Calculate Molecular Weight of Protein

Protein Molecular Weight Formula:

\[ MW = \sum(MW_{aa}) - 18 \times (length - 1) \]

Molecular Weight:

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1. What is Protein Molecular Weight?

The molecular weight of a protein is the sum of the masses of all its atoms. In practice, it's calculated by adding up the weights of the amino acids and subtracting the weight of water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ MW = \sum(MW_{aa}) - 18 \times (length - 1) \]

Where:

\( MW \) — Molecular weight (g/mol)
\( MW_{aa} \) — Molecular weight of individual amino acids (g/mol)
\( length \) — Number of amino acids in the protein
18 — Molecular weight of water (g/mol)

Explanation: Each peptide bond formation results in the loss of one water molecule (H₂O, MW=18 g/mol). For a protein with n amino acids, there are (n-1) peptide bonds.

3. Importance of Molecular Weight Calculation

Details: Knowing a protein's molecular weight is essential for laboratory work including SDS-PAGE, protein purification, mass spectrometry, and biochemical characterization.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "MAKER"). The sequence is case-insensitive and non-letter characters are automatically removed.

5. Frequently Asked Questions (FAQ)

Q1: Does this include post-translational modifications?
A: No, this calculator only considers the unmodified polypeptide chain. Modifications like phosphorylation or glycosylation would increase the actual molecular weight.

Q2: Why subtract water molecules?
A: During peptide bond formation, one water molecule is lost per bond. This must be accounted for in the total molecular weight calculation.

Q3: What about N-terminal and C-terminal groups?
A: The calculation assumes standard protonated N-terminus and deprotonated C-terminus at physiological pH.

Q4: How accurate is this calculation?
A: It provides a theoretical average molecular weight. Actual experimental values may vary slightly due to isotopic distributions.

Q5: Can I use three-letter amino acid codes?
A: No, this calculator only accepts single-letter codes (A, R, N, D, C, E, Q, G, H, I, L, K, M, F, P, S, T, W, Y, V).