1. What is Protein Isoelectric Point?

The isoelectric point (pI) is the pH at which a protein carries no net electrical charge. It is a fundamental property used in protein purification, characterization, and analysis.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• pKa values — Acid dissociation constants for ionizable groups
• Adjusted sum — Calculated based on amino acid composition

Explanation: The calculation considers all ionizable amino acid side chains (Asp, Glu, His, Lys, Arg, Cys, Tyr) and their respective pKa values to determine the pH where net charge is zero.

3. Importance of pI Calculation

Details: Knowing the isoelectric point is crucial for protein purification techniques like isoelectric focusing, ion exchange chromatography, and predicting protein solubility and behavior at different pH levels.

4. Using the Calculator

Tips: Enter a valid protein sequence using standard single-letter amino acid codes (ACDEFGHIKLMNPQRSTVWY). The sequence should contain only these characters without spaces or numbers.

5. Frequently Asked Questions (FAQ)

Q1: What are typical pI values for proteins?
A: Most proteins have pI values between 4-7, but values can range from 3 to 12 depending on amino acid composition.

Q2: Why do different proteins have different pI values?
A: pI depends on the number and type of ionizable amino acids (acidic vs basic) in the protein sequence.

Q3: How accurate is this calculation?
A: This provides an estimation. Actual pI can be influenced by protein structure, modifications, and environmental factors.

Q4: What are the main ionizable amino acids?
A: Aspartic acid (D), glutamic acid (E), histidine (H), lysine (K), arginine (R), cysteine (C), and tyrosine (Y).

Q5: Can I use this for modified amino acids?
A: This calculator uses standard pKa values for unmodified amino acids. Modifications may affect the actual pI.