Example Data Table
| Example | Sequence | Use case |
|---|---|---|
| Lysozyme fragment | KVFGRCELAAAMKRHGLDNYR |
Basic rich fragment |
| Acidic peptide | MDEDEDEEDALAEK |
Low pI example |
| Balanced peptide | ACDEFGHIKLMNPQRSTVWY |
Mixed residue set |
Formula Used
Acidic group charge: -n / (1 + 10pKa - pH)
Basic group charge: n / (1 + 10pH - pKa)
Total charge: Sum of terminal, acidic, and basic charges.
pI: The pH where total protein charge is zero.
How to Use This Calculator
- Paste a one-letter protein sequence or FASTA record.
- Select a pKa data set.
- Enter a target pH for charge estimation.
- Adjust termini and custom pKa values when needed.
- Press Calculate to view the pI result.
- Use CSV or PDF buttons for export.
Protein pI Calculation Guide
What pI Means
The isoelectric point is the pH where a protein has no net charge. At this point, positive and negative groups balance. A protein may show lower solubility near this value. This makes pI useful in purification planning. It also helps explain migration in electrophoresis.
Why Sequence Matters
Each amino acid sequence has a different charge pattern. Aspartic acid and glutamic acid add negative charge. Lysine, arginine, and histidine add positive charge. Cysteine and tyrosine can also lose protons. Terminal groups affect small peptides strongly. Their effect becomes smaller in long proteins.
Advanced Inputs
This tool allows several pKa tables. It also supports custom pKa values. That is useful for special buffers or modified proteins. You can include or exclude terminal groups. This helps when termini are blocked. The target pH field estimates charge at any chosen pH.
Result Meaning
A high pI usually means more basic residues. A low pI usually means more acidic residues. The reported charge is model based. Real proteins may behave differently. Folding, salt, temperature, and nearby residues can shift pKa values. Treat the result as a strong estimate.
Laboratory Use
Protein pI helps choose buffers and separation methods. It can guide ion exchange chromatography. A protein below its pI tends to be positive. A protein above its pI tends to be negative. This rule helps select exchanger type. Use exported files for records and reports.
FAQs
1. What is protein pI?
Protein pI is the pH where total net charge is zero. It depends on ionizable residues, terminal groups, and selected pKa values.
2. Which residues affect pI most?
Aspartic acid, glutamic acid, lysine, arginine, histidine, cysteine, and tyrosine affect charge. N-terminal and C-terminal groups also matter.
3. Can I paste FASTA format?
Yes. Header lines starting with the greater-than symbol are ignored. The calculator reads the remaining sequence letters.
4. Why are unknown residues ignored?
Unknown letters do not have fixed charge or mass values. The calculator reports them and excludes them from numerical calculations.
5. What does custom pKa mean?
Custom pKa lets you override the selected data set. Use it for special assumptions, modified termini, or local laboratory standards.
6. Is the calculated pI exact?
No. It is a model estimate. Real pI can shift due to folding, ionic strength, post-translational modifications, and experimental conditions.
7. Why include target pH?
Target pH shows expected net charge at a chosen buffer pH. This helps with purification and solubility planning.
8. Can I export the result?
Yes. Use the CSV button for spreadsheets. Use the PDF button for a compact report file.