Isoelectric Point is the basic concept in the field of biochemistry and protein chemistry which plays an important role in understanding the behaviour of molecules in different environments. The isoelectric point is a property of an Amino Acid or protein that is defined as the pH at which a protein molecule carries an equal positive and negative charge.

In this article, we will learn about the isoelectric point as well as the formula for the isoelectric point and also provide a step-by-step guide to help you easily calculate the isoelectric point. So, let’s start with the definition of isoelectric point first.

What is Isoelectric Point (pI)?

The intermediate pH at which the amino acid shows no tendency to migrate towards any of the electrodes and exists in equilibrium when placed in an electric field. As different amino acids have different side chains, which may be acidic, basic, or neutral that affect their overall charge properties, the pI differs among different amino acids.

For amino acids, the isoelectric point is the average of the pK values of the two ionizable groups.

Note:

• If pH < pK_a then the solution is acidic.
• If pH > pK_a then the solution is basic.

Read more about pH.

Isoelectric Point Definition

The pH at which a molecule carries an equal amount of positive and negative charge is called the Isoelectric point.

It is represented as pI. The isoelectric point can be defined for protein as well as for amino acids.

Isoelectric Point Formula

Isoelectric Point Formula is used to calculate the isoelectric point for various different substances and is given as:

pI = (pK_a +pK_b)/2

Where,

• pI is the Isoelectric Point of Any Substance
• pK_a is the negative logarithm value (base 10) of the acid dissociation constant i.e., K_a
• pK_b is the negative logarithm value (base 10) of the base dissociation constant i.e., K_b

How to Calculate Isoelectric Point?

To calculate the isoelectric point we need to follow the below-listed steps:

• Assign pK_a Values
• Calculating Their Average

Assign pK_a values

The first step to calculate the isoelectric point is to assign the pK_a values of the molecules. The pK_a value is nothing but an ionizable group where a molecule is carrying a half-positive charge and a half-negative charge.

Calculating Their Average

After assigning the pK_a values we need to calculate their average. This will provide us with the result of the isoelectric point(pI).

pI = (pK_a +pK_b)/2

Isoelectric Point of Amino Acid

The isoelectric point (pI) of an amino acid is the pH at which it carries no net electrical charge. At this pH, the amino acid exists as a Zwitter Ion, which is a molecule with both a positive and a negative charge.

In the Zwitter ion form, the amino acid has both a positively charged amino group and a negatively charged carboxyl group. The pI varies among different amino acids due to differences in their side chains, which may be acidic, basic, or neutral, affecting their overall charge characteristics.

The below listed are some of the amino acids and their approximate values.

• Glycine (Gly): pI = 6.06
• Lysine (Lys): pI = 9.60
• Arginine (Arg): pI = 10.76
• Histidine (His): pI = 7.59

Read More about Amino Acid.

Isoelectric Point of Protein

A protein’s isoelectric point (pI) is the pH at which the protein carries no net electrical charge. It is a characteristic property of a protein and is determined by the overall composition of its constituent amino acids and their respective pI values.

Since proteins are composed of multiple amino acids with different pI values, calculating the exact pI of a protein can be complex and may require computational methods or experimental techniques.

Some of the examples of proteins along with their approximate isoelectric point values are listed below:

• Myglobin: pI = 7.0
• Hemoglobin: pI = 7.2
• Cytochrome C: pI = 10.3
• Albumin: p! = 4.9

Learn more about Proteins.

Solved Problems on Isoelectric Point

Problem 1: Find the isoelectric point of the amino acid where pK_a= 4 and pK_b=9.

Solution:

Formula: pI = (pK_a +pK_b)/2

pI = (4+9)/2

pI = 13/2 = 6.5

Problem 2: Find the isoelectric point of amino acid where pK_a‘s are 2.0, 10.5, and 3.8.

Solution:

Formula: pI = (pK_a +pK_b)/2

Here we will take pK_a as 2.0 and pK_b as 2.0 and 3.8.

pI = (2.0 + 3.8)/2

pI = 2.9

Problem 3: The value of pK_a and pK_b is given as 2.3 and 9.7 then find the isoelectric point.

Solution:

Formula: pI = (pK_a +pK_b)/2

pI = (2.3 + 9.7)/2

pI = 6

Problem 4: Find the pK_b of amino acid where pK_a= 1.88 and the isoelectric point is 2.77.

Solution:

Formula: pI = (pK_a +pK_b)/2

2.77 = (1.88 + pK_b)/2

pK_b = 3.65

Problem 5: Find the isoelectric point of amino acid where pK_a‘s are 2.19, 4.25 and 9.62

Formula: pI = (pK_a +pK_b)/2

Here pK_a and pK_b will be 2.19 and 4.25.

pI = (4.5 + 5.67)/2

pI = 5.085

FAQs on Isoelectric point

1. Define Isoelectric Point.

The pH at which a molecule carries an equal amount of positive and negative charge is called the Isoelectric point. It is represented as pI. The isoelectric point can be defined for protein as well as for amino acids.

2. What is Isoelectric Point of Glycine?

The isoelectric point of Glycine 6.06.

3. Define Zwitter Ion.

It is an ionic form that is neutral but carries an equal amount of positive and negative charge.

4. What is Isoelectric Point of Histidine?

The isoelectric point of histidine is 7.59 .

5. What is Isoelectric Point of Casein?

The isoelectric point of casein is found in the range of 4.6 to 5.2.

6. What is Isoelectric Point of Arginine?

The isoelectric point of arginine is 10.76.

7. What is Isoelectric Point of Lysine?

The isoelectric point of Lysine is 9.74.