Isoelectric point
The isoelectric point (IEP or pI) is the pH value at which the number of positive and negative charges on an amphoteric molecule is exactly the same on a statistical mean.
Isoelectric point in proteins
Dissolved proteins form polyvalent ions. Their negative charges come from the carboxy groups of the glutamic acid and aspartic acid residues. The chain's carboxy group also contributes a negative charge. The positive charges are provided by the basic amino acids arginine , lysine and histidine as well as by the amino-terminal amino acid. The charge state of these acidic and basic groups depends on the pH value : a low pH value suppresses the dissociation of the acidic groups, while at a high pH value the basic groups are uncharged. From this it can be concluded that the solubility of proteins is lowest at the isoelectric point. The calculation of the isoelectric point of proteins is not trivial due to configuration influences.
Isoelectric point in amino acids
Amino acids are present in aqueous solution as zwitterions at the pH value of the isoelectric point .
See also
- Henderson-Hasselbalch equation
- Isoelectric focusing
- Acid constant
- 2D gel electrophoresis
- Mass spectrometry
Web links
- IPC - Isoelectric Point Calculator - Calculate the isoelectric point of the protein with over 15 methods
- CurTiPot - a spreadsheet for calculating acid-base equilibria (charge versus pH plot of amphoteric molecules, e.g. amino acids)
- SWISS-2DPAGE - a database with isoelectric points from two-dimensional polyacrylamide gel electrophoresis (~ 2000 proteins)
- Proteome-pI - a proteome isoelectric point database (predicted isoelectric point for all proteins)
Individual evidence
- ↑ Entry on isoelectric point . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03275 Version: 2.3.3.
- ↑ Compare protein pKa calculations in the English language Wikipedia.