Protein A - FC binding...what really happens?

Mike Clark mrc7 at
Fri Aug 2 05:16:12 EST 2002

In article <20020801162158.12788.qmail at>,
<URL:mailto:tmorris at> wrote:
> Dear Colleagues:
> I had always assumed that each IgG FC region could only bind one protein
> A molecule. T he FC portion is referred to in literature in the singular
> sense, even though it deriv es from portions of two heavy chains, and
> this seems to be consistent with crystallogr aphic binding models as best
> as I can understand them (Deisenhofer, 1981, Biochemistry ).

It is worse than you thought. Not only are there two potential Protein A
binding sites in the Fc but thre are also two other potential binding
sites, one in each Fab!

Each molecule of Protein A, depending upon its source, might also have more
than one binding site for Ig!

Some V-region subgroubs are able to bind to Protein A which means that only
a proportion of antibodies bind Protein A in the Fab.

Mike                            <URL:>
M.R. Clark, PhD. Division of Immunology
Cambridge University, Dept. Pathology
Tennis Court Rd., Cambridge CB2 1QP
Tel.+44 1223 333705  Fax.+44 1223 333875

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