1. What is Protein Molecular Weight?

Protein molecular weight is the sum of the masses of all atoms in a protein molecule. It's typically measured in Daltons (Da) or grams per mole (g/mol). The weight is calculated by summing the weights of individual amino acids and subtracting the weight of water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the protein molecular weight formula:

Where:

• \( MW \) — Molecular weight of the protein (g/mol)
• \( MW_{aa} \) — Molecular weight of each amino acid (g/mol)
• \( length \) — Number of amino acids in the protein
• 18 — Molecular weight of water (g/mol) lost per peptide bond

Explanation: For each peptide bond formed, one water molecule (18 g/mol) is lost. A protein with n amino acids has n-1 peptide bonds.

3. Importance of Protein Weight Calculation

Details: Knowing a protein's molecular weight is essential for laboratory techniques like SDS-PAGE, size-exclusion chromatography, mass spectrometry, and protein quantification.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (e.g., "MAKEGT"). The sequence should only contain standard amino acid letters (A-Z, except B, J, O, U, X, Z).

5. Frequently Asked Questions (FAQ)

Q1: Why subtract water molecules?
A: During protein synthesis, each peptide bond formation releases one water molecule (condensation reaction), so we subtract these lost molecules.

Q2: Does this include post-translational modifications?
A: No, this calculator only considers the unmodified polypeptide chain. PTMs like phosphorylation or glycosylation would add additional weight.

Q3: How accurate is this calculation?
A: It provides a theoretical molecular weight based on amino acid composition. Actual experimental values may differ slightly due to protein folding and other factors.

Q4: What about N-terminal and C-terminal groups?
A: This calculation assumes standard protonated N-terminus and deprotonated C-terminus at physiological pH.

Q5: Can I calculate weight for modified amino acids?
A: This calculator uses standard amino acid weights only. For modified residues, you would need to adjust the weights manually.