Isoelectric point

The isoelectric point, or pI, is the pH at which a protein has zero net charge. When the pH is higher than the isoelectric point, the protein has negative charge, and when lower, positive charge. You can calculate the isoelectric point of your protein easily using on-line resources.

1. First, get the one-letter amino acid sequence of your protein. To do this for a solved structure, open the page for the pdb ID at the RCSb Protein Data Bank. There on the left, click the FASTA sequence link under the structure tab and download all chains in FASTA format. Block the sequence of the chain of interest (excluding the comment line beginning >) and copy it to the clipboard.

2. Second, go to the EMBL WWW Gateway to Isoelectric Point Service, paste your sequence in the box, and press the button. 3. Warning: the sequence you paste in must be in UPPER CASE one letter code. If you paste in a lower case sequence, you'll get pI = 6.014999, which is for the backbone only, because it doesn't recognize lower case amino acids!