MW ≈ Number of amino acids × 110 Da (average)
The average molecular weight of an amino acid residue (after losing water during peptide bond formation) is approximately 110 Da. Thus, protein MW ≈ number of amino acids × 110. This is an estimate — actual MW depends on amino acid composition and post-translational modifications (glycosylation, phosphorylation). Example proteins: insulin 5.8 kDa (51 aa), GFP 27 kDa (238 aa), IgG antibody 150 kDa (~1330 aa total). SDS-PAGE separates proteins by size against a molecular weight marker ladder. For precise MW, use mass spectrometry (MALDI-TOF or ESI-MS). The largest known human protein is titin at ~3,800 kDa (34,350 amino acids).