The following jpeg encoded image files are the models of human IgG1 where various residues have been marked in a different colour to illustrate features of the molecule. These IgG models are based on some model structures which I have been using in my undergraduate teaching. These other models are stored in a different location and can be accessed from my main menu . To download higher resolution jpeg encoded files just click on the gif images below
All of the above indicated residues have been implicated as being important in some IgG functions. I have mapped them all onto a model of human IgG1. This figure is based upon information in a review I have written and the colour figure here can be used in conjunction with the text in this review, replacing the monochrome image used in the book.
see Clark, M (1997), Chemical Immunology 65, 88-110 Antibody Engineering IgG Effector Mechanisms . You might also like to download the PDB format file of the model which I generated in order to produce the above figure.
I also have an online version of an earlier review on IgG structure and function which I wrote with Judith Greenwood and which usefully can be read in conjunction with reference to the above figure.
The neonatal Fc Receptor FcRn and the related catabolic Fc receptor FcRc which are probably the same receptor performing two functions (see the update section of the June issue of Immunology Today, Clark, M (1996), Immunology Today 17, 251 Two different roles for the neonatal IgG Fc receptor FcRn? or Clark M (1996), Lancet 347, 1104 One IgG receptor, two different functions (News item) or the review article on IgG functions, Clark, M (1997), Chemical Immunology 65, 88-110 Antibody Engineering IgG Effector Mechanisms ). The receptor is structurally related to MHC Class I antigens with an alpha heavy chain and a beta-2-microglobulin light chain. As you can see from the figure, two receptor binding sites exist in the IgG molecule in the interface between the C H 2 and C H 3 domains. Several Histidine residues are involved in the interaction and this makes the binding pH dependent. Interestingly this one of the sites on IgG molecules recognized by the bacterial proteins Protein A and Protein G which are also pH dependent.
You may also be interested in a rotating image of the above model which I have produced.
This shows that the Protein A binding site is the same as the conserved FcRn/FcRc binding site within the Fc. The fragment of Staphlococcus aureus Protein A is shown in green on the left.
When I have time I will add more text in explanation of all the above but you may also be interested in the section on Immunoglobulins from the text book Immunology by Janis Kuby and published by W.H. Freeman and Co., some of which is made available on the web.