Protein Molecular Weight Calculator Expasy

Protein Molecular Weight Calculation:

\[ MW = \sum (amino\ acid\ residues) - (water\ molecules \times 18.015) \]

Molecular Weight:

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1. What is Protein Molecular Weight?

The molecular weight (MW) of a protein is the sum of the masses of all its amino acid residues plus the mass of one water molecule (for the N-terminal H and C-terminal OH). This calculator uses the standard amino acid molecular weights from Expasy.

2. How Does the Calculator Work?

The calculator uses the following formula:

\[ MW = \sum (amino\ acid\ residues) + 18.015 - (peptide\ bonds \times 18.015) \]

Where:

Each amino acid has a specific molecular weight
18.015 Da is added for the terminal groups
18.015 Da is subtracted for each peptide bond (n-1 bonds for n residues)

3. Importance of MW Calculation

Details: Knowing a protein's molecular weight is essential for gel electrophoresis, mass spectrometry, protein purification, and biochemical characterization.

4. Using the Calculator

Tips: Enter the protein sequence in single-letter code (case insensitive). The calculator automatically removes non-alphabet characters before processing.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculates the theoretical MW of the unmodified polypeptide chain only.

Q2: What about selenocysteine (U) or pyrrolysine (O)?
A: These rare amino acids are not included in the standard calculation.

Q3: How accurate is this calculation?
A: This gives the theoretical average MW. Actual MW may vary slightly due to isotopic distribution.

Q4: Does it account for disulfide bonds?
A: No, disulfide bonds between cysteines are not accounted for in this calculation.

Q5: What's the difference between MW and molar mass?
A: In this context, they are essentially the same - the mass of one mole of the protein in grams.