MW proteína (sequência AA)

Protein molecular weight from amino acid sequence

A protein's molecular weight (MW) comes from adding up the residue weights, then subtracting the water that each peptide bond releases: MW = Σ MWaa − (n−1)·18.015 Da, with n the residue count and 18.015 Da the mass of water. A handy shortcut: the average residue weighs about 110 Da, which puts a 100-aa protein near 11 kDa. A few examples drive it home — human insulin (51 aa) ≈ 5.8 kDa, serum albumin (585 aa) ≈ 66 kDa, apolipoprotein B-100 (4 536 aa) ≈ 515 kDa. People treat ExPASy ProtParam as the standard reference; it draws monoisotopic and average masses from the same residue table.

Applications: bioinformatics, biotech, mass spec

Getting MW from a sequence is everyday work in bioinformatics (UniProt annotation, ProtParam), in protein design and vaccine development where you handle recombinant antigens and therapeutic IgG ≈ 150 kDa, and in mass spectrometry, where you match an observed m/z against the predicted MW to make an ID. It also guides SDS-PAGE planning: you pick a percentage range that resolves the band size you expect.

FAQ

Why does my observed MW differ from the predicted value? Post-translational modifications such as glycosylation, phosphorylation and signal peptide cleavage, along with disulfide bonds, all nudge the apparent mass away from the bare calculation.

Average or monoisotopic mass? Go with average for SDS-PAGE and low-resolution MS, and switch to monoisotopic once you're on high-resolution MS like FT-ICR or Orbitrap.

How accurate is the residue-based formula? For the unmodified polypeptide it lands within 0.01%, which is plenty. When the number is off, the cause is almost always biology rather than the arithmetic.