Apparent MW of protein on-PAGE gel > real MW?

Wayne J Becktel wbecktel at
Fri Mar 11 08:50:48 EST 1994

     Globular proteins have, on average, approximately the same number of 
negatively and positively charged groups with the population being roughly 12% 
of the total number of amino acids for each.  SDS binds, again on average, one 
molecule of the detergent for every two amino acids.  Thus, for a 100 residue 
globular protein, one might expect to find 12 positively and 12 negatively 
charged amino acids and about 50 SDS molecules associated with the protein.

     Very basic proteins, such as histones, do not have an "average" 
composition of charged amino acids and run slow on SDS/PAGE gels.  On the other
hand, chemical modification of proteins containing Cys residues with iodo 
acetic acid will increase the negative charge and cause those proteins to have 
lower apparent molecular weights.

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