1. What is Protein Molecular Weight?

Protein molecular weight is the sum of the atomic weights of all atoms in a protein molecule. For proteins, this is calculated from the amino acid sequence, accounting for water molecules lost during peptide bond formation.

2. How Does the Calculator Work?

The calculator uses the protein molecular weight formula:

Where:

• \( MW \) — Protein molecular weight (g/mol)
• \( AA\ MW \) — Molecular weight of individual amino acids (g/mol)
• \( n \) — Number of amino acids in the sequence
• \( 18 \) — Molecular weight of water (g/mol)

Explanation: The formula sums the molecular weights of all amino acids in the protein, then subtracts the weight of water molecules lost during the formation of (n-1) peptide bonds.

3. Importance of Protein Molecular Weight

Details: Knowing a protein's molecular weight is essential for various applications including protein purification, gel electrophoresis, mass spectrometry, biochemical assays, and pharmaceutical development.

4. Using the Calculator

Tips: Enter the protein sequence using single-letter amino acid codes (A, R, N, D, C, E, Q, G, H, I, L, K, M, F, P, S, T, W, Y, V). The sequence should not contain spaces or special characters.

5. Frequently Asked Questions (FAQ)

Q1: Why subtract (n-1)×18 from the total weight?
A: During protein synthesis, each peptide bond formation results in the loss of one water molecule (MW = 18 g/mol). For n amino acids, (n-1) peptide bonds are formed.

Q2: Does this calculation account for post-translational modifications?
A: No, this calculator provides the theoretical molecular weight of the unmodified polypeptide chain. Post-translational modifications (phosphorylation, glycosylation, etc.) will increase the actual molecular weight.

Q3: What about disulfide bonds?
A: Disulfide bond formation involves the loss of two hydrogen atoms, which is not accounted for in this simple calculation. For precise calculations with disulfide bonds, specialized tools should be used.

Q4: Are terminal groups considered?
A: This calculation assumes standard amino and carboxyl termini. For proteins with modified termini (acetylation, amidation, etc.), additional adjustments would be needed.

Q5: How accurate is this calculation?
A: This provides a theoretical molecular weight that is typically within 1-2% of the actual value for unmodified proteins, making it suitable for most laboratory applications.