How To Calculate The Isoelectric Point

The isoelectric point (pl) is the pH (index of solution acidity) at which a molecule in solution has zero net charge. This value is particularly important in biochemistry as a fundamental characteristic of proteins. Proteins have a positive net charge at pH of solution below the isoelectric point; they are negatively charged if pH is above it. The isoelectric point is the function of amino acid composition of a protein. Therefore, the amino acid protein sequence is necessary for the pI calculation.

Step 1

Decide how to obtain the protein sequence. If the sequence is already available, go directly to Section 2. Otherwise, obtain it from the Universal Protein Resource database (see Resources).

Step 2

Enter a protein name as keywords in the query field and click "Search."

Step 3

Scroll down the search results and find an appropriate entry. Write down the database accession number.

Step 4

Navigate to the ExPASy Server Computing Tool (see Resources).

Step 5

Enter the database accession number from Step 4 in the field and "Click here to compute pI/Mw."

Step 6

Click "Submit" on the next screen.

Step 7

Read the isoelectric point (pI) value.

Step 8

If the sequence was already available, navigate to the ExPASy Server Computing Tool (see Resources).

Step 9

Enter the sequence manually in the field or copy and paste from a file. Click "Click here to compute pI/Mw."

Step 10

Read the isoelectric point (pI) value.