Question

In lysine, the \(\mathrm{p} \mathrm{K}_{\mathrm{a}}\) of the side chain is about \(10.5 .\) Assuming that the \(\mathrm{p} K_{\mathrm{a}}\) of the carboxyl and amino groups are 2 and \(9,\) respectively, the pl of lysine is closest to: (A) 5.5 (B) 6.2 (C) 7.4 (D) 9.8

Short answer

The pI of lysine is closest to 9.8 (option D).

Step-by-step solution

Understand pI Calculation

The isoelectric point (pI) of an amino acid is the pH at which the molecule carries no net charge. For amino acids with ionizable side chains, one must consider the carboxyl group, the amino group, and the side chain's pKa values.

Identify pKa Values

For lysine: pKa of carboxyl group is 2, pKa of amino group (alpha-amino) is 9, and pKa of the side chain (epsilon-amino) is 10.5.

Determine Ionization States

At the isoelectric point, lysine should have no net charge. This typically happens between the pKa values of the groups that are closest together. Given the pKa values, the relevant ones are 9 (alpha-amino) and 10.5 (epsilon-amino).

Calculate the pI

To find the pI, take the average of the pKa values closest to each other: \[ pI = \frac{(9 + 10.5)}{2} = 9.75 \] Thus, the pI is closest to 9.8.

Key concepts

amino acid pKa

The pKa value of an amino acid gives information about the pH at which a specific group on the amino acid will accept or donate a proton. This value is crucial because it helps determine the ionization state of the amino acid at given pH levels. Amino acids contain at least two groups that can ionize: the carboxyl group and the amino group. Each group has its own pKa value.
For lysine, the side chain also has an ionizable group with its own pKa value. In the context of calculating the isoelectric point, knowing the pKa values is essential because it shows at what pH levels the amino acid will carry no net charge.

lysine

Lysine is a basic amino acid with a side chain that contains an additional amino group, known as the epsilon-amino group. This characteristic makes lysine unique compared to many other amino acids because it has three ionizable groups: the carboxyl group, the alpha-amino group, and the epsilon-amino group.
The pKa values for lysine's ionizable groups are:

• Carboxyl group: pKa ≈ 2
• Alpha-amino group: pKa ≈ 9
• Epsilon-amino group: pKa ≈ 10.5

These pKa values are critical for calculating the isoelectric point, which determines the pH at which lysine will carry no net charge.

amino group ionization

Amino group ionization refers to the process where amino groups in amino acids accept or donate protons based on the surrounding pH. When calculating the isoelectric point, we need to consider the ionization states of these groups.
For lysine, both the alpha-amino group (with pKa ≈ 9) and the epsilon-amino group (with pKa ≈ 10.5) are relevant. At lower pH levels, the amino groups are protonated and positively charged. As the pH rises above each group's pKa, these groups lose protons and become neutral. Understanding these transitions helps us pinpoint the pH at which lysine has no net charge, a crucial factor for determining the isoelectric point.

biochemistry calculations

Biochemistry calculations often involve determining the pH, pKa values, and isoelectric points (pI) of amino acids and proteins. For calculating the pI of an amino acid like lysine, you need to average the pKa values of the groups that are closest to each other on the pH scale.
For example, in lysine, the relevant pKa values are 9 (for the alpha-amino group) and 10.5 (for the epsilon-amino group).
To find the pI, we calculate the average of these two values:
\[ pI = \frac{(9 + 10.5)}{2} = 9.75 \]
Thus, the pI of lysine is closest to 9.8. These kinds of calculations are fundamental in biochemistry, as they help in understanding the behavior of amino acids and proteins at various pH levels.