Protein pI Calculator
Calculate the isoelectric point (pI) of a protein from its amino acid sequence.
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What it does
The protein pI calculator determines the isoelectric point of a polypeptide from its sequence — the pH at which the molecule carries no net electrical charge. At that pH the protein stops migrating in an electric field and its solubility typically reaches a minimum. Biochemists use it to predict migration in isoelectric focusing and two-dimensional gels, to choose the buffer pH for ion-exchange chromatography, to judge whether a purification step will precipitate the target, and to see how adding or removing charged residues shifts the behaviour of an engineered variant. Alongside the pI value this tool reports the net charge at pH 7, the charge-versus-pH curve, and the pKa constants it actually used.
How it works
The calculation considers every ionizable group together: the N-terminal alpha-amino group, the C-terminal alpha-carboxyl, and the side chains of aspartate (D), glutamate (E), cysteine (C), tyrosine (Y), histidine (H), lysine (K) and arginine (R) — nine in principle, fewer in practice when a residue is absent. How much charge each group carries at a given pH follows the Henderson-Hasselbalch relation: acidic groups go negative by 1/(1+10^(pKa-pH)) while basic groups go positive by 1/(1+10^(pH-pKa)). Summing those fractional charges gives the net charge at that pH. Net charge falls monotonically as pH rises but has no closed-form inverse, so the tool binary-searches over pH for the point where the sum is zero — the pI. The terminal pKa values are residue-specific, so the N-terminus is 9.8 on an alanine but 10.8 on a cysteine, and the termini therefore contribute differently when you change the sequence. Finally, different programs ship different default constants, which is why the same protein can come out a few tenths of a pH unit apart in different tools; every constant actually applied here is listed below the result.
Worked example
Take the tripeptide Lys-Lys-Lys (KKK). Each lysine side chain has a pKa of 10.53, the N-terminal alpha-amino group 9.0, and the C-terminal alpha-carboxyl 2.18. At pH 7 all three side chains are essentially fully protonated and carry +1 each, while the C-terminus is already fully deprotonated and carries -1, giving a net charge of about +3.0. Raising the pH strips protons from the side chains, and at about pH 10.84 the positive charge they contribute exactly offsets the terminal negative charge, so the net charge reaches zero — that is the pI. Swapping in Asp-Asp-Asp (DDD) reverses the picture: those side chains have a pKa of 3.65 and are fully deprotonated at pH 7, so the net charge is about -3.0 and the pI falls near 2.49. A hexahistidine tag (HHHHHH) sits in between, with side-chain pKa 6.0, a net charge of about +0.54 at pH 7 and a pI near 7.99. The pattern is consistent: the pI is governed almost entirely by which charged residues are present and how many, not by the length of the chain.
When to use it
Choosing the pH range for isoelectric focusing or two-dimensional gel separation is the clearest use: a protein is only retained on an immobilised pH gradient if its pI falls inside the strip range. For ion-exchange chromatography the working pH has to sit at least one unit away from the pI — below it for cation exchange, above it for anion exchange — otherwise the protein carries almost no net charge, binds poorly, or precipitates during low-salt elution. Storage and lysis buffers follow the same rule: keeping the pH a full unit away from the pI markedly reduces aggregation. When engineering a variant, comparing the pI of mutant and wild type is a fast way to catch a mutation that unintentionally alters the charge profile. The pI also helps explain anomalous migration on SDS-PAGE, predicts solubility in a given buffer, and guides elution conditions for a band recovered from an isoelectric focusing gel.
FAQ
- How is a protein isoelectric point calculated?
- Every ionizable group — the N-terminal alpha-amino group, the C-terminal alpha-carboxyl, and the side chains of Asp, Glu, Cys, Tyr, His, Lys and Arg — contributes a fractional charge given by the Henderson-Hasselbalch relation, and those contributions are summed at each pH. The net charge falls monotonically as pH rises, so the tool binary-searches over pH for the point where the sum reaches zero; that pH is the pI. The pKa constants actually used are listed beneath the result.
- Which amino acids affect the isoelectric point the most?
- Asp (pKa 3.65) and Glu (4.25) are fully deprotonated at physiological pH and are the dominant source of negative charge, pulling the pI down. Lys (10.53) and Arg (12.48) are essentially fully protonated at that pH and are the dominant source of positive charge, pushing the pI up. His (pKa 6.0) sits close to physiological pH and is therefore only partly charged, so its influence is more moderate. Cys (8.33) and Tyr (10.07) contribute according to how close the solution pH is to their own pKa.
- Which pKa values does this calculator use?
- It uses a widely used set of standard constants: for acidic side chains Asp 3.65, Glu 4.25, Cys 8.33 and Tyr 10.07, and for basic side chains His 6.00, Lys 10.53 and Arg 12.48. The N- and C-terminal pKa values are residue-specific — the N-terminus is 9.8 on Ala but 10.8 on Cys, and the C-terminus is 1.88 on Asp but 2.36 on Leu. Tools that ship a different constant set can return a pI a few tenths of a unit away, which is why every value actually applied is listed beneath the result.
- What does the net charge at pH 7 tell me?
- It shows where the protein sits electrically under physiological conditions. A positive net charge means it migrates towards the cathode at pH 7 and binds only weakly to a cation exchanger; a negative value means it migrates towards the anode and suits anion exchange. The magnitude also indicates how far the protein is from its isoelectric point — the closer the net charge is to zero, the more likely it is to aggregate or precipitate. For experiments run near pH 7, such as cell lysis, immunoprecipitation and most enzyme assays, this number is often more directly useful than the pI itself.
- Why do different tools give me a different pI for the same protein?
- Two reasons, mostly. First, the pKa constant sets differ: values published for the same group by different sources can disagree by 0.3 to 0.5 of a pH unit, and that disagreement propagates straight into the pI. Second, implementations differ in how they treat the termini and unusual residues — some use fixed N- and C-terminal pKa values, others make them residue-specific; some count His as a basic residue, others do not. A pI that differs by a few tenths between tools is therefore normal, and any comparison should first establish which constants each program used. This page lists every pKa value it applies so that such a comparison is possible.
- Which sequence formats does the calculator accept?
- Both 1-letter (MKNFTGALA) and 3-letter codes are accepted, and three-letter input may be separated (Met-Lys-Asn-Phe) or run together (MetLysAsnPhe). Whitespace, line breaks and digits are ignored. Tokens that cannot be recognised are listed in a warning and excluded from the calculation, so you never receive a result based on a silently truncated sequence. There is no hard length limit, though a sequence of many tens of thousands of residues will make the interface pause briefly while the binary search runs.