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.
