Rule of thumb for protein mass calculations: average residue mass When estimating the mass of a polypeptide from its amino acid count, what average mass per residue is typically used?

Introduction / Context:
Rapid back-of-the-envelope mass estimates are common in proteomics and molecular biology. A practical constant is the average mass of an amino acid residue in a polypeptide.

Given Data / Assumptions:

• Residue mass excludes water lost during peptide bond formation.
• Typical cellular amino acid composition assumed.

Concept / Approach:
While free amino acids average about 128–138 Da, formation of a peptide bond removes one molecule of water (approximately 18 Da). Empirically, the average residue mass in proteins is approximated as 110 Da, allowing quick estimates: mass ≈ number_of_residues * 110 Da.

Step-by-Step Solution:
Recognize “residue” = amino acid minus H2O after polymerization.Use the 110 Da convention for a representative proteome composition.Apply in practice: a 300-residue protein ≈ 33,000 Da (33 kDa).

Verification / Alternative check:
Detailed calculations using exact sequence and modification states refine this number but rarely deviate massively for rough estimates.

Why Other Options Are Wrong:
118 or 150 Da overestimate typical residue mass; 80 or 95 Da underestimate and would produce large errors.

Common Pitfalls:
Forgetting to include tags, post-translational modifications, or disulfide-linked partners in mass estimates.

Final Answer:
110 daltons.