You can manipulate the structure directly by dragging with the mouse. Mouse drag down with Shift key to zoom. Try gestures on touch screen devices. Click on the links below the black box to see other useful views of the structure. Advanced users can select the Console from the function menu and directly enter JSmol commands.
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 main form of circulating antibody. It cannot bind antigen or complement.
Disulfide bonds are shown in yellow spacefill. The complementarity determining regions (CDR) of the heavy chains are in contact across the +z axis and the CDRs of the light chains contact across the -z axis. In the absence of antigen, steric self-complementarity of the heavy chains favours this quaternary isomer over others. The complement-binding domains of the heavy chains (C-terminal ends) are held apart, preventing binding of complement. Where the complement-binding domain joins to the rest of the heavy chain, Pro240 and Pro241 are in the cis conformation to allow Cys239 to bond to Cys242.
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
Further details of JSmol available here