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The a2b2 quaternary structure of IgG suggests that, like hemoglobin, IgG exists in two states described as quaternary isomers. Geometrically, as for hemoglobin, there is a third possible isomer, but there are grounds to suppose it does not exist in nature. This structure is the proposed minor form of circulating antibody. It can bind antigen and complement. According to the principles of chemical equilibrium, binding antigen makes this isomer the major form and the binding of complement is greatly increased.
Binding complement requires two IgG molecules to be present.
Show complement-binding domains of a second IgG (chains F and G in the file)
/ Hide F and G
Disulfide bonds are shown in yellow spacefill. The complementarity determining regions (CDR) of the heavy and light chains are in close proximity across the z axis. Antigen is bound by the random entry of a surface loop or loops into the gap between heavy and light chains. If the loop is complementary to the CDRs, it stays there permanently and the equilibrium between trans and cis isomers is shifted towards cis. The complement-binding structure formed from two IgG assemblies works in the same way.
Where the complement-binding domain joins to the rest of the heavy chain, Pro240 and Pro241 are in the trans conformation to allow Cys239 and Cys242 to bind to their counterparts on the other heavy chain.
For further information about this protein see the posts of 24 June 2009, 24 November 2012 and 28 November 2012 on nativeproteins.blogspot.com Or contact the author, Dr Don Vanselow at dvanselow@hotmail.com
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