isoelectric point (pI), we take the average of the two pKa values identified in Step 3:

\[ pI = \frac{pKa2 + pKa3}{2} = \frac{8.95 + 10.79}{2} \]

### Step 5: Perform the calculation
Now, we perform the calculation:

\[ pI = \frac{8.95 + 10.79}{2} = \frac{19.74}{2} = 9.87 \]

### Conclusion
The isoelectric point (pI) of lysine is **9.87**.

---