In biochemistry, pI refers to the isoelectric point. It's the pH at which a molecule, typically a protein or amino acid, carries no net electrical charge—meaning its positive and negative charges are balanced. At a pH below the pI, the molecule carries a net positive charge; above the pI, it carries a net negative charge. This property is crucial in various biochemical techniques like isoelectric focusing and protein purification.
Understanding pI
The isoelectric point (pI) is a critical characteristic of proteins and amino acids. Several resources define it as follows:
- ScienceDirect Topics: The isoelectric point (IEP) reflects a surface's response to an electrolyte, usually water. (https://www.sciencedirect.com/topics/chemistry/isoelectric-point)
- Wikipedia: The isoelectric point (pI, pH(I), IEP) is the pH where a molecule has no net electrical charge. (https://en.wikipedia.org/wiki/Isoelectric_point)
- Master Organic Chemistry: The isoelectric point, pI, is the pH where negative and positive charges are balanced in a zwitterionic amino acid. (https://www.masterorganicchemistry.com/2023/02/09/isoelectric-point-calculation/)
Knowing the pI is essential for:
- Protein purification: Techniques like isoelectric focusing exploit the differences in pI values to separate proteins.
- Protein solubility: A protein's solubility is often minimal at its pI.
- Intracellular localization: The pI can provide insights into a protein's location within a cell. (https://pmc.ncbi.nlm.nih.gov/articles/PMC8667598/)
Several online tools calculate the theoretical pI of proteins based on their amino acid sequence, such as the Expasy Compute pI/Mw tool. (https://web.expasy.org/compute_pi/) A database, Proteome-pI, even compiles predicted isoelectric points for numerous proteomes. (https://pubmed.ncbi.nlm.nih.gov/27789699/)
pI and pH Relationship
The relationship between pI and the surrounding pH is crucial:
- pI > pH: The protein carries a net positive charge.
- pI < pH: The protein carries a net negative charge.
- pI = pH: The protein carries no net charge.
