Hello,
I am currently working with BSA (UniProtKB - P02769 (ALBU_BOVIN)) at different pH for zeta potential measurements. I am therefore interested in its charge as a function of pH and its pI. I find in all the publications values of pI around 4.5 whereas with ProtPi and Expasy I get rather 5.9. Does anyone have an explanation?
Thank you for your help
The discrepancy may be between experimental measurements (4.5) and the calculated value based on the amino acid sequence (5.9). Calculated values can differ from experimental values because calculated values do not take into account that some charged residues are buried within the protein, which can change their pKas significantly.
Adam is partially correct. There are 3 isoforms of BSA that can be separated by capillary electrophoresis (see link), these have slightly different pI's. BSA is coded for by multiple genes in the cow that can vary in sequence. There are also post-translational modifications of BSA that can affect its pI. You also need to make sure that you are looking at the pI of the mature protein. BSA is exported from the cells and the signal peptide is removed. There are nearest neighbor effects on the pKa of many amino acids, so when you look up the pKa values for a residue in a protein, you get a range, not one pKa number. Finally, make sure that the pI software is accounting for cysteine and tyrosine, sometimes these aren't counted as ionizable residues in some pI estimation software.
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