Protein Molecular Weight Calculator
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Protein Molecular Weight Calculator
Approximate Molecular Weight:
Da
Understanding Protein Molecular Weight Calculation
Proteins are fundamental molecules in biology, composed of chains of amino acids. The molecular weight of a protein is a crucial characteristic, often expressed in Daltons (Da) or kilodaltons (kDa). It's essential for various biochemical and biophysical applications, including SDS-PAGE analysis, mass spectrometry, and understanding protein function and interactions.
The Calculation Method
The approximate molecular weight of a protein can be calculated by summing the average molecular weights of the individual amino acids in its sequence. This calculation typically accounts for the loss of a water molecule (H₂O) during the formation of each peptide bond that links amino acids together.
For a polypeptide chain with N amino acids, there are N-1 peptide bonds. The formula for the approximate molecular weight is:
Molecular Weight ≈ Σ(Average Molecular Weight of Amino Acid) - (N-1) * Molecular Weight of H₂O
Where:
Σ denotes the sum over all amino acids in the sequence.N is the total number of amino acids in the sequence.- The molecular weight of water (H₂O) is approximately 18.015 Da.
Average Amino Acid Residue Weights
Different amino acids have different side chains, contributing to their unique average molecular weights when they are part of a peptide chain. The standard average residue weights (in Da) for the 20 common amino acids are approximately:
- Alanine (A): 71.08
- Arginine (R): 129.14
- Asparagine (N): 114.10
- Aspartic Acid (D): 115.09
- Cysteine (C): 103.14
- Glutamic Acid (E): 129.12
- Glutamine (Q): 128.13
- Glycine (G): 57.05
- Histidine (H): 137.14
- Isoleucine (I): 113.16
- Leucine (L): 113.16
- Lysine (K): 128.17
- Methionine (M): 131.19
- Phenylalanine (F): 147.17
- Proline (P): 97.12
- Serine (S): 75.07
- Threonine (T): 99.13
- Tryptophan (W): 186.21
- Tyrosine (Y): 163.18
- Valine (V): 99.13
Note: These are average residue weights. For precise calculations, isotopic masses may be considered, but for most general purposes, these average values suffice.
Use Cases for Protein Molecular Weight
- Electrophoresis: Predicting protein migration patterns on SDS-PAGE gels.
- Mass Spectrometry: Verifying protein identity and purity.
- Drug Discovery: Understanding protein-ligand interactions and dosage calculations.
- Protein Engineering: Designing proteins with specific properties.
- Bioinformatics: Analyzing protein databases and predicting protein behavior.
This calculator provides a quick and convenient way to estimate the molecular weight of a protein based on its amino acid sequence, leveraging established biochemical principles.
function calculateMolecularWeight() {
var sequenceInput = document.getElementById("aminoAcidSequence");
var resultDiv = document.getElementById("result");
var resultValueSpan = document.getElementById("result-value");
var sequence = sequenceInput.value.trim().toUpperCase();
if (sequence === "") {
alert("Please enter an amino acid sequence.");
return;
}
// Average molecular weights of amino acid residues in Daltons (Da)
// These are average weights, and isotopic variations exist.
var aminoAcidWeights = {
'A': 71.08, // Alanine
'R': 129.14, // Arginine
'N': 114.10, // Asparagine
'D': 115.09, // Aspartic Acid
'C': 103.14, // Cysteine
'E': 129.12, // Glutamic Acid
'Q': 128.13, // Glutamine
'G': 57.05, // Glycine
'H': 137.14, // Histidine
'I': 113.16, // Isoleucine
'L': 113.16, // Leucine
'K': 128.17, // Lysine
'M': 131.19, // Methionine
'F': 147.17, // Phenylalanine
'P': 97.12, // Proline
'S': 75.07, // Serine
'T': 99.13, // Threonine
'W': 186.21, // Tryptophan
'Y': 163.18, // Tyrosine
'V': 99.13 // Valine
};
var totalWeight = 0;
var numberOfAminoAcids = sequence.length;
var invalidChars = [];
for (var i = 0; i 0) {
alert("Invalid amino acid code(s) found in sequence: " + invalidChars.join(', ') + ". Please use standard one-letter codes (A, R, N, D, C, E, Q, G, H, I, L, K, M, F, P, S, T, W, Y, V).");
resultDiv.style.display = 'none';
return;
}
// Subtract the weight of water molecules lost during peptide bond formation
// For N amino acids, there are N-1 peptide bonds.
var waterWeight = 18.015; // Approximate molecular weight of water (H2O)
if (numberOfAminoAcids > 1) {
totalWeight -= (numberOfAminoAcids – 1) * waterWeight;
} else if (numberOfAminoAcids === 1) {
// For a single amino acid, no peptide bond is formed, so no water is lost.
// The calculated totalWeight already reflects the residue weight.
// However, if the input is a single amino acid, its full molecular weight
// (not residue weight) is typically what's desired.
// This calculation gives residue weight. For full MW of single amino acid:
// Add H and OH back (H2O).
// For simplicity and common usage in protein MW, we'll stick to residue summation
// and water loss for >1 amino acid. For a single AA, residue weight is often used.
// If full MW of a free amino acid is needed, it's: residue_weight + 18.015 (water)
// But for context of a protein chain, residue weight sum minus water is standard.
}
// Ensure the result is not negative due to potential calculation errors with very short sequences
if (totalWeight < 0) totalWeight = 0;
resultValueSpan.textContent = totalWeight.toFixed(2); // Display with 2 decimal places
resultDiv.style.display = 'block';
}