charge on protein

[Dr Engelbert Buxbaum]

Am 18.08.2008, 08:32 Uhr, schrieb Rashmi Srivastava  
<rashmi1208 from googlemail.com>:

> What simple methods can I use to determine the overall charge on a  
> membrane protein??

There is none. For a review on the problem see D.J. Winzor: Determination  
of the net charge (valence) of a protein: a fundamental but elusive  
parameter, Anal.Biochem. 325 (2004) 1-20.

Be particularly warry about all these naive computer programs that claim  
to calculate charge or pI from sequence. The pKa values tabulated for  
amino acid side chains are valid only in aqueous solution. Inside a  
protein the pKa may differ by 2-3 pH-units because of hydrophobic  
environment and the presence of other charged residues. This is of  
fundamental importance for the reaction mechanism of many enzymes.  
Example: Bacillus circulans xylanase Glu-172 pKa = 6.7 acts as proton  
donor for acid/base catalysis, Glu-78 pKa = 4.6 is ionised and stabilises  
the positively charged intermediate.